Metabolism Sessions 1-6 Flashcards
Which is the natural stereoisomer form of a monosaccharide?
D
How do carbohydrates exist?
As mono, di or polysaccharides
In a typical western diet, how many grams make up the carb intake?
300
What makes up 1% of wet weight in the body?
Carbohydrates
How is the energy from carbohydrates stored in the body?
First ~300g as glycogen, nucleic acids, glycolipids and glycoproteins then as TAGs in adipose tissue
At what level is glucose always present in the blood?
~5 mmol per litre
What can excessive dietary intake of galactose, fructose and glucose lead to respectively in the blood?
Galactosaemia
Fructose intolerance
Diabetes mellitus
What glucose level in the blood confers with diabetes?
> or equal to 7 mmol per litre
What bond is found in disaccharides?
O-glycosidic
What are the monomers of sucrose, lactose and maltose respectively?
Sucrose: glucose and fructose
Lactose: galactose and glucose
Maltose: 2x glucose
Where does maltose come from in the diet?
Digestion product of dietary starch
When is a disaccharide non-reducing?
If aldehyde and/or ketone groups of both monomers are involved in forming the glycosidic bond
Give an example of a non-reducing disaccharide.
Sucrose
What are mainly homopolymers?
Disaccharides
What is the structure glycogen?
Glucose units linked by alpha 1-4 and alpha 1-6 bonds which give a highly branched structure
Where is glycogen stored?
In granules in the liver and skeletal muscle
What is the structure of starch?
Amylose with alpha 1-4 binds and amylopectin with alpha 1-4 and alpha 1-6 bonds
What does hydrolysis of starch release in the GI tract?
Glucose and maltose
Why can the human body not digest cellulose?
It does not have the enzymes needed to break beta 1-4 bonds present between glucose molecules
If it can’t be digested, why do we need to eat cellulose?
Important for normal GI function
Where is starch and glycogen digested to dextrins?
In the mouth by salivary amylase
Where are dextrins digested to monosaccharides?
In the small intestine by pancreatic amylase
What is attached to the brush border membrane of epithelial cells to aid digestion?
Pancreatic amylase, isomaltase, sucrose and lactose
How does a low level of lactase cause diarrhoea?
Lactose remains in colon lumen and increase osmotic pressure to water moves into the lumen
How does a low level of lactose cause bloating?
Colonic bacteria digest the lactose present in the colonic lumen and produce hydrogen, carbon dioxide and methane
How are glucose, galactose and fructose absorbed into the blood?
Actively transported into absorbitive cells lining gut and then move by facilitated diffusion into blood and tissue
What does facilitated diffusion of glucose into the tissues require?
GLUT1-5
Which GLUT transporter is upregulated in response to high levels of insulin in skeletal muscle and adipose tissue?
4
Can all tissues remove glucose, galactose and fructose from the blood?
Yes
Where is the major site of fructose and galactose metabolism?
Liver
Roughly how much glucose is needed per day to supply the tissues who have an absolute need for glucose?
180 g
Which areas of the body have an absolute glucose requirement?
RBCs WBCs Kidney Lens of eye CNS
What is the central pathway of glucose metabolism which happens in all tissues?
Glycolysis
What are the functions of glycolysis?
Oxidise glucose
Produce NADH
Net synthesis of 2 ATP
Produce glycerol phosphate and 2,3-BPG
What are the main features of glycolysis?
Exergonic
Oxidative
No loss of carbon dioxide
Irreversible pathway
What is the purpose of converting glucose to G6P during glycolysis?
Makes it ionic so that it doesn’t cross the membrane
Increases reactivity so can follow a different pathway
Allows formation of high phosphoryl-group transfer potential compounds
Why are steps 1 and 3 in the glycolytic pathway irreversible?
Have a very negative delta-G
Which step of the glycolytic pathway is the committing step to glycolysis?
3
What does the initial investment of energy in glycolysis do to glucose?
Makes it less stable so it is more easily catabolised
What occurs in the second phase of glycolysis?
C3 –> 2C3 which are interconvertible
Small amount of reducing power is captured
Substrate level phosphorylation allows for ATP synthesis
Which glucose metabolism pathway is cytosolic?
Glycolysis
Which glucose metabolism pathway is cytoplasmic?
Pentose Phosphate pathway
Where does the pentose phosphate pathway occur?
Liver
RBCs
Adipose tissue
What are the functions of the pentose phosphate pathway?
Produce NADPH for reducing power for anabolic processes
Produce C5 sugar ribose for nucleotide synthesis
What is the NADPH generated in the pentose phosphate pathway used for?
Lipid synthesis in liver and adipose tissue
Maintain free -SH groups on cysteine residues in RBCs
Detoxification
What are the features of the pentose phosphate pathway?
High activity in dividing tissues
Carbon dioxide produced so irreversible
Considered in two phases
What happens in the first phase of the pentose phosphate pathway?
G6P makes C5 sugar phosphate, NADPH, hydrogen ions and carbon dioxide
Which enzymes are involved in the first phase of the pentose phosphate pathway?
Glucose-6-phosphate dehydrogenase
6-phosphogluconate dehydrogenase
What happens in the second phase of the pentose phosphate pathway?
Complex series of reactions to convert unused C5 sugar phosphates to glycolysis intermediates
How is the second phase of the pentose phosphate pathway regulated?
Controlling activity of G6PD with NADP+ which activates it and NADPH which inhibits it
What are the products of the second phase of glycolysis?
ATP
1,3-BPG
Phosphoenolpyruvic acid
What is glycerol phosphate needed for?
TAG synthesis in liver and adipose tissue
What is glycerol phosphate produced from in adipose tissue?
Dihydroxyacteone phosphate
Why is TAG synthesis not as dependent on rate of glycolysis in the liver as it is in adipose tissue?
Liver has glycerol kinase which it can use to phosphorylase glycerol
What is 2-3 BPG produced from in RBCs?
1,3-bisphosphoglycerate
What is 2,3-BPG?
An important oxygen affinity regulator
What does muscle tissue possess that enables the high energy of hydrolysis phosphate bond in ADP to drive ATP synthesis in an emergency?
Myokinase
How is glycolysis regulated by ATP levels?
Inhibited when high
Stimulated when low
Why do most of the metabolic reactions in glucose metabolism not need to be regulated?
They are close to equilibrium
How can phosphofructokinase be regulated?
Inhibited by high ATP levels or glucagon
Stimulated by high AMP or insulin
What are the products of anaerobic glycolysis?
Lactate
ATP
Water
What normally produces ~50 g of lactate per day?
RBCs Skin Brain Skeletal muscle GI tract
How is lactate dealt with once it has been released into the circulation?
Converted to glucose in liver and kidney
Converted back to pyruvate and oxidised to carbon dioxide in heart muscle
How does lactic acidosis occur?
If plasma lactate concentration >5 mmol per litre it exceeds renal threshold and affects buffers
What can cause lactic acidosis?
Compromised circulation Lack of thiamine Shock Strenuous exercise Liver disease Hearty eating
What appears in the urine in lactic acidosis?
Lactate
How is galactose liberated?
Digestion of lactose
Where is galactose metabolised?
Mainly liver but also kidney and GI tract by soluble enzymes
What are the products of galactose metabolism?
G6P and ADP
What does epimerase do?
UDP-glucose UDP-galactose
Why is it important for the epimerase reaction to be reversible?
So galactose can be formed during lactation
So galactose can form UDP-glucose in glycogen synthesis
Which two enzymes can patients with galactosaemia lack?
Kinase or transferase
Which is the more common enzyme deficiency in galactosaemia?
Transferase
What accumulates in the tissues of patients lacking kinase enzyme in galactosaemia?
Galactose
What accumulates in the tissues of patients lacking the transferase enzyme in galactosaemia?
Galactose and galactose-1-phosphate
What happens to the accumulated metabolites in transferase deficient galactosaemia?
Reduced by aldose reductase to galacitol which depletes NADPH
What are the consequences of NADPH reduction in transferase deficient galactosaemia?
Raised intraocular pressure –> blindness
Non-enzymatic glycosylation of lens proteins –> cataracts
Inappropriate disulphide bond formation –> cataracts
Why are the liver, kidney and brain damaged in transferase deficient galactosaemia?
Pi sequestered so not available for ATP synthesis
Why does galactose spill into pathways it is not normal seen in during galactosaemia?
High Km values
Why does treating galactosaemia patients with a galactose free diet not cause a galactose deficiency?
UDP-glucose is present and this can be converted by epimerase
How is fructose produced in the body?
Hydrolysis of sucrose and ingestion of fruit
How is fructose metabolised?
Soluble enzymes in the liver catalyse its conversion to glyceraldehyde 3-phosphate, an intermediate of glycolysis
What causes G6PD deficiency?
Point mutation causing an X-linked gene defect
Which populations is G6PD more prevalent in?
Mediterranean
Black USA males
What are the consequences of G6PD deficiency?
Insufficient NADPH levels
Inappropriate disulphide bridges form
Haemoglobin and other proteins become X-linked
What are Heinz bodies?
Insoluble aggregates of X-linked proteins in RBCs which cause haemolysis
What can cause acute haemolytic episodes in G6PD deficiency?
NADPH level reducing chemicals e.g. antimalarials, sulphonamides, glycosides
What happens to pyruvate before it can enter the TCA cycle?
Converted to acetyl CoA by pyruvate dehydrogenase
Why is pyruvate metabolism sensitive to vitamin B deficiency?
4 B vitamins are needed as cofactors for pyruvate dehydrogenase
Why is the PDH reaction irreversible?
Carbon dioxide is lost
What controls the PDH reaction?
PDH allosterically activated by ADP
PDH allosterically inhibited by acetyl-CoA, ATP and NADH
What does the allosteric inhibition of PDH by acetyl CoA allow?
Acetyl-CoA from beta-oxidation to be used instead of that from glycolysis under certain conditions
How does insulin activate PDH?
Promotes dephosphorylation
Where does the conversion of pyruvate to acetyl CoA take place?
Mitochondrial matrix after transportation of pyruvate from the cytoplasm
What does PDH deficiency lead to?
Lactic acidosis
Why is the TCA cycle unidirectional?
Conversion of removed acetyl to carbon dioxide which is lost
Why does the TCA cycle require NAD+ and FAD?
It is oxidative
Where does the TCA cycle take place?
Mitochondrial matrix
How does the TCA cycle produce energy?
ATP/GTP
Precursors for biosynthesis
What are the products of one molecule of glucose entering the TCA cycle?
6 NADH
2 FAD2H
2 GTP
Which molecules does the TCA cycle produce precursors of biosynthesis for?
Fatty acids
Amino acids
Haem
Glucose
How is the TCA cycle regulated?
Isocitrate dehydrogenase is allosterically activated by ADP and allosterically inhibited by NADH
Which enzyme in the catabolism of glucose must gluconeogenesis bypass?
PDH
What is the TCA cycle the central pathway of metabolism for?
Sugars Fatty acids Ketone bodies Amino acids Alcohol
How do the intermediates of the TCA cycle act?
Catalytically - no net synthesis or degradation
Why are there no known genetic defects to the TCA cycle?
They would be lethal
How many molecules of ATP are produced per molecule of glucose in that has entered the TCA cycle?
32
Where does oxidative phosphorylation take place?
Inner membrane of the mitochondria
Why is oxygen required in oxidative phosphorylation?
Final electron acceptor
What happens to NADH and FAD2H in oxidative phosphorylation?
Re-oxidised
How is energy used in oxidative phosphorylation?
30% to move H+ across membrane
Why are humans warm blooded?
70% of the energy in oxidative phosphorylation is dissipated as heat
What creates the proton motive force in oxidative phosphorylation?
Anions left on inside of inner membrane create [H+] gradient
Why does ATP synthesis occur in oxidative phosphorylation?
Return of proteins is energetically favoured electrically and chemically but can only happen via ATP synthase
How is discharge of the proton gradient prevented?
Impermeable membrane
How much ATP is generated by oxidation of 2 moles of NADH?
5 moles
How much ATP is generated by oxidation of 2 moles of FAD2H?
3 moles
How does the transfer of electrons differ between FAD2H and NADH?
FAD2H is lower energy so transfers electrons at a lower energy complex
How is oxidative phosphorylation regulated?
High ATP means no substrate for ATP synthase –> inward H+ flow stops, proton gradient rises and feedback inhibition takes place so e- transport is stopped
How are oxidative phosphorylation and electron transport usually regulated?
By mitochondrial [ATP] as they are tightly coupled
How do cyanide ions and carbon monoxide interfere with the electron transport chain?
Bind to cytochrome oxidase and prevent transfer of e- to oxygen therefore the pmf is not established
What does cytochrome oxidase contain which allows it to be used in electron transport?
Haem group that can change oxidation state
What percentage of BMR can be attributed to proton leak?
~20-25%
How do uncouples work?
Increase permeability of the IMM to protons so the gradient is dissipated and there is no pmf for ATP synthesis
Why are overweight people warmer than lean individuals?
Fatty acids act as uncouplers so more energy is dissipated as heat due to leaky IMM
What are dinitrophenol and dinitrocresol examples of?
Uncouplers
What genetic defects can reduce electron transport and ATP synthesis?
PTCs and ATP synthase encoded by mtDNA
What affects the efficiency of energy transfer and can be varied in some tissues?
Tightness of oxidative phosphorylation and electron transport coupling
Where is brown adipose tissue found in newborns?
Around vital organs
What does brown adipose tissue contain that allows uncoupling of oxidative phosphorylation from electron transport?
Thermogenin (UCP1)
How does thermogenin in brown adipose tissue cause an independent in temperature?
NA stimulates lipase –> FA from TAGs oxidised –> UCP1 activated which transports H+ back to mitochondria causing energy of pmf to be released as heat
How do the proteins in oxidative phosphorylation compare to those in substrate level phosphorylation?
OP: membrane associated complexes
SLP: soluble enzymes in cytoplasm and mitochondrial matrix
How does energy coupling in oxidative phosphorylation compare to substrate level phosphorylation?
OP: indirectly through generation and utilisation of pmf
SLP: directly through formation of high energy of hydrolysis bonds
How does the ability of substrate level phosphorylation and oxidative phosphorylation compare?
OP: cannot occur in oxygen absence
SLP: occurs to limited extent in oxygen absence
Which is the major process for ATP synthesis in cells needing lots of energy?
Oxidative phosphorylation
Why do carbohydrates require less oxygen than fatty acids to be metabolised?
They are already partially oxidised
Why are lipids more reduced that carbohydrates?
They have less oxygen and more hydrogen atoms per carbon atom
What are the four fatty acid derivatives?
FA
TAGs
Phospholipids
Eicosanoids
What is the function of eicosanoids?
Transmitter b/w neighbours
What are the 4 derivatives of hydroxy-methyl–glutamic acid?
Ketone bodies
Cholesterol
Cholesterol esters
Bile acids and salts
What are cholesterol esters used for?
Cholesterol storage
Which hydroxy-methyl-glutamic acid derivative is used in lipid digestion?
Bile acids and salts
What does vitamin A deficiency cause?
Scaly skin
What does vitamin E deficiency cause?
Muscular dystrophy
What does vitamin K deficiency cause?
Haemorrhaging anaemic babies
What are TAGs?
Hydrophobic molecules stored in anhydrous form in fatty globules in adipose tissue until there is an increase in metabolic load
What stimulates reduced TAG storage?
Glucagon Adrenaline Cortisol Growth hormone Thyroxine
What happens to the glycerol part of TAGs?
Hydrolysed in GI tract from TAG –> enters blood –> taken up by liver
What happens to the fatty acid part of TAGs?
Hydrolysed from TAG in GI tract –> carried by chylomicrons in blood –> taken up by adipose tissues
What are chylomicrons in the blood recognised by?
Tissues that need triglycerides
What is released by adipose tissue into the blood that can be taken up by muscles to provide energy?
Fatty acids
What are fatty acids carried by in the blood?
Plasma proteins e.g. Albumin
What happens to TAGs in the small intestine?
Hydrolysed by pancreatic lipase to release glycerol and FA using bile salts and colipase
What is colipase?
Protein factor
What happens to glycerol in the liver?
Turned into glycerol phosphate which can be used in glycolysis or TAG synthesis
What is the difference between saturated and unsaturated fatty acids?
Saturated: all single carbon bonds
Unsaturated: one or more double carbon bonds
Why are certain polyunsaturated fatty acids essential?
Mammals cannot introduce double carbon bonds beyond C9
What happens with lipolysis in stress response?
FA released and non-covalently bound to albumin and carried in blood to liver, heart and skeletal muscle for beta-oxidation
Glycerol is released into blood where it travels to the liver to be oxidised/converted to glucose/TAGs
Where does FA oxidation take place?
Cytoplasm
What is the function of fatty acyl CoA synthase?
Link FA to CoA by forming a high energy sulphide bond using ATP
Why do activated FAs not readily cross the IMM?
Hydrophobic acids w/a hydrophobic cofactor
How is the rate of FA oxidation controlled?
Need for cartinine shuttle to transport fatty-aceyl CoA across the IMM
What happens in the transport of fatty acetyl-CoA across the IMM?
Molecule modified into cartinine derivative which is recognised by the transporter –> into matrix where enzyme converts back to acetyl CoA so it is in its metabolic form
What prevents newly synthesised FA from being immediately oxidised?
Inhibition of fatty acyl-CoA transport by FA synthesis intermediate malonyl CoA
How do defects in the fatty acyl-CoA transport system present?
Poor exercise tolerance
Increase in fat droplets in muscle cells
What is acetyl CoA a catoabolism product of?
FA
Sugars
Alcohol
Amino acids
Why is beta-oxidation of a fatty acid said to be a spiral cycle?
Gets shorter w/each turn until there is only one C2 unit left
Why can beta-oxidation not occur in the absence of oxygen?
Requires mitochondrial NAD+ and FAD
Is there any direct ATP synthesis in beta-oxidation of FA?
Nope
What happens to all intermediates and carbon atoms in beta-oxidation of FA?
Intermediates linked to CoA
Carbon atoms converted to acetyl-CoA
Is more energy derived from FA oxidation or glucose oxidation?
FA oxidation
What can be said about the substrate and product in beta-oxidation of FA?
Both activated
Which ketone bodies are synthesised in the liver from acetyl CoA?
Acetoacetate
Beta-hydroxybutyrate
How is acetone formed?
Spontaneous, non-enzymatic decarboxylation of acetoacetate
Why are ketone bodies present in plasma and excreted in urine?
Soluble in water
What occurs if ketone bodies are present in the blood above renal threshold?
Ketonuria
At what concentration is the plasma lactate level kept son start at?
Why level of ketone bodies is usually seen in the blood?
What is physiological ketosis?
Ketone bodies at 2-10 mM in the blood, seen in starvation
Why is pathological ketosis?
Ketone bodies at >10mM in blood seen in untreated type I diabetics with pear-drop smelling breath
What happens in the fed state to HMG-CoA which has been formed from acetyl CoA?
Goes to form cholesterol to increase the insulin/glucagon ratio
What happens in the starvation state to HMG-CoA?
Forms ketone bodies to decrease the insulin/glucagon ratio
How do statins work to lower cholesterol?
Inhibit HMG-CoA reductase to prevent conversion of HMG-CoA to mexalonate and then cholesterol
How are ketone bodies synthesised?
Acetyl CoA –> synthase –> HMG-CoA –> lyase —> acetoacetate –> beta-hydroxybutyrate
How is ketone body synthesis controlled?
Isocitrate dehydrogenase which uses feed forward allosteric inhibition
What is required for ketone body synthesis?
FA for oxidation in liver after excess lipolysis in adipose
How can ketone bodies be used as a source of energy?
All tissues w/mitochondria (incl. CNS) convert to acetyl-CoA at a rate proportional to plasma concentration
How is acetone transported around the body?
In blood as it is soluble
What can beta-hydroxybutyrate carry?
Hydrogen
Why are ketone bodies used in starvation/diabetes?
Prime aim is to spare glucose for brain
What processes can spare glucose?
Glycogenolysis
Gluconeogenesis
Ketogenesis
Lipolysis
Why does the body preferentially break down muscle in starvation?
It does not want to lose tissue mass
What are the 4 basic stages of catabolism?
1: breakdown to building block molecules
2: breakdown to metaboli intermediates, release of reducing power and energy
3: TCA cycle, release energy and reducing power
4: conversion of reducing power to ATP
What energy stores make up the textbook 70 kg man?
~15 kg TAGs
~0.4 kg glycogen
~6 kg muscle protein
How do the typical energy stores change in obesity?
Glycogen and muscle protein masses stay the same but mass of TAGs greatly increases
How long after a meal does glycogenolysis occurs?
2 hrs
How long after a meal does gluconeogenesis occur?
8-12 hours
Why doe glycogen have a minimal osmotic effect?
It’s structure
What forms the straight chain alpha 1-4 bonds in glycogen?
Glycogen synthase
What forms the branching alpha 1-6 bonds in glycogen?
Branching enzyme
What ratio are the alpha 1-4 and 1-6 bonds present in?
10:1
Why is the amount of glycogen stored in liver and skeletal muscle limited?
High polarity attracts a lot of water
What can glycogen storage diseases cause?
Tissue damage w/excessive storage
Fasting hypoglycaemia
Poor exercise tolerance
Abnormal storage affecting liver and/or muscle
Why is glycogen never completely degraded?
Small amount needed as primer
What can the body not do in von Gierke’s disease?
Breakdown glycogen
How is glycogen synthesised?
Glucose + ATP –> G6P –> G1P + UTP + water –> UDP-glucose –> add on to existing glycogen
Which enzymes are needed for synthesis of glycogen?
Hexokinase/glucokinase
Phosphoglucomutase
Glycogen synthase/branching enzyme
What inhibits glycogen synthesis?
Phosphorylation by glucagon or adrenaline
What activates glycogen synthesis?
Dephosphorylation by insulin
How is glycogenolysis controlled with respect to glycogen synthesis?
Reciprocally
Which is the rate limiting step of glycogenolysis?
Conversion of glycogen to G1P by glycogen phosphorylase/de-branching enzyme
Which enzyme converts G1P to G6P?
Phosphoglucomutase
What is found only in the liver which can convert G6P from glycogenolysis into glucose?
Glucose-6-phosphatase
What precursors are used mainly in the liver but also the kidney cortex for gluconeogenesis after 8 hours of fasting?
Non-carbohydrate
Which enzyme controls the rate limiting step in galactose metabolism?
Fructose-1,6-bisphosphosphatase
Which enzyme is used for galactose, fructose and glycerol in gluconeogenesis?
PEPCK
How do the stimulators of gluconeogenesis act on the enzymes involved?
Increase expression and activity
When can the energy from TAG storage in adipose be mobilised?
~20 mins of aerobic exercise
Stress
Starvation
Second half of gestation
Why can FA not move through the blood-brain barrier to the CNS?
Endothelial cells in BV have very tight junctions
What is acetyl CoA converted into using ATP and NADPH in the cytoplasm of liver cells?
Fatty acids
How is acetyl-CoA transported out of the mitochondria if it is too big to pass through as it is?
Combines w/oxaloacetate to form citrate which can be transported, then reforms in cytoplasm
How are FA synthesised once acetyl-CoA has been transported into the cytoplasm?
Acetyl-CoA converted to malonyl-CoA
Fatty acid synthase adds one C2 unit to growing FA per cycle
What is the growing fatty acid linked to in FA synthesis?
Acyl carrier protein (ACP)
Which enzyme controls the important regulatory step in FA synthesis?
Acetyl carboxylase
How is acetyl carboxylase controlled?
Citrate and insulin activate
AMP, glucagon and adrenaline inhibit
(Hormones cause covalent modification)
Are fatty acid oxidation and synthesis reversals of each other?
Nope
Why is the catabolism and anabolism of the same substrate usually not just a reversal of the same process?
Allows greater flexibility, better control and thermodynamically irreversible steps can be bypassed
What is special about lysine, isoleucine, leucine, threonine, valine, tryptophan, phenylalanine, methionine and under certain conditions histidine, arginine, tyrosine and cysteine?
They are essential
What are amino acids used in the synthesis of?
Proteins Purines Pyrimidines Porphyrins Creatine Neurotransmitters Hormones
When is the nitrogen balance negative?
During starvation
When is the nitrogen balance positive?
During growth
What release free amino acids by breaking peptide bonds?
Proteases
Peptidase said
What stimulates uptake of amino acids into skeletal muscle, adipose, and the liver as well as stimulating protein synthesis?
Insulin
Growth hormone
Which hormone stimulates proteolysis and can be raised by steroid drugs?
Cortisol
What happens to amino acids once they’ve been released from the polypeptide?
Amino group is removed and carbon skeleton enters TCA cycle
What types of carbon skeleton can be left when the amino group is removed from a free amino acid?
Glucogenic
Ketongenic
How do Ketongenic amino acids enter the TCA cycle?
Produce acetyl CoA which goes on to make ketone bodies
How do goucogenic amino acids enter the TCA cycle?
Produce pyruvate, oxaloacetate, fumarate, alpha-ketoglutarate or succinate to undergo gluconeogenesis
What is special about the catabolism of isoleucine, threonine, phenylalanine, tyrosine and tryptophan?
They can take either goucogenic or ketongenic pathways
What two ways can be used to transform excess amino acids metabolised in the liver into a less toxic form?
Transamination
Deamination
Which enzymes are used in transamination what can be used to measure liver function?
ALT
AST
Why does transamination disrupt energy supply?
It reduces TCA cycle activity
What does ALT convert?
Alanine to glutamate
What does AST convert?
Glutamate to aspartate
What stimulates transaminase in the liver?
Cortisol
What happens to the amine group removed from amino acids at physiological pH?
Converted to very toxic and soluble ammonium ion
Who can toxic ammonium ions be removed?
Via urea or glutamine
What is required to form glutamine in order to remove toxic ammonium ions?
Glutamate
Ammonia
ATP
Which enzyme is needed for glutamine synthesis?
Glutamine synthetase
How is excess glutamine dealt with?
Released from cells into blood where it travels to the kidneys to enter urine and the liver to form urea
Why is urea a good way to get rid of ammonium ions?
High N content
Non-toxic
V. water soluble
Chemically inert in humans
What nitrogen containing compound can bacteria in the gut break down?
Urea
What are the products of the urea cycle?
Urea Fumarate ADP AMP Pi
How may enzymes are there in the urea cycle?
5
Why is the urea cycle not regulated by feedback inhibition?
Function is disposal so controlled by protein intake
Where does the urea cycle take place?
Ornithine part in mitochondrial matrix and rest in cytosol
What is the result of a complete loss of any one enzyme in the urea cycle?
Death
Why does partial loss of an enzyme in the urea cycle cause vomiting, lethargy, irritability, mental retardation, seizures and coma?
Causes hyperanaemia which increases intermediates that interfere with the TCA cycle
How is a deficiency of an enzyme in the urea cycle treated?
Low protein diet and replacing amino acids w/keto acids
How is phenylketonuria detected?
Excess phenylketones excreted in urine and high levels in the blood
What causes phenylketonuria?
Autosomal recessive defect of gene on chromosome 12 which causes lack of phenylalanine hydroxylase enzyme
How is phenylketonuria treated?
Low phenylalanine diet
Why is mental retardation seen in PKU?
Tyrosine is not formed which is needed to synthesise noradrenaline, adrenaline and dopamine for CNS development
What causes homocystinuria?
Autosomal recessive defect on chromosome 21 causes complete deletion or reduced activity of cystathionine beta-synthase
What is made instead of cystathionine in homocystinuria?
Methionine
What does homocystinuria affect in the body?
Fibrillin-1 protein structure so tissue, muscle, CNS and CVS affected
Symptoms of which other disease involving chromosome 21 are linked with homocystinuria?
Down’s syndrome
What other disease presents similarly to homocystinuria and can be confused on diagnosis?
Marian’s
How do the relative toxicities of methionine and homocysteine compare?
Methionine is harmful at high levels but less so than homocysteine
What does cystathionine-beta-synthase need to function?
Active vitamin B
What treatment is given in homocystinuria where there is reduced activity of CBS?
Vitamin B supplement
Which important gas signalling molecule is linked with arginine?
Nitric oxide
What functions does nitric oxide have?
Vasodilator - maintenance of B.P.
Neurotransmitter
Inflammatory mediator
Memory storage
Which important gas signalling molecule is cysteine associated with?
Hydrogen sulphide
What is the function of hydrogen sulphide?
Vasodilator
Neuromodulator
Cytoprotective agent against oxidation (it’s a reducing agent)
What is creatinine?
Breakdown product of creatine and creatine phosphatase in muscle cells
How does the release of creatinine vary?
Constant unless muscle is wasting when it becomes elevated
What is the importance of creatinine excretion over 24 hrs by the kidneys being proportional to muscle mass ?
Means urine concentration is a stable marker of urine dilution, blood concentration and measure kidney function
When is the level of urinary creatinine excretion over 24 hrs not proportional to the muscle mass of an individual?
If they are a carnivore or are wasting
What are the five classes of lipids?
Phospholipids Cholesterol esters Cholesterol Fatty acids Triacyglycerols
What is cholesterol he precursor to?
Steroid hormones and bile acids
What is the function of bile acids?
Emulsify fats in the GI tract
Where does cholesterol come from?
Some from the diet but mainly synthesised in the liver
How can a fatty acid be used to eliminate the polar part of a cholesterol molecule?
The OH on the cholesterol can be esterified with a FA
What is the most abundant protein in the blood?
Albumin
What does albumin carry?
~2% of lipids, mainly FA
What is the problem with using albumin as transport for lipids?
It has a limited capacity of ~3 mmol per litre so becomes saturated if lipid levels are too high
How is ~98% of lipid carried in the blood?
As lipoprotein particles
What is a lipoprotein?
Particle mainly constructed in the liver consisting of a TAG and cholesterol hydrophobic core surrounded by a protein, phospholipid and cholesterol coat
What is the function of the protein component of an apolipoprotein?
Stabilisation during transit
What is the role of phospholipids and cholesterol in the apolipoprotein?
Structural integrity
What is an apoprotein?
Specialised protein for lipid transport
How do different types of lipoproteins vary?
Size (5-100 nm) Lipid and protein composition (2-55% protein) Density Surface charge Function
How can lipoproteins be separated?
Electrophoresis or ultracentrifugation
What does each class of lipoprotein particle have?
Own set of specific apolipoproteins
What are the two general functions of apolipoproteins?
Activate enzymes
Recognise CSM receptors
Where can ApoA (I) be found and what is its function here?
Intestine
Structural HDL component
Where is ApoB100 found and what is its function?
Liver
VLDL
Where can ApoC be found and what is its function?
Liver
Cofactor activator of lipoprotein lipase/inhibits LPL
Where is ApoD found and what is its function?
Brain and testes
HDL
Where is ApoE found and what is its function?
Liver, binding to LDL receptors
What is the function of ApoH?
Inhibits coagulation factors
What happens if there are polymorphisms in LDL receptors?
Early onset CVD
Order the classes of chylomicrons in increasing density.
VLDL
LDL
HDL
What are chylomicrons?
Recombined TAGs and apoprotein
What forms chylomicrons?
Enterocytes - simple columnar cells w/glycocalyx found on the intestinal lining
What is the function of chylomicrons?
Combine dietary TAGs split by pancreatic lipase and reformed w/apoprotein so to carry lipids form the diet to tissues
How long are chylomicrons usually present for in the blood following a meal?
4-6 hours
Where are VLDLs formed?
In liver for energy store
What are VLDLs rich in?
TAGs
What is the function of VLDLs?
Combine TAGs synthesised in the liver w/specific apoproteins to carry lipids from liver to tissues
Which is more important, the absolute LDL and HDL levels or the ratio between them?
Ratio
Where are LDLs formed?
In the liver
What are LDLs rich in?
Cholesterol
What is the function of LDLs?
Combine liver cholesterol w/specific apoproteins (esp. apoB100) to carry it to the tissues
What is the structure of an LDL?
Phospholipid she’ll w/lots of cholesterol, filled w/cholesterol esters
What are high levels of LDLs associated with?
High risk of atherosclerosis
Where are HDLs formed?
Tissues
What is the function of HDLs?
Combine excess cholesterol in the tissues w/specific apoproteins to carry it to the liver so it cannot cause damage by being in the blood
What enzyme do endothelial cells have on their outside?
Lipase
What catches cholesterol and binds it?
Chylomicrons and VLDL
What happens to VLDL remnants?
Removed by liver or converted to other lipoprotein particles
What is necessary for recognition of cholesterol from LDL?
ApoB100
What controls ApoB100 receptor expression in the cell?
[cholesterol]
What happens when the LDL complex binds to the receptor on a cell?
Receptor/LDL complex taken in by endocytosis –> cholesterol ester is released and cleaved into cholesterol and FA
How are HDL particles loaded?
Nascent HDL shells made in the liver and VLDL tenants sequester cholesterol from capillaries which then mature into HDL particles
What is hyperlipoproteinaemia?
Raised levels of one or more lipoprotein classes
What causes hyperlipoproteinaemia?
Problems w/receptors, lipase enzymes or ApoE (for example) which results in over production or under removal
What lifestyle alterations can be sufficient to treat hyperlipoproteinaemia?
Reduce cholesterol and saturated lipid intake
Increase exercise
Stop smoking
What drugs can be used to treat hyperlipoproteinaemia?
Statins
What effects do statins have?
Inhibit HMG CoA reductase
Increase LDL receptor expression in hepatocytes
Increase vascular lipase secretion
Possible anti-inflammatory action
What effects does hypercholesterolaemia have around the body?
Cholesterol deposited on eyelids (xanthelasma), on tendons, corneal arcus
What is the pathogenesis of atherosclerosis?
Oxidised LDL in intima –> macrophages become foam cells –> accumulation in intima of BV walls –> fatty streak –> atheroma
What are foam cells?
Fat filled macrophages
What is an early warning sign of atherosclerosis?
Angina
What happens if an atherosclerotic plaque ruptures?
Clotting cascade is initiated and thrombus forms
Why do statins have a wide spectrum of side effects?
Pathway they act on is wide branching
Give an example of a useful species produced by the cholesterol pathway.
Coenzyme Q10
Why are statins taken before bed?
Most cholesterol is produced in the liver at night
Are statins universally effective?
No, varies from patient to patient so trial and error for treatment plan
What is the problem with statins and risk of CVD?
Statins save lives at high risk of CVD but controversy over whether to prescribe routinely for >10% risk
What is a dangerous byproduct of ATP production?
Reactive oxygen species
How are reactive oxygen species formed surfing ATP synthesis?
Electrons drop out of the electron transport chain and combine with oxygen
What do superoxide radicals damage?
DNA and CSM
How does superoxide dismutase (SOD) act as a defence mechanism against superoxide radicals?
Converts them to hydrogen peroxide which catalase can then turn into oxygen and water
Where is catalase found?
Widely distributed in cells
What ROS is produced by ionising radiation?
Hydroxyl radicals
What do hydroxyl radicals cause in RBCs?
Haemolysis
Why are antioxidants needed to eliminate hydroxyl radicals?
They cannot be removed enzymatically
How is nitric oxide, which is a free radical, used physiologically in the body?
Used for signalling
How is nitric oxide formed?
From arginine by inducible NO synthase
What is formed if hydrogen peroxide combines with oxygen?
Peroxynitrite
What is peroxynitrite involved in?
Inflammation
What exogenous agents can overwhelm the tissue’s defence against oxidising agents?
Drugs e.g. antimalarials
Toxins e.g. paraquat
What are the five most common reactive oxygen species?
Superoxide Hydrogen peroxide Hydroxyl Nitric oxide Peroxynitrite
What are the five cellular defences against ROS?
SOD Catalase NADPH Glutathione Antioxidants e.g. those found in grapes
Why is glutathione a very effective reducing agent?
Thiol group donates its hydrogen easily
How does glutathione act as the first line of defence against ROS?
Donates H from thiol group –> oxidised to glutathione disulphide which can be reduced by GSH reductase to be constantly replenished to glutathione
When can glutathione not be reformed in RBCs?
G6PDH deficiency
How is NADPH mainly produced?
Pentose phosphate pathway
What does NADPH use to act as a second line of defence against ROS?
G6PD
What does NADPH replenish?
GSH
What other types of antioxidants are there apart from glutathione and NADPH?
Dietary vitamins
Flavenoids
Minerals
Which dietary vitamins are antioxidants?
A, C and E
Give two examples of flavonoids which are found in coloured fruit and veg.
Polyphenols
Beta-carotene
Name two minerals which may be required by some enzymes for antioxidant function.
Selenium
Zinc
What is lipid peroxidation?
Reaction of unsaturated lipids w/ROS to form lipid peroxide so
Why does lipid peroxidation lead to early stages of CVD?
Damages CSM and plaques form as macrophages recognise and engulf faulty lipids
What is oxidative burst?
Rapid release of superoxide and hydrogen peroxide from leucocytes to kill local pathogens
What is NADPH oxidase?
A very complex protein in CSM that fires rapidly produced ROS inwards
What conditions are associated with oxidative stress?
Cancer Emphysema Pancreatitis CVD Crohn's RA T1DM Alzheimer's Ischaemia/reperfusion injury
How does reperfusion injury cause oxidative stress?
Clot bust –> sudden increase in oxygen to ischaemic tissue –> increase in ROS
What is pharmacodynamics?
What the drug does to the body
What is pharmacology?
Study of how chemical agents affect the function of living organisms
What is pharmacokinetics?
What the body does to the drug
What four things does pharmacokinetics consider?
Absorption
Distribution
Metabolism
Elimination
Are most drugs water soluble or lipid soluble?
Lipid
Why is a large proportion of a penicillin dose lost in the urine?
It is water soluble
Why can most drugs not be excreted directly by the kidney?
Lipid soluble so are filtered out by kidney but then reabsorbed so
What path must drugs take to be excreted via the kidney?
Parent drug molecule –> polar, lipid insoluble, water soluble derivative –> excretion via kidney
How does the pharmacological activity of metabolites usually compare to the parent drug molecule?
Less active
What is special about prodrugs?
They must be metabolised to become active
Why might patients belonging to the Chinese population not be able to be treated with codeine?
Lack conversion enzyme so cannot metabolise it to morhpine
What is primidone metabolised to?
Phenoarbitone
What is the toxic metabolite of pethidine?
Norpethidine
What happens during phase I of drug metabolism?
Add or exposure of a reactive group by oxidation, reduction or hydrolysis
What is the main site of phase I drug metabolism?
Microsomes on ER in hepatocytes
Where, other than the liver, can phase I drug metabolism take place?
GI tract
Kidney
Lung
Plasma
Why does heroin have such a short half life?
Its ester bonds are rapidly broken down by cholinesterase enzymes in the plasma
Give an example of an opioid drug which skips phase I metabolism.
Morphine
What is present in the microsomes of ER in hepatocytes that is used in phase I of drug metabolism?
Cytochrome P450 enzyme system (CYP)
What accounts for ~55% of drug metabolism in the phase I pathway?
Isoform CYP3 A4
What is abundant in hepatocytes which is a CYP cofactor that has both oxidative and reductive functions?
NADPH
What happens in phase II of drug metabolism?
Conjugation - altered molecule is combined w/water soluble group so it can be eliminated by the kidney
Where is the most common site of conjugation which uses cytosolic enzymes?
Liver
What reactions can be used in phase II drug metabolism?
Glucuronidation
Sulphate conjugation
Glutathione conjugation
What high energy form of metabolite is used as a cofactor for glucuronidation?
Uridine diphosohate glucuronic acid
What is glucuronic acid?
Metabolite of glucose which is readily available to make drug molecules water soluble
What genetic factors can vary drug metabolism in the population?
Polymorphisms
Lack main enzyme for acetylation in phase II - slow acetylators
Low levels of pseudocholinesterase
What causes short term muscle relaxants to act for much longer in some individuals?
Have low levels of pseudocholinesterase in plasma
What environmental influences vary drug metabolism in the population?
Metabolism of one drug can affect metabolism of another
Inhibition e.g. fruit juices
Induction by an agent of enzymes in the liver
Why are smokers and drinkers problematic for anaesthetists?
Smoking and alcohol consumption induces enzymes in the liver so anaesthetics are more rapidly broken down
What path do drugs take in the first pass effect?
Lumen of ileum –> venous blood –> hepatic portal vein –> liver
What is the first pass effect?
Any drug absorbed into the blood is excessively metabolised by the liver
Why are large doses of paracetamol needed?
90% is metabolised by first pass effect so only 10% of a dose is effective
What is pharmacoepidemiology?
Study of drug effects in a large population
What is pharmacovigilance?
Reporting of adverse drug reactions (ADRs) post-marketing
Give two drugs and their effects that were significant to pharmacovigilance.
Thalidomide - affected limb buds which did not arise in animal testing
Mefloquine - no wide malarial resistance but causes lots of side effects
What happens to a therapeutic dose of paracetamol?
Undergoes phase II metabolism only - glucuronidation and sulphation
Why does paracetamol bypass phase I metabolism?
It already has an OH reactive group
What happens in a toxic dose of paracetamol?
Glucuronidation and sulphation are saturated so paracetamol undergoes phase I metabolism
What is formed when paracetamol undergoes phase I metabolism?
NAPQI, a toxic metabolite
Why is NAPQI formation in paracetamol overdose so dangerous?
It is a toxic metabolite
It undergoes glutathione conjugation which kills liver cells by oxidative stress
How is alcohol dealt with by the body?
90% metabolised
What is alcohol converted to as it is metabolised?
Alcohol –> acetylaldehyde –> acetic acid
Which enzymes are involved in alcohol metabolism?
Alcohol dehydrogenase
Aldehyde dehydrogenase
CYP2E1
What causes hangovers?
Build up of toxic metabolite acetylaldhyde
What can acetic acid from alcohol metabolism be used for?
FA and ketone body synthesis
What is the metabolism rate for alcohol?
~7 g per hour
What action of alcohol dehydrogenase can lead to excess fat deposition, decreased FA oxidation and therefore hypoglycaemia, gout and lactic acidosis?
Decreases NAD+ to NADH ratio
What causes fatty liver in alcoholics?
Fat deposition and acetylaldehyde damage
What can fatty liver lead to?
Alcoholic hepatitis
Alcoholic cirrhosis
What treatment is given in severe alcohol dependency?
Disulfiram
How does Disulfiram work?
Inhibits aldehyde dehydrogenase which causes a build up of acetaldehyde resulting in a week long hangover
What do the cost and effects on the CNS in alcoholism cause?
Poor dietary habit which leads to GI tract disturbances such as folic acid deficiency leading to anaemia, vitamin deficiency, diarrhoea and lack of appetite
