Metabolism Flashcards
1
Q
State the products of pyruvate metabolism by: i. pyruvate dehydrogenase ii. lactate dehydrogenase
A
acetyl CoA lactate (lactic acid)
2
Q
Explain why epigenetics is different to genetic mutation.
A
Genetic mutation involves changes in the nucleotide sequence (changes in the DNA sequence), whereas epigenetics involves methylation of DNA and changes in histone structure that affect gene transcription.
3
Q
Define the term Basal Metabolic Rate (BMR) and list factors that may affect it.
A
BMR is the energy required to maintain life i.e. - for the functioning of the various tissues of the body at physical, digestive and emotional rest. (1 mark) Body weight Body temperature Gender Thyroid status
4
Q
Briefly explain how uncoupling proteins (UCPs) are involved in heat generation in the body.
A
UCPs allow a leak of protons across the membrane (1 mark), reducing the p.m.f, and the energy is dissipated as heat rather than ATP production. (1 mark) UCP1 is expressed in brown adipose tissue and is involved in thermogenesis. (1 mark)
5
Q
Briefly explain how the metabolism of alcohol can cause damage to the liver.
A
•The intermediate metabolite of alcohol metabolism, acetaldehyde, is toxic to liver cells. •The increased availability of acetyl-CoA affects liver metabolism. The conversion of alcohol to acetaldehyde by alcohol dehydrogenase also produces NADH. •The decreased NAD+/NADH ratio favours the formation of triacylglycerols which accumulate in the liver cells, leading to ‘fatty liver’, dyslipidaemia, insulin resistance
6
Q
Fluoroacetate is a plant toxin that is used as a pesticide. It inhibits aconitase of the TCA cycle. What effect will fluoroacetate have on aerobic metabolism?
A
Aerobic metabolism will be inhibited, so decreasing ATP production. Anaerobic metabolism will increase and cause lactic acidosis.
7
Q
List the most common reactions involved in phase I of drug metabolism
A
Oxidation, reduction, hydrolysis. Other reactions do occur, but these are the most common. The purpose of phase 1 is to add or expose a reactive group on the drug molecule.
8
Q
List agents in cells which protect against reactive oxygen species.
A
Three of: Superoxide dismutase Catalase Glutathione NADPH Antioxidant vitamins (e.g. C and E) Other antioxidants in the diet (e.g. polyphenols)
9
Q
What type of epithelium lines the gut?
A
Simple columnar
10
Q
Describe the key features of electron transport and explain how the proton motive force (p.m.f) is produced.
A
In Electron transport electrons are transferred from NADH (and FAD2H) sequentially through a series of multi-component complexes to molecular oxygen with the release of free energy. The free energy is used to move protons from the inside to the outside of the inner mitochondrial membrane. The membrane itself is impermeable to protons and as electron transport proceeds the proton concentration on the outside of the inner membrane increases. The chemical bond energy of the electrons is transformed into an electro-chemical potential difference of protons. This is known as the proton motive force (p.m.f).
11
Q
Explain why cortisol, a glucocorticoid, can have mineralocorticoid and androgen-like effects when present in high concentrations.
A
The actions of cortisol on target tissues are mediated by binding to receptors in the cytoplasm/nucleus. All steroid hormone receptors have similar basic structure with hormone and DNA binding domains. The hormone binding domains of the mineralocorticoid and androgen receptors have over 60% sequence homology with the hormone-binding domain of the glucocorticoid receptor. Thus, cortisol can bind to these receptors to a limited extent causing their partial activation.
12
Q
Explain why individuals with a defect in the enzyme lecithin-cholesterol acyltransferase produce unstable lipoproteins of abnormal structure. What are the clinical consequences of this defect?
A
•Lipoproteins particles are only stable if they maintain their spherical shape and this is dependent on the ratio of core to surface lipids. As the lipid from the hydrophobic core is removed and taken up by tissues the lipoprotein particles become unstable as the ratio of surface to core lipids increases. Stability can be restored if some of the surface lipid is converted to core lipid. This is achieved by the enzyme LCAT which is important both in the formation of lipoprotein particles and in maintaining their structure. The enzyme converts cholesterol (a surface lipid) to cholesterol ester (a core lipid) using fatty acid derived from lecithin (phosphatidylcholine). •Deficiency of the enzyme results in unstable lipoproteins of abnormal structure and a general failure in the lipid transport processes. Lipid deposits occur in many tissues and atherosclerosis is a serious problem.
13
Q
What is hyperlipoproteinaemia?
A
Any condition in which, after a 12 hour fast, the plasma cholesterol and/or plasma triglyceride is raised.
14
Q
Explain why insulin & C-peptide are secreted from the beta-cell in equimolar amounts.
A
Insulin is synthesised as the precursor molecular proinsulin. This molecule contains the A and B chains of insulin joined together by a connecting peptide. The conversion of proinsulin to insulin occurs in the storage vesicles and involves proteolysis. The products are insulin, C-peptide and 4 basic amino acids produced in equimolar amounts. Since these are produced in the storage vesicles they are secreted together during exocytosis.
15
Q
What role does calcitonin have in the regulation of serum calcium levels?
A
•Sometimes called the third hormone and is thought to lower serum calcium levels in other mammals. •It does not seem to have much of a role in humans. Higher levels in pregnancy may protect bone from excessive resorption. •The role of calcitonin is controversial in humans.
16
Q
What is gluconeogenesis and why is it necessary? Name the hormones that stimulate the process and those that inhibit it.
A
Gluconeogenesis is the production of glucose from precursors such as lactate, pyruvate, glycerol and certain amino acids. It is necessary to provide glucose for glucose-dependent tissues such as the CNS and red blood cells during starvation when the liver stores of glycogen have been exhausted. Insulin inhibits gluconeogenesis Cortisol and glucagon stimulate gluconeogenesis
17
Q
Describe how cortisol is transported in the blood and how it affects its target tissues.
A
Cortisol, like all steroids, is lipophilic (hydrophobic) and must be transported bound to plasma proteins. The major transport protein is transcortin and this carries ~90% of the plasma cortisol the remaining ~10% being free and biologically active. Cortisol can cross the plasma membranes of target cells and bind to cytoplasmic receptors. The hormone/receptor complex then enters the nucleus to interact with specific regions of DNA. This interaction changes the rate of transcription of specific genes and may take time to occur.
18
Q
Primary coenzyme Q is an autosomal recessive genetic condition that affects CoQ synthesis. What are the functional consequences of this deficiency?
A
Primary CoQ deficiency will prevent electron transfer from complexes I and II, decreasing ATP production. This will result in muscle weakness and exercise intolerance.
19
Q
Describe the role of the hypothalamus in the control of pituitary function.
A
The hypothalamus releases a number of substances that act on the anterior pituitary cells, and are known as Releasing or Inhibiting Hormones depending on whether they stimulate or inhibit the release of pituitary hormones. Releasing and Inhibiting hormones travel to the pituitary gland via specialised blood vessels known as the hypophyseal portal vessels. Releasing and inhibiting hormones allows the brain to control pituitary hormone secretion, and explains, for example how the secretion of hormones can change during stress. Examples of releasing or inhibiting hormones include: •Thyrotrophin Releasing Hormone (TRH) - stimulates TSH release •Corticotrophin Releasing Hormone (CRH) -stimulates ACTH release •Somatotrophin Releasing Hormone (SRH) - stimulates GH release •Somatostatin - inhibits GH release
20
Q
Under anaerobic conditions, the pyruvate produced by glycolysis in skeletal muscle may be reduced to lactate. What advantage is this to the muscle cells?
A
•There is a fixed amount of NAD+ + NADH in the cell. •The reactions of glycolysis require the presence of NAD+ which is converted to NADH. If all of the NAD+ is converted to NADH glycolysis would stop because of lack of NAD+. •This does not normally occur because, in the presence of oxygen, NADH is converted back to NAD+ by electron transport in the mitochondria. However, in the absence of oxygen (anaerobic conditions) or mitochondria (red blood cell) electron transport cannot occur. •Under these conditions pyruvate is converted to lactate via the enzyme lactic dehydrogenase (LDH) using NADH which is oxidised to NAD+ (2 marks): CH3COCOOH + NADH + H+ ↔ CH3CHOHCOOH + NAD+ •This enables glycolysis to continue so that it can provide the cell with ATP via substrate level phosphorylation.
21
Q
Briefly explain why the rate at which patients metabolise drugs can vary.
A
Variation in drug metabolism is due to genetic effects and environmental effects. General genetic variation in the population (polymorphisms) means that enzyme expression varies slightly and thus the rate of drug metabolism varies. Some people may have gene deletions and lack a key enzyme involved in drug metabolism, which can affect metabolism of certain drugs significantly. Some drugs or agents can inhibit enzymes in the cytochrome P450 system, which can affect the metabolism of other drugs given at the same time. Some drugs are well known to cause induction of enzymes in the liver, which increases the rate of metabolism of other drugs given at the same time.
22
Q
Briefly explain how an oxidative burst is produced by some leukocytes.
A
Some cells of the immune system, such as neutrophils and monocytes, when stimulated can rapidly produce a release of ROS which is known as an oxidative burst. The oxidative burst is produced by a membrane-bound enzyme complex termed NADPH oxidase. This enzyme is present in the cell membrane and it transfers electrons from NADPH across the membrane to couple these to molecular oxygen to generate superoxide radicals.
23
Q
Describe the relationship between electron transport and ATP synthesis. Explain how this relationship is altered during thermogenesis in brown adipose tissue mitochondria.
A
The chemical bond energy of the e- in NADH and FAD2H is used to drive ATP synthesis in the final stage of catabolism (oxidative phosphorylation). This occurs in mitochondria and involves highly organised multi-component systems. Two processes are involved electron transport and ATP synthesis. The p.m.f. created by electron transport, forces protons back into the mitochondrial matrix through an ATP synthase complex driving the synthesis of ATP from ADP and Pi. Normally ET and ATP synthesis are tightly coupled and controlled such that one does not occur without the other. The inner mitochondrial membrane of brown adipose tissue has, in addition to the ATP synthase complex, a special proton conductance protein (thermogenin) that allows the controlled re-entry of protons into the mitochondrial matrix without driving ATP synthesis i.e. it acts to uncouple ATP synthesis from ET. This protein is used to activate heat production (non-shivering thermogenesis) in cold environments. In response to cold, norepinephrine is released from the sympathetic nervous system and stimulates lipolysis releasing fatty acids to provide fuel for oxidation in brown adipose tissue. As a result of b-oxidation of the fatty acids NADH and FAD2H are formed, driving ET and increasing the p.m.f. However, norepinephrine also activates thermogenin allowing the protons to re-enter the mitochondrial matrix without driving ATP synthesis. This dissipates the p.m.f as heat (Marks p320 and Fig. 20.13).
24
Q
What three synthetic pathways do muscles use to generate ATP during exercise, and what are their effective time courses?
A
ATP synthetic pathways: Creatine phosphate Lactic acid system Oxidative phosphorylation
25
Define lactic acidosis and explain why it may occur.
Lactic acidosis is an elevation of plasma lactate that affects the buffering capacity of the plasma i.e. there is a fall in plasma pH due to the accumulation of lactic acid. Situations in which there may be a marked increase in plasma lactate due to increased production include strenuous exercise (up to 10g/min), hearty eating, shock and congestive heart disease. Increases due to decreased utilisation occur in liver disease, thiamine deficiency and during alcohol metabolism.
26
What would be the expected signs and symptoms in a patient with a deficiency in the ability to store or utilise glycogen?
Deficiency in the ability to store or use glycogen in the liver would result in fasting hypoglycaemia. Deficiency in muscle would result in muscle weakness (exercise intolerance).
27
How and where is vitamin D formed/ absorbed?
Formed in the skin via UV light or absorbed in the gut from the diet
28
Compare and contrast the functions of liver and skeletal muscle glycogen.
Glycogen is a storage form of glucose. However, there is no specialised glycogen storage tissue and it has to be stored in tissues that have other important functions. The liver can store up to ~100g glycogen, while skeletal muscle can store up to ~300g glycogen. Glycogen is degraded to glucose 1-phosphate in skeletal muscle in response to exercise and in the liver in response to fasting or as part of the stress response (“fright, fight or flight response”). In both tissues glucose 1-phosphate is converted to glucose 6-phosphate. In muscle, the glucose 6-phosphate enters glycolysis and is used to provide energy for the exercising muscle. Thus, muscle glycogen represents a store of glucose 6-phosphate that can only be used by the muscle cells. In liver during fasting or during stress the glucose 6-phosphate is converted to glucose by the enzyme glucose 6-phosphatase (this enzyme is absent from muscle): glucose 6-phosphate + H2O glucose + Pi catalysed by glucose 6-phosphatase The glucose is released into the blood stream and transported to other tissues. Thus liver glycogen represents a store of glucose that can be made available to all tissues of the body.
29
Describe, in outline, the processes that enable the triacylglycerols stored in adipose tissue to be used by skeletal muscle cells.
During prolonged aerobic exercise, starvation, stress situations and lactation, adipose tissue triacylglycerols are hydrolysed by the enzyme hormone-sensitive lipase to release fatty acids and glycerol. This process is known as lipolysis. It is activated by adrenaline, glucagon, growth hormone, cortisol and thyroxine and inhibited by insulin. The fatty acids are carried to tissues such as muscle via the blood stream bound non-covalently to albumin. The albumin-bound fatty acids are variously called non-esterified fatty acids (NEFA) or free fatty acids (FFA). The glycerol is not used by muscle cells but is instead metabolised by liver cells. Many tissues including heart muscle and skeletal muscle use the fatty acids as a source of energy. The process by which fatty acids are oxidised to release energy is known as -oxidation and it occurs in mitochondria. In order to be metabolised the fatty acids are activated by linking them to CoA in a reaction that requires ATP. The activated fatty acids are then transported into the mitochondria via a specialised transport process that uses carnitine. The rate of fatty acid transport into the mitochondrion determines the rate of their subsequent oxidation. Once inside the mitochondrion the oxidation of fatty acids occurs via a sequence of reactions (-oxidation pathway) that oxidises the fatty acid and removes a C2 unit (acetate). The shortened fatty acid is cycled through the reaction sequence repeatedly removing a C2 unit each turn of the cycle until only two carbon atoms remain. The overall reaction for -oxidation of stearic acid (C-18) is: C17H35COOH + 9 CoA + ATP + 8 FAD + 8 NAD+ + 8 H2O 9 CH3CO CoA (acetyl CoA) + AMP + 2 Pi + 8 FAD2H + 8 NADH + 8 H+
30
Why does a deficiency in pyruvate dehydrogenase complex activity cause lactic acidosis?
A deficiency results in a rise in pyruvate, thereby upregulating other reactions of pyruvate metabolism such as reduction to lactate leading to lactic acidosis.
31
How does the acetylcysteine work?
Its mode of action is through increasing glutathione levels and binding to the toxic metabolic breakdown products of paracetamol.
32
Outline the metabolic responses to feeding and fasting and describe how they are controlled.
Effects of feeding: Absorption of glucose, amino acids and lipids from the gut raises their blood concentration. As blood glucose rises, the endocrine pancreas responds by releasing insulin. Insulin increases glucose utilisation by muscle and adipose tissue. Insulin promotes production of glycogen in liver and muscle. Insulin promotes amino acid uptake and protein synthesis. Insulin promotes storage of fats. Effects of fasting: As blood glucose falls insulin secretion is depressed. Glucose uptake by tissues especially by muscle and adipose tissue is depressed. Falling blood glucose stimulates glucagon secretion i.e. insulin/anti-insulin ratio ¯. Glycogenolysis in the liver is stimulated to maintain blood glucose for the brain and other glucose dependent tissues. Lipolysis in adipose tissue is stimulated to provide fatty acids for use by tissues. Gluconeogenesis begins to increase to maintain supplies of glucose for the brain.
33
What are the effects of aldosterone?
Increased Na reabsorption in the kidney Decreased K absorption in the kidney
34
Compare and contrast the functions of glycolysis in adipose tissue, skeletal muscle and red blood cells.
Glycolysis is used to produce ATP by substrate level phosphorylation in all three tissues: In red blood cells it is the only mechanism for ATP production. In skeletal muscle it enables ATP production to occur under anaerobic conditions. In adipose tissue it is a minor route for ATP production. Glycolysis is used to produce useful intermediates in red blood cells and adipose tissue: 2,3-bisphosphoglycerate is produced from 1,3-bisphosphoglycerate in red blood cells and is important in regulating (decreases) the oxygen affinity of haemoglobin. Glycerol phosphate is produced from dihydroxyacetone phosphate in adipose tissue and is used in the esterification of fatty acids to produce triacylglycerols.
35
Give 2 reasons why finding out the time of ingestion of the paracetamol overdose is important?
If the paracetamol is taken less than one hour ago activated charcoal can be used if \> 150 mg/kg paracetamol is ingested. Patients are at risk of liver damage and requiring acetylcysteine which can be identified from a single measurement of the plasma paracetamol concentration providing it is not less than 4 hours since the tablets were ingested. Less than 4 hours may underestimate the paracetamol level. If it has been longer than 15 hours, then the patient may not benefit from acetylcysteine and if this is the case then you would need to get advice from the National Poisons Information Service.
36
Outline the pathways by which tissues obtain the cholesterol they need.
Direct synthesis from acetyl CoA within tissues. Obtain cholesterol synthesised in the liver via LDLs.
37
List four of the signs and symptoms you might expect to find in a 46 year old female who had the following endocrine test results on a fasting blood sample: Total plasma T4 48 nmol/L (normal range 54-142 nmol/L) Free T4 9 pmol/L (normal range 10-26 pmol/L) TSH 15 mU/L (normal range 0.3-3.8 mU/l) Explain, in general terms, why these signs and symptoms occur.
T4 decrease and TSH increase - suggests hypothyroidism due to defect in thyroid gland. •Weight gain due to reduced BMR. •Cold intolerance due to reduced BMR. •Lethargy/tiredness due to reduced uptake of nutrients by muscle. •Bradycardia – slow heart rate due to reduced responsiveness to catecholamines and reduced heart muscle protein synthesis. •Dry skin and hair loss due to reduced synthesis of proteins. •Slow reflexes and clumsiness due to reduced sensitivity to catecholamines. •Constipation due to reduced responsiveness of GI tract Hoarse voice.
38
Explain how tissues obtain the lipids they need from lipoproteins.
•Triacylglycerols are obtained from chylomicrons and VLDLs by the extracellular enzyme lipoprotein lipase present in the capillary bed of the tissue. •This hydrolyses triacylglycerols to fatty acids and glycerol. •The fatty acids are taken up by tissues and re-esterified to triacylglycerols using glycerol phosphate derived from glucose metabolism. •Cholesterol is obtained from LDLs by receptor mediated endocytosis. •The LDL particles bind to LDL receptors on the surface of target cells. •The receptor with its bound LDL is taken into the cell by endocytosis. •The endosome is attacked by lysosomal enzymes releasing free cholesterol in the cell and destroying the receptor protein. •The cholesterol is converted to cholesterol esters for storage. •When the cell has enough cholesterol, cholesterol inhibits the synthesis of new LDL receptors and the uptake of cholesterol is reduced.
39
What three effects does parathyroid hormone initiate in response to low calcium ions in extracellular fluid?
Mobilization of calcium from bone Enhancing absorption of calcium from the small intestine Suppression of calcium loss in urine (renal effect)
40
List four key components of control systems.
• communication • control centre • receptor • effector.
41
List five of the signs and symptoms you might expect to find in a 10 year old male who had the following test results on a fasting blood sample: Plasma glucose = 10 mmol/L (reference range = 3.9-6.1 mmol/L) Serum insulin = 5 pmol/L (reference range = 35-145 pmol/L) Why do these signs and symptoms occur?
Results suggest type 1 diabetes - lack of insulin in young patient producing hyperglycaemia. Hyperglycaemia caused by reduced glucose uptake in muscle & adipose tissue, reduced storage of glycogen in muscle & liver and increased liver gluconeogenesis. Glycosuria = glucose in urine - blood glucose concentration exceeds renal threshold of ~8mM Polyuria = excess urine production - due to osmotic effects of glucose in urine retaining water in the urine Polydipsia = Thirst - due to excess water loss in urine and osmotic effect of glucose on thirst centres Weight loss - lack of insulin leads to increased lipolysis (loss of adipose tissue) and proteolysis (loss of muscle) Ketoacidosis - nausea, vomiting - lack of insulin and increased lipolysis switches liver to produce ketones from fatty acids Tiredness/weakness - loss of muscle mass, dehydration Disturbances to vision, drowsiness, confusion - dehydration and ketoacidosis Acute clinical emergency refer to diabetologists at hospital. Treatment will include insulin, control of diet, exercise and monitor for development of complications.
42
Explain why blood ammonia levels are normally kept low and describe the processes involved.
Ammonia levels are kept low as ammonia is toxic especially to the CNS. It interferes with mitochondrial energy metabolism by removing a-ketoglutarate (forms glutamate) from the Krebs cycle. This reduces Krebs cycle activity, interferes with ATP production from glucose and inhibits brain function. Ammonia reacts with water to produce the ammonium and hydroxyl ions thus it is basic and part of its effects on the CNS may be related to pH changes. Ammonia is removed from the body by: Conversion to urea in the liver. The urea is removed from the body in the urine via the kidney. Conversion to glutamine using glutamate and used for purine and pyrimidines synthesis. Excretion as ammonium ion in urine.
43
Which set of clinical findings in a blood sample is more suggestive of liver disease? A.) Increased ALT, AST and bilirubin, or B.), increased ALT, no change in AST or bilirubin?
Both ALT and AST leak into the blood when liver cells are damaged. The increased bilirubin indicates a problem with hepatic metabolism.
44
What is haemoglobin A1c (HbA1c)? Explain how it can be used as an index of glycaemic control.
•Glucose reacts non-enzymatically with the terminal valine of haemoglobin to form glycosylated haemoglobin (HbA1c). •Extent of glycosylation depends on blood glucose concentration and half-life of haemoglobin (~60days). •Poor glycaemic control = high blood glucose = high HbA1c. Good glycaemic control = normal blood glucose = normal HbA1c (~5%).
45
Explain why some women develop gestational diabetes.
In some women the endocrine pancreas is unable to respond to the metabolic demands of pregnancy and the pancreas fails to release the increased amounts of insulin required. As a consequence there is a loss of control of metabolism, blood glucose increases and diabetes results (Gestational Diabetes).
46
List features of the gut mucosae that increase its surface area for absorption of nutrients.
Micro villi, villi, plicae circulares.
47
Compare and contrast triacylglycerols and glycogen as energy storage materials in humans.
Triacylglycerols and glycogen are both energy storage molecules. Triacylglycerols are the major energy storage molecules in the body (a 70kg man would normally store 10-15kg compared to 0.4 kg of glycogen). Triacylglycerols are stored in a highly specialised tissue (adipose tissue) and glycogen is stored in tissues such as the liver and skeletal muscle which have other important functions. Triacylglycerols are a more efficient from of energy storage as they are hydrophobic and are stored in an anhydrous form while glycogen is polar and is stored with water. In addition, triacylglycerols are more reduced that glycogen and contain more stored energy per C-atom than glycogen.
48
State immediate management strategies for sickle cell crisis.
Oxygen, Pain relief, Intravenous fluids, antibiotics, (transfusion).
49
What are the possible fates of the lactate produced by skeletal muscle under anaerobic conditions?
Lactate is released from muscle cells and carried in the blood to the liver and heart muscle. In both tissues it is converted back to pyruvate by LDH. In heart muscle the pyruvate is converted to acetyl~CoA that is subsequently oxidised via the TCA cycle to provide energy. In the liver pyruvate may also be oxidised to provide energy but most will be converted to glucose via the gluconeogenic pathway. A third possibility in the liver is oxidation to acetyl~CoA which may be used for lipid biosynthesis (fatty acids, ketone bodies or cholesterol).
50
Why might triacylglycerols rich in MCFAs be used in the treatment of disorders of lipid digestion and/or absorption or of chylomicron metabolism?
Tgs rich in MCFAs might be used in the treatment of disorders of lipid metabolism eg pancreatitis because they are rapidly degraded by gastric lipases, so eliminating the need for pancreatic lipase, they are taken up by enterocytes and released directly into the blood and do not require incorporation onto chylomicrons. Coconut oil contains a high concentration of Tgs rich in MCFAs and is often used in medical therapy for these disorders.
51
Describe how the secretion of cortisol is controlled.
•Adrenocorticotrophic Hormone (ACTH or corticotrophin) secreted from the corticotrophs of the anterior pituitary is the main factor controlling the secretion of cortisol. •The secretion of ACTH is under the control of corticotrophin releasing factor (CRF), a 41 amino acid polypeptide produced in the hypothalamus. •CRF is secreted in response to physical (temperature, pain), chemical (hypoglycaemia) and emotional stressors. •There is also negative feedback by glucocorticoids on both the hypothalamus and pituitary.
52
Outline the important roles of pyruvate dehydrogenase in glucose metabolism.
PDH is a multi-enzyme complex that catalyses the overall reaction: CH3COCOOH + CoA + NAD+® CH3CO~CoA + CO2 + NADH + H+ The PDH reaction cannot be reversed in the cell. There are two major consequences of this: the loss of CO2 from pyruvate is irreversible. acetyl~CoA cannot be converted to pyruvate and therefore cannot be converted to glucose. The reaction is therefore subject to control mechanisms that ensure: acetyl~CoA from the b-oxidation of fatty acids rather than from glucose is used in stage 3 of catabolism (acetyl~CoA inhibits the enzyme allosterically). the reaction is sensitive to the energy status of the cell (ATP and NADH inhibit and ADP activates the enzyme allosterically). the enzyme is activated when there is plenty of glucose to be catabolised (insulin activates the enzyme by promoting its dephosphorylation).
53
List the products that can be synthesised from acetyl CoA and explain why it cannot be converted to glucose in humans.
Acetyl CoA is produced by the catabolism of fatty acids, sugars, alcohol and certain amino acids and can be oxidised via stage 3 of catabolism. It is also an important intermediate in lipid biosynthesis. The major site of lipid synthesis in the body is the liver (some in adipose tissue) and most lipids (not polyunsaturated fatty acids) can be synthesised. Acetyl CoA cannot be converted to pyruvate in man because the enzyme pyruvate dehydrogenase is irreversible and there is no other mechanism for by-passing this enzyme. Since it cannot be converted to pyruvate it cannot be converted to glucose.
54
Describe the processes that produce ammonia in the body.
Ammonia is produced in the body by the deamination of amino acids. It is also absorbed from the gut where it is produced by bacterial action. Several deaminase enzymes of varying specificity are found in the liver and kidney that react with amino acids to remove the NH2-group as free NH3 (NH4+): L and D-amino acid oxidases are low specificity enzymes that convert amino acids to keto acids and NH3. Glutaminase is a high specificity enzyme that converts glutamine to glutamate + NH3. Glutamate dehydrogenase is a high specificity enzyme that catalyses the reaction: Glutamate + NAD+ + H2O -ketoglutarate + NH4+ + NADH + H+ It is important in amino acid metabolism by the liver as it is involved in both the disposal of amino acids (glutamate -ketoglutarate + NH4+) and the synthesis of non-essential amino acids (-ketoglutarate glutamate).
55
List two histological differences between brown and white fat.
White fat: peripheral nucleus, larger size, unilocular fat; brown fat: more central nucleus, smaller, multilocular fat. Brown fat cells are darker due to more mitochondria and greater blood supply.
56
What are the questions on the CAGE questionnaire?
Have you ever felt you should Cut down on your drinking? (Yes/No) Have people Annoyed you by criticising your drinking? (Yes/No) Have you ever felt bad or Guilty about your drinking? (Yes/No) Have you ever had a drink first thing in the morning to steady your nerves or to get rid of a hangover (Eye opener)? (Yes/No)
57
Explain the clinical consequences of severe protein deficiency in children.
Protein deficiency results in an inadequate intake of essential amino acids. This leads to a reduced rate of protein synthesis and a reduced rate of synthesis of other nitrogen containing compounds. The signs and symptoms could include: •Growth failure (height and weight below normal). •Impaired physical development (tiredness, weakness and poor exercise tolerance due to reduced muscle mass). •Impaired mental development (low IQ). •Negative nitrogen balance due to Nin \< Nout •Oedema due to reduced albumin synthesis in the liver. •Increased risk of infection due to reduced immunoglobulin synthesis. •Anaemia due to reduced haemoglobin synthesis. •Fatty liver due to reduced lipoprotein synthesis.
58
Outline the major ultrastructural features of the beta-cell that relate to the synthesis, storage and secretion of insulin.
Many mitochondria - active proteins synthesis, storage and secretion all require energy. Extensive RER - active synthesis of protein for export. Extensive Golgi - active formation of hormone storage vesicles. Many storage vesicles - storage of large amounts of hormone ready for secretion. Many microtubules & microfilaments - active secretory tissue (exocytosis).
59
Briefly explain the relationship between NADPH and glutathione.
There is a recycling system between NADPH and glutathione. NADPH reduces oxidised glutathione to its reduced form, via GSH reductase. The reduced glutathione is then available to be oxidised by reactive oxygen species, thus removing ROS.
60
List the most common enzymes involved in phase 1 ethanol metabolism
Alcohol dehydrogenase aldehyde dehydrogenase CYT2E1
61
Where and why is vitamin D converted to 25-hydroxyvitamin D?
Longer half-life of 2 weeks and is rapidly converted in the liver
62
How does type 2 diabetes mellitus differ from type 1 diabetes mellitus
•More gradual onset, often presenting with complications (visual problems, thrush, infections) •Caused by insulin resistance and/or defective insulin secretory response. •Normally adult onset, often obese individuals. •Do not usually develop ketoacidosis. Treated, at least initially with diet, drugs and exercise. •Strong genetic component.
63
In a blood test, what would the GP request be measured that would represent alcoholism?
•Gamma glutamyl transferase •mean corpuscular volume (size of the blood cells) •Aspartate amino transferase AST •Alanine amino transferase ALT •Alkaline phosphatase •Carbohydrate deficient transferrin
64
Explain how lipids are transported around the body.
•Hydrophilic lipids are transported in association with protein: ~98% as specialised lipoprotein particles ~2% bound non-covalently to albumin (fatty acids) Lipoprotein particles consist of non-covalent assemblies of triacylglycerols, phospholipids cholesterol, cholesterol esters and protein. There are 4 major classes: Chylomicrons transport dietary lipids (mostly triacylglycerols). VLDLs transport lipids (triacylglycerols) synthesised in the liver. LDLs transport cholesterol synthesised in the liver. HDLs transport cholesterol from peripheral tissues to the liver for disposal.
65
List four of the signs and symptoms you might expect to find in a 30 year old female who had the following endocrine test results on a fasting blood sample: Total serum T4 200 nmol/L (normal range 54-142 nmol/L) Total serum T3 5 nmol/L (normal range 0.8-2.5 nmol/L) TSH \<0.05 mU/L (normal range 0.3-3.8 mU/l) Explain, in general terms, why these signs and symptoms occur.
T3 and T4 elevated and TSH low suggests hyperthyroidism due to overactive thyroid. Common signs and symptoms: •Weight loss •Heat intolerance, sweating, warm vasodilated hands. •Irritability, emotional lability •Tachycardia (noticeable heart beat) often irregular. •Fatigue and weakness •Increased bowel movements - increased appetite •Menstrual dysfunction •Hyper-reflexive •Possible tremor of outstretched hands Signs and symptoms of hyperthyroidism caused by: •generally catabolic effects of T3 on tissue metabolism which result in increased metabolic rate. •effects of T3 which potentiate the actions of the catecholamines especially on the sympathetic nervous system. •specific effects of T3 on certain target tissues such as bone and the central nervous system.
66
Which types of fatty acids must be supplied by the diet?
Linoleic and linoleic acid, as they cannot be synthesised by humans. These are essential fatty acids.
67
Explain, in outline, why cardiac arrest affects the heart and central nervous system more rapidly than it affects skeletal muscle.
Cardiac muscle and the central nervous system are functionally highly specialized tissues that do not contain significant stores of fuel or oxygen. In addition, they have very limited capacity for anaerobic metabolism. Thus, they require a continuous supply of fuel and oxygen that comes to them via the circulatory system. Skeletal muscle is also a functionally highly specialized tissue. However, it does store significant amounts of fuel (glycogen) and oxygen (myoglobin). In addition, it has a limited capacity (~5 min) for anaerobic metabolism. Any disruption in the supply of fuel and oxygen to the heart and CNS, such as that caused by a cardiac arrest, rapidly affects their ability to maintain intracellular ATP levels. This leads to reduced functional activity, failure to maintain ionic gradients (especially of Na+ and K+) and reduced membrane stability. This may produce permanent structural damage. Skeletal muscle is less sensitive to the short-term disruption of its blood supply caused by a cardiac arrest as it has supplies of fuel and oxygen and can metabolise anaerobically for a few minutes. However, skeletal muscle will also eventually lose its ability to function if its blood supply does not return to normal.
68
•total serum cholesterol = 12 mmol/L (reference range = 3.5 – 6.5 mmol/L) •serum triacylglycerol = 1.0 mmol/L (reference range = 0.7 - 2.0 mmol/L) •serum lipoprotein profile = increased amounts of Low Density Lipoprotein particles (LDL) What type of hyperlipoproteinaemia is this patient likely to be suffering from?
Type IIa or familial hypercholesterolaemia
69
Explain why ketoacidosis develops in untreated type 1 diabetes mellitus.
Absence of insulin has a number of effects including: 1. increased rate of lipolysis in adipose tissue which releases large amounts of fatty acids, the substrate for ketone body formation. 2. activation of the ketogenic enzymes in the liver.
70
Briefly explain how an overdose of paracetamol can cause damage to the liver.
•With high levels of paracetamol the normal phase II pathway is saturated, so metabolism switches to a phase I pathway which produces a toxic product, N-acetyl-p-benzo-quinone imine (NAPQI) that is toxic to hepatocytes. (2 marks) •NAPQI undergoes phase II conjugation with glutathione and so depletes the hepatocytes of this important anti-oxidant defence. (1 mark)
71
•total serum cholesterol = 12 mmol/L (reference range = 3.5 – 6.5 mmol/L) •serum triacylglycerol = 1.0 mmol/L (reference range = 0.7 - 2.0 mmol/L) •serum lipoprotein profile = increased amounts of Low Density Lipoprotein particles (LDL) What other analysis should you have requested and why?
Blood glucose to check for fasting sample and/or diabetes.
72
Explain why cyanide is toxic to cells.
•Cyanide blocks NADH and FAD2H oxidation by inhibiting the respiratory chain at cytochrome oxidase/cytochrome aa3. •This prevents generation of the proton motive force and hence ATP synthesis. Without ATP generation cell structure and function are impaired and cell death rapidly ensues.
73
Explain why high circulating levels of ACTH can lead to increased pigmentation in certain areas of the body.
Pigment (melanin) production by melanocytes is activated by the hormones MSH (Melanocyte stimulating hormone). ACTH is a 39 amino acid, single chain polypeptide hormone released from the anterior pituitary. The initial biosynthetic precursor is a 241 amino acid protein called pro-opiomelanocortin (POMC). Post-translational processing of POMC at different sites produces a range of biologically active peptides including ACTH and MSH. The MSH sequence of 13 amino acids is contained within the ACTH sequence in POMC giving ACTH some MSH-like activity when present in excess. The clinical consequences of over-secretion of ACTH therefore include increased pigmentation due to partial MSH activity.
74
Outline the major structural differences between the various classes of steroid hormones.
Steroid hormones differ in the: •number of C-atoms (C27,21,19,18) •presence of functional groups •distribution of C=C double bonds.
75
Which chemical messengers are involved in the control of serum calcium (Ca2+) levels?
Parathyroid hormone and calcitriol
76
Define ‘oxidative stress’.
Oxidative stress occurs when the production of ROS is excessive or antioxidant levels are low in a cell, and the balance is shifted in favour of ROS. Usually, cells have sufficient antioxidant power to cope with the normal production of ROS.
77
Describe how the chemical structures of hormones determine how they are transported and the way they interact with their target tissues.
Hydrophilic hormones (proteins & catecholamines) are transported dissolved in the plasma. However, they cannot cross the plasma membrane of target cells and they must therefore interact with cell surface receptors. The interaction of the hormone with its plasma membrane receptor produces a change in the intracellular concentration of one or more signal molecules known as second messengers. Hydrophobic hormones (steroids and thyroid hormones) cannot dissolve in the plasma and they need to be transported bound to specialised transport proteins. However, they can cross the plasma membrane of target cells. They interact with intracellular receptors that may be in the cytoplasm or nucleus. The hormone/receptor complex associates with specific regions of DNA leading to a change in the rate of transcription of specific genes and to changes in the rate of synthesis of specific proteins. The activity of these proteins determines the response of the target tissue.
78
Explain why faecal fat might be measured if lipid malabsorption might be suspected.
Faecal fat will be increased with defects in lipid digestion, for example cystic fibrosis, or absorption because dietary fats primarily excreted rather than absorbed
79
What is creatinine?
Breakdown product of creatine (and creatine phosphate). • Produced by a spontaneous reaction at a constant rate; unless muscle is wasting • Excreted via kidneys. • Creatinine excretion per 24h is proportional to muscle mass of the individual. • Providesameasureofmusclemass • Creatinine concentration in urine is a marker of urine dilution (normally low, so a reference marker). • Used to estimate true urinary loss of many substances; • eghormonesinpregnancy
80
What role does gluconeogenesis play in a long term fast/starvation?
During starvation, hepatic and renal gluconeogenesis provides sustained glucose synthesis that maintains blood glucose levels, so ensuring glucose availability for tissues such as the brain and red blood cells which require a continuous supply.
81
Describe and account for five of the signs and symptoms you might expect to find in a male patient who had the following endocrine test results on a 9am fasting blood sample: Cortisol = 1400 nmol/L (reference range = 150-700 nmol/L)
Clinical effects of excess cortisol secretion. - Essentially Cushing’s syndrome: •Increased muscle proteolysis and hepatic gluconeogenesis may lead to hyperglycaemia with associated polyuria and polydipsia ("steroid diabetes"). •Increased muscle proteolysis leads to wasting of proximal muscles and produces thin arms and legs and muscle weakness. •Negative nitrogen balance due to loss of tissue protein. •Increased lipogenesis in adipose tissue leads to deposition of fat in abdomen, neck and face producing characteristic body shape and moon-shaped face and weight gain. •Purple striae found on lower abdomen, upper arms and thighs reflect the catabolic effects on protein structures in the skin and lead to easy bruising because of thinning of skin and subcutaneous tissue. •May be back pain and collapse of ribs due to osteoporosis caused by disturbances in calcium metabolism and loss of bone matrix protein. •Mineralocorticoid effects may produce hypertension due to sodium and fluid retention. •Immunosuppressive, anti-inflammatory and anti-allergic reactions of cortisol lead to increased susceptibility to bacterial infections and may produce acne.
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Miss Oban is a 35-year-old woman who has had a baby 18 months ago, presents with a six-week history of weight loss, poor energy, anxiety, heat intolerance and palpitations. Her GP finds that she is anxious, sweaty and has a fast heart rate. She has tremors when she holds her hands out. Her eyes are swollen. She has an enlarged thyroid gland (goitre). The GP does some blood tests and the results are as follows: Na 136 moml/l K 4.5 moml/l Urea 5.8 mmol/l Creatinine 86 μmol/L (reference range 55-105) Free T4 76 pmol/l (reference range 10-16) Free T3 26 pmol/l (reference range 4-8) TSH \<0.01 mU/l (reference range 0.4-4.5) How do you treat this person?
Beta blockers, anti-thyroid medications such as Carbimazole, Propylthiuracil Radioiodine, surgery
83
By what process do RBCs metabolise glucose in the absence of oxidative phosphorylation in mitochondria?
ATP is generated by glycolysis. Resulting pyruvate is converted to lactate. RBCs also use the pentose phosphate pathway.
84
The National Screening Committee recommends that new-born blood spot screening include:
• Congenital Hypothyroidism (CHT) • Sickle Cell disorders • Cystic Fibrosis (CF) Six inherited metabolic diseases • phenylketonuria(PKU-Phenylalanine) • medium-chain acyl-CoA dehydrogenase deficiency (MCADD) • maple syrup urine disease (MSUD - leucine, isoleucine, valine) • isovalericacidaemia(IVA-leucine) • glutaric aciduria type 1 (GA1 - lysine & tryptophan) • homocystinuria(HCU)
85
Describe the long-term clinical consequences of persistent hyperglycaemia.
Macrovascular complications: • increased risk of stroke • increased risk of myocardial infarction • poor circulation to the periphery especially feet Microvascular complications: • Eye disease, glaucoma, cataracts, retinopathy. • Kidney disease (nephropathy) • Peripheral nerve disease (neuropathy) - loss of sensation, erectile dysfunction. • Diabetic feet - risk of ulceration and infection
86
Describe the typical signs and symptoms of untreated type 1 diabetes mellitus.
•Hyperglycaemia due to lack of insulin - reduced uptake of glucose in muscle and adipose tissue, reduced synthesis of glycogen and increased gluconeogenesis •Glycosuria due to hyperglycaemia exceeding renal threshold •Polyuria (large volumes of urine) due to glycosuria •Polydipsia (thirst) due to polyuria •Weight loss due to muscle (proteolysis) and adipose tissue (lipolysis) breakdown •Ketoacidosis and ketonuria (nausea, vomiting, confusion) due to lack of insulin and increased availability of fatty acids switching liver to produce ketones from fatty acids •Often presents as acute clinical emergency in the young.
87
Diabetes carries a significant risk of microvascular complications. List three examples of microvascular complications.
Retinopathy. Neuropathy, Nephropathy
88
Explain why the pituitary is considered by some to be the "Master Endocrine Gland".
Unlike other endocrine glands the pituitary releases a number of trophic hormones that control hormone secretion from several other endocrine glands. The trophic hormones include: •Thyroid Stimulating Hormone = Thyrotrophin (TSH) - affects thyroid gland •Adrenocorticotrophic Hormone = Corticotrophin (ACTH) - affects adrenal gland •Growth Hormone = Somatotrophin (GH) - affects liver production of somatomedins •Luteinizing Hormone (LH) - affects ovary and testis hormone production. •Follicle stimulating Hormone (FSH) - affects ovary and testis function
89
List five of the signs and symptoms you might expect to find in a 60 year-old male who had the following test results on a fasting blood sample: Plasma glucose = 8 mmol/L (reference range = 3.9-6.1 mmol/L) Serum insulin = 250 pmol/L (reference range = 35-145 pmol/L) Blood Haemoglobin A1c = 10% (reference range = 3.8-6.4%)
Results suggest type 2 diabetes due to insulin resistance - hyperglycaemia in the presence of high insulin i.e. insulin not working properly. Signs & symptoms include: • Weight gain (obesity) • Glycosuria, Polyuria & Thirst • Tiredness/weakness • Visual problems - retinopathy, cataracts • Thrush, skin infections • Erectile dysfunction - neuropathy
90
Explain how some amino acids can be converted to glucose (glucogenic), some can be converted to ketone bodies (ketogenic) and others can be converted to both glucose and ketone bodies (glucogenic and ketogenic).
An early step in the catabolism of an amino acid is the removal of the amino group (-NH2). This is converted to urea (CO(NH2)2) and excreted from the body in the urine. The remaining C-skeletons of the amino acids are converted to one or more of the following organic precursors: pyruvate, oxaloacetate, fumarate, -ketoglutarate, succinate, acetyl CoA. Amino acids that produce acetyl CoA (e.g. leucine, lysine) are described as ketogenic as acetyl CoA can be used for the synthesis of ketone bodies. Amino acids that give rise to the other products (glutamic, aspartic, serine) are described as glucogenic as they can be used for glucose synthesis by gluconeogenesis. Some of the larger amino acids (isoleucine, threonine, phenylalanine, tyrosine and tryptophan) are both ketogenic and glucogenic as they give rise both to acetyl CoA and one of the other organic precursor molecules.
91
Describe the functions and actions of adrenaline (epinephrine) in humans.
Adrenaline is released as part of the fright, flight or fight response in man and it is secreted in response to stress situations. It has effects on cardiovascular system ( Increase cardiac output, Increase blood supply to muscle), central nervous system (Increase mental alertness), carbohydrate metabolism (Increase glycogenolysis) and lipid metabolism (Increase lipolysis). The major target tissues for its action are liver, skeletal muscle, heart muscle and the central nervous system.
92
List signs or symptoms of the very high blood cholesterol that you might expect to find in this patient?
Corneal arcus Xanthelasma and/or Tendon xanthoma Accelerated development of atherosclerotic diseases
93
What classes of lipoprotein are present in the serum from a fasting blood sample taken from a normal individual and what are their functions?
LDL, VLDL and HDL (Not chylomicrons).
94
Secretion of insulin is regulated rather than constitutive, describe these two modes of protein secretion.
In regulated secretion vesicles release the protein in response to stimulation by a signal; for example a hormone, neurotransmitter or in the case of insulin an increase in glucose, while constitutive secretion is an unregulated ongoing process.
95
List the functions of the TCA cycle.
The TCA cycle has both catabolic and anabolic roles. The catabolic role is to oxidise the acetyl group (CH3CO-) of acetylCoA to two molecules of carbon dioxide, with the concomitant reduction of NAD & FADHand formation of GTP. The NADH and FADH2 are subsequently oxidised via the respiratory chain to generate ATP by oxidative phosphorylation. The anabolic role is to provide intermediates for the synthesis of various important molecules such as haem, fatty acids, glutamate and aspartate The TCA cycle plays also an important role in the conversion of several glucogenic amino acids to glucose.
96
Sickle cell disease is the name for a group of inherited conditions that affect the red blood cells. Why do Suzie’s red blood cells appear elliptical?
The point mutations in the haemoglobin molecules produce an abnormal protein structure, which when they become self-assembled inside red blood cells it results in the formation of rigid fibril structures. These fibril structures damage to the cell membrane making the red blood cells less deformable and make them sickle shaped.
97
How does the H2 breath test help provide a diagnosis of lactose intolerance?
Lactose passes through the small intestine to the colon undigested. Colonic bacteria digest it to short chain fatty acids and H2 which can be tested in the breath.
98
How are T3 and T4 transported in the blood and how do they produce effects in target tissues?
T3 and T4 are hydrophobic molecules and are transported in the blood bound to proteins (thyronine binding globulin, pre-albumin and albumin). Only a small amount (\<1%) of T3 and T4 is free in solution and it is this free hormone that is biologically active. T3 has a slightly lower affinity for the transport proteins than T4 and hence a greater percentage is free and its half-life in the circulation is therefore shorter (2 days compared to 8 days for T4). The normal plasma concentration of free T3 is 1.2-3.8 pmoles/l and T4 is 13-30 pmoles/l. T3 and T4 cross the plasma membrane of target cells either by diffusion or by a specific carrier. They then bind to specific cytoplasmic binding proteins that concentrate the hormone and carry them to specific high affinity receptors. These receptors are located in the nucleus and possibly mitochondria. The receptors are proteins and they have a 10-fold greater affinity for T3 than T4. The receptors have a number of domains. Binding of T3 to the hormone-binding domain is thought to produce a conformational change in the receptor that unmasks the DNA-binding domain. Interaction of the hormone-receptor complex with DNA (nuclear or mitochondrial) increases the rate of transcription of specific genes that are then translated into protein. The increased rate of protein synthesis stimulates oxidative energy metabolism in the target cells to provide the extra energy required for protein synthesis. In addition, protein synthesis produces increased amounts of specific functional proteins leading to increased cell activity and an increased demand for energy.
99
Briefly explain how superoxide radicals are produced by mitochondria.
During oxidative phosphorylation about 0.1 - 2% of electrons do not reach the end of the electron transport chain and they prematurely reduce oxygen to from superoxide radicals, O2-.
100
Outline treatment options for a patient with hyperlipidaemia.
1. Reduce/eliminate cholesterol from the diet and reduce the intake of triacylglycerols especially those containing saturated fatty acids. 2. Statins (simvastatin) to reduce endogenous synthesis of cholesterol in tissues especially the liver. 3. Bile salt sequestrants (cholestyramine) to increase disposal of cholesterol from the body.
101
How would you expect such an individual to respond to a stress such as trauma or a severe infection?
Release of adrenal steroids is part of the normal response to a stress such as infection or trauma. Patients with Addison’s disease may not therefore respond appropriately. This may lead to an Addisonian Crisis with nausea, vomiting, fever, hypotension and vascular collapse. This is an emergency situation and treatment with fluid replacement (5% dextrose in normal saline) and cortisol is life-saving.
102
Name the anti-oxidant vitamins.
vitamin C (ascorbic acid) (water soluble) vitamin E (a-tocopherol) (lipid soluble) vitamin A (retinol)
103
Compare and contrast the processes of oxidative phosphorylation and substrate level phosphorylation.
Oxidative phosphorylation Produces ATP from ADP and Pi Requires membrane associated complexes (inner mitochondrial membrane) Energy coupling occurs indirectly through generation and subsequent utilisation of a proton gradient (p.m.f). Cannot occur in absence of oxygen. Efficiency of energy conservation ~33% - considerable heat production Major process for ATP synthesis in cells that require large amounts of energy. Substrate level phosphorylation Produces ATP from ADP + phosphorylated organic compound. Requires soluble enzymes (cytoplasmic and mitochondrial matrix) Energy coupling occurs directly through formation of a high energy of hydrolysis bond (phosphoryl-group transfer). Can occur to a limited extent in absence of oxygen. Efficiency of energy conservation ~60% - low heat production. Minor process for ATP synthesis in cells that require large amounts of energy.
104
Describe and account for five of the signs and symptoms you might expect to find in a female patient who had the following endocrine test results on a 9am fasting blood sample: Cortisol = 78 nmol/L (reference range = 150-700 nmol/L) ACTH = 120 ng/L (reference range = 10-80 ng/L)
•Low cortisol in presence of high ACTH suggests Addison’s Disease. Too little cortisol secretion, that was caused by auto-immune destruction of the adrenal gland, would also involve the loss of the mineralocorticoids producing a complex situation that may present as an acute emergency (Addisonian Crisis) or as a chronic debilitating disorder (Addison’s disease): •Insidious onset with initial non-specific symptoms of tiredness, extreme muscular weakness, anorexia, vague abdominal pain, weight loss and occasional dizziness. •Extreme muscular weakness and dehydration. •A more specific sign is the increased pigmentation, particularly on the exposed areas of the body, points of friction, buccal mucosa, scars and palmar creases due to ACTH-mediated melanocyte stimulation. •Decreased blood pressure due to sodium and fluid depletion. Reduced vascular tone. •Postural hypotension due to fluid depletion. •Hypoglycaemic episodes especially on fasting. •These effects may lead to nausea, vomiting, extreme dehydration, hypotension, confusion, fever and even coma (Addisonian crisis). This constitutes a clinical emergency that must be treated with intra-venous cortisol and fluid replacement (dextrose in normal saline) to avoid death.
105
Explain why the process of fatty acid synthesis (lipogenesis) is not simply a reversal of the process of fatty acid degradation (β-oxidation).
The process of fatty acid degradation is an exergonic process and involves reactions that are not freely reversible in the cell. Thus, the process cannot be easily reversed in the cell. The process for fatty acid synthesis must therefore use different reactions. These differences also allow: greater flexibility (substrates and intermediates can be different). better control (can be controlled independently or co-ordinately).
106
How is the activity of the thyroid gland controlled?
The secretion of T3 and T4 is under the control of the hypothalamus and anterior pituitary gland. The hypothalamic factor is Thyrotrophin-Releasing Hormone (TRH). This is released from cells in the dorsomedial nucleus of the hypothalamus under the influence of the circulating levels of T3 and T4 (negative feedback), stress (increases release)) and temperature (fall in temp increases release). The TRH travels in the hypothalamic/pituitary portal system to affect the thyrotrophs in the anterior pituitary. Under the influence of TRH the thyrotrophs release Thyroid Stimulating Hormone (TSH) which travels in the blood to affect the follicular cells of the thyroid gland. Thyroid Stimulating Hormone is released in low-amplitude pulses following a diurnal rhythm with higher levels attained during the night and decreasing in the early hours of the morning. TSH interacts with receptors on the surface of the follicle cells and stimulates all aspects of the synthesis and secretion of T3 and T4. In addition, TSH has trophic effects on the gland which result in increased vascularity, increase in size and number of the follicle cells. These trophic effects can result in an enlarged thyroid (goitre).
107
Describe in general terms how amino acids are degraded in the body and list the products of their degradation.
Degraded to smaller molecules largely in the liver. Each amino acid has its own pathway of catabolism but the pathways share common features: The C-atoms are converted to intermediates of carbohydrate metabolism (glucogenic amino acids) or lipid metabolism (ketogenic amino acids). The N-atoms are usually converted to urea for excretion in the urine but some may be excreted directly as ammonia and some may be converted to glutamine and used for the synthesis of purines and pyrimidines. The first step in the various pathways usually involved the removal of the –NH2 group by transamination or deamination. Products = urea, pyruvate, acetyl CoA, -ketoglutarate, oxaloacetate, succinate and fumarate.
108
In the GI tract what proteins are responsible for the transport of glucose, galactose and fructose into the mucosal cell? What protein allows for the transport of these three sugars into the portal circulation?
The intestine can only absorb monosaccharides (glucose, fructose and galactose). Na+/glucose cotransport (SGLT1) transport glucose and galactose. Fructose and glucose enters via facilitated transport (GLUT5). GLUT2 allows the transport of glucose, galactose and fructose into the portal circulation.
109
Define 'metabolic syndrome'.
Metabolic syndrome is the co-occurrence in the same individual of a number of cardiovascular risk factors such as dyslipidaemia and hypertension, usually in association with overweight or obesity and a sedentary life style.
110
Outline the metabolic responses to starvation and describe how they are controlled.
Should fasting proceed beyond 10hr the changes associated with starvation begin to occur. Insulin levels continue to fall, glucagon, growth hormone and cortisol levels increase. Muscle proteolysis increases to provide amino acids for gluconeogenesis. Gluconeogenesis increases to maintain blood glucose for the CNS. Lipolysis continues to increase to provide fatty acids for tissues. Ketogenesis in the liver produces ketone bodies that can be used by the CNS and other tissues. As ketogenesis increases so rate of gluconeogenesis decreases but it remains active and the kidney contributes to gluconeogenesis.
111
List the most common reactions involved in phase II of drug metabolism
Conjugation with glucuronic acid (glucuronidation), sulphate, glutathione. Glucuronidation is the most common reaction, but other reactions are possible. The purpose of phase II metabolism is to conjugate a water-soluble group with the drug molecule directly or with the metabolite from phase 1.
112
Describe the role of glutamine synthetase.
Glutamine synthetase catalyses the reaction between ammonia and glutamate to glutamine in the peripheral tissues. Glutamine is then transported in the blood to the liver and kidneys.
113
How would debranching enzyme deficiency affect glycogen structure? What are the clinical consequences?
Debranching enzyme is bifunctional, a deficiency (Cori disease) results in glycogen with short branches. Hypoglycaemia and muscle weakness can result in a decreased ability to mobilise stored glycogen.
114
Bill, an agricultural worker, was employed by a farmer in East Anglia to spray 400 acres of wheat with a pesticide containing an aromatic weak acid, dinitrocresol (DNC). He failed to wear any protective clothing. After several days work, Bill fell ill with a very high temperature and profuse sweating. He was admitted to hospital, but in spite of strenuous attempts to decrease his body temperature, he fell into a coma and died. Autopsy showed a striking absence of subcutaneous fat. How would you explain these symptoms?
Aromatic weak acids like 2-DNC (and 2,4-DNP) readily penetrate the mitochondrial inner membrane and act as uncoupling agents, i.e. they collapse the proton motive force and hence cause uncontrolled respiration to occur. This consumes large amounts of metabolic fuels (particularly fatty acids which are derived from fats stored in adipose tissue - hence the absence of subcutaneous fat) and of oxygen (this would lead to hypoxia but is prevented by increased pulmonary activity). As much less ATP than normal is made by oxidative phosphorylation under these conditions, a considerable amount of energy is lost as heat and the body temperature rises. When attempts to combat this by increased sweating eventually fail, the patient falls into a coma and dies.
115
How does the cell ensure that the glucose taken in by a GLUT remains inside rather than diffusing back out?
Hexokinases irreversibly catalyses the phosphorylation of intracellular glucose to glucose 6 phosphate, so trapping it inside the cell because there is no membrane transporter for phosphorylated sugars. Note there are four isoforms of hexokinase, 1-3 are found in most tissues and hexokinase 4 (glucokinase) is found in the liver and pancreatic beta cells.
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