Metabolism

Question 1

Gain an overview of what we mean by “metabolism”

Answer 1

Metabolism is the set of life-sustaining chemical reactions in organisms.

Question 2

Comprehend that metabolic pathways are often a series of enzyme catalysed reactions

Answer 2

These are series of enzymatic reactions producing specific products. They are branched and interconnected. Metabolites are known as reactants, intermediates and products. These carry by cell type, nutritional status and developmental stage.

Question 3

Explain the role of glycolysis and to identify the key regulatory steps

Answer 3

This is employed by all tissues for glucose oxidation to provide energy (ATP). All sugars can be ultimately converted to glucose. It happens with an adequate supply of oxygen and mitochondria. Pyruvate is the end product.

Step 1- Phosphorylation of glucose
Step 2- Isomerization of Glucose-6-phosphate
Step 3- Phosphorylation of fructose-6-phosphate
Step 4- Cleavage of fructose 1, 6-diphosphate
Step 5- Isomerization of dihydroxyacetone phosphate
Step 6- Oxidative Phosphorylation of Glyceraldehyde 3-phosphate
Step 7- Transfer of phosphate from 1, 3-diphosphoglycerate to ADP
Step 8- Isomerization of 3-phosphoglycerate
Step 9- Dehydration 2-phosphoglycerate
Step 10- Transfer of phosphate from phosphoenolpyruvate

Question 4

Create an energy balance sheet for glycolysis

Answer 4

The energy balance sheet for Glycolysis:

1 Glucose. (-2 ATP).2 G3P. +2 NADH (1 per G3P). +2 ATP. +2 ATP. 2 pyruvate. 2 ATP. 2 NADH.

Question 5

Explain what is meant by flux through a metabolic pathway and how it is controlled

Answer 5

Living systems maintain a steady state of flux through a metabolic pathway. The rates of synthesis and breakdown of metabolites maintain concentrations. A steady state far from equilibrium allows useful work. Flux is determined by rate determining steps. This can be controlled through long and short term strategies (allosteric, covalent modification, substrate cycles, genetic control)

Question 6

Understand the structure (and role) of ATP

Answer 6

its structure the nitrogenous base, adenine; the sugar, ribose; and a chain of three phosphate groups bound to ribose. atp provides energy to cells for respiration and active trasnport.

Question 7

Explain what carbohydrates are

Answer 7

A carbohydrate is a biomolecule consisting of carbon, hydrogen and oxygen atoms, they are an energy store.

Question 8

Recognise the structure of D-glucose

Answer 8

C6H12O6, right, glucose forms a 6 membered ring called pyranose

Question 9

Describe glycogen’s role

Answer 9

glycogen is an unsoluble glucose store that doesn’t affect osmosis, used during exercise.

Question 10

Describe the events of glycogen’s synthesis and its dismantling

Answer 10

glycogen synthesis is known as glycogenesis. Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to chains of glycogen for storage. This process is activated during rest periods following the Cori cycle, in the liver, and also activated by insulin in response to high glucose levels.

Glycogenolysis is the biochemical pathway in which glycogen breaks down into glucose-1-phosphate and glycogen. The reaction takes place in the hepatocytes and the myocytes. The process is under the regulation of two key enzymes: phosphorylase kinase and glycogen phosphorylase

Question 11

Understand the hormonal and allosteric regulation of glycogenesis and glycogenolysis

Answer 11

The key hormones involved are insulin, glucagon and adrenaline. Hormones act through changes to the phosphorylation state of enzymes.

Glycogenesis is inhibited when the substrate and the energy level is high. Glycogenolysis is increased when the glucose and the energy levels are low. This permits rapid response and can override hormone-mediated covalent regulation.

Question 12

Provide examples of allosteric effectors and their impact under high energy and low energy states

Answer 12

In the high energy state, [glucose] high, [G6P] high, [ATP] high, glycogen phosphorylase inhibited by G6P and ATP (and glucose in liver), glycogen synthase activated by G6P, and glycogen synthesis predominates.

In the low energy state, [glucose] low, [G6P] low, [ATP] low, muscle [AMP] high, muscle glycogen phosphorylase activated by AMP (raised by contraction), and glycogenolysis predominates.

Question 13

Describe the basic structure of triacylglycerol and saturated and nonsaturated fatty acids and how they are named.

Answer 13

TG are composed of a 3 carbon glycerol to which 3 fatty acids are esterified

saturated - no double bonds, unsaturated - double bonds present

Question 14

Explain the role of fatty acid oxidation in human metabolism.

Answer 14

Fatty acid oxidation is a major source of ATP in tissues such as liver, skeletal muscle, and heart; especially in fasting conditions where glucose availability is limited, fatty acids are used as the main source.

Question 15

Describe the fundamental features of fatty acid beta oxidation, the end products and how it is regulated.

Answer 15

Oxidation generates a double bond on carbon 2 from bond and FADH2.

Hydration generates an OH group on carbon 3 (the beta carbon) using H2O.

Oxidation then generates a carbonyl of carbon 3 and NADH.

Thiolytic cleavage generates acetylCoA and a shortened fatty acrylic CoA, which is 2 carbons shorter.

This repeats to create two acetyl CoA molecules in fatty acids with an even numbered chain. It’s net yield is 129 ATP, much higher than that of glucose.

Fatty acid oxidation and fatty acid synthesis are linked reciprocally.

Question 16

Explain the role of fatty acids in the diet.

Answer 16

Essential fatty acids, such as omega-3 fatty acids, serve important cellular functions. They are a necessary part of the human diet because the body has no biochemical pathway to produce these molecules on its own.

Question 17

Understand the basics of fatty acid synthesis.

Answer 17

Fatty acid synthesis is the creation of fatty acids from acetyl-CoA and NADPH through the action of enzymes called fatty acid synthases. This process takes place in the cytoplasm of the cell. Most of the acetyl-CoA which is converted into fatty acids is derived from carbohydrates via the glycolytic pathway.

Question 18

Understand the basics of cholesterol biosynthesis.

Answer 18

Biosynthesis of cholesterol generally takes place in the endoplasmic reticulum of hepatic cells and begins with acetyl- CoA, which is mainly derived from an oxidation reaction in the mitochondria. Acetyl-CoA and acetoacetyl-CoA are converted to 3-hydroxy- 3-methylglutaryl-CoA (HMG-CoA) by HMG-CoA synthase.

Question 19

Describe the structure of lipoproteins.

Answer 19

Lipoproteins are complex particles that have a central hydrophobic core of non-polar lipids, primarily cholesterol esters and triglycerides. This hydrophobic core is surrounded by a hydrophilic membrane consisting of phospholipids, free cholesterol, and apolipoproteins

Question 20

Explain how dietary lipid is transported to the liver.

Answer 20

Triglycerides and cholesteryl esters are transported in the core of plasma lipoproteins. The intestine secretes dietary fat in chylomicrons, lipoproteins that transport triglyceride to tissues for storage. Dietary cholesterol is transported to the liver by chylomicron remnants which are formed from chylomicrons.

Question 21

Explain how endogenous lipid is transported to peripheral tissues.

Answer 21

Excess carbohydrate and lipid are converted to TGs in the liver. Cholesterol is synthesized in the liver or delivered on chylomicrons remnants.

Question 22

Explain how cholesterol is transported from peripheral tissues to the liver.

Answer 22

HDL serves as a chemical shuttle that transports excess cholesterol from peripheral tissues to the liver. This pathway is called the reverse cholesterol transport (RCT) system. In this system, plasma HDL takes up cholesterol from the peripheral tissues, such as fibroblasts and macrophages.

Question 23

Describe the design, strengths and limitations of RCTs

Answer 23

design: always intervention and control group, maybe more.
strengths: equal chance for participants to get allocated the control group or the treatment group, Allocation concealment masks the randomisation process before randomisation occurs, Blinding prevents ascertainment bias
limitations: blinding not always possible, allocation concealment doesn’t always occur

Question 24

Explain the importance of randomisation, allocation concealment and blinding

Answer 24

randomisation - random distribution of characteristics between the two groups. The only differences seen as therefore due to the intervention.

allocation cocnealment - patients and study group don’t know which group they have been randomised to

blinding - prevents ascertainment bias

Question 25

Describe what makes a suitable outcome measure in an RCT

Answer 25

An ideal outcome is appropriate and relevant to the patient or clinician, valid, attributable and sensitive to the effect of treatment, reliable and robust to different people doing the measuring, and simple and cheap to measure.

Question 26

Discuss how to deal with losses to follow-up and non-compliance with treatment

Understand the difference between explanatory and pragmatic trials

Answer 26

pre-protocol analysis removes patients that didn’t comply with the study protocol. trail is explanatory.

intention to treat analyisis includes non-compliers. Non-compliers tell us something about acceptability of treatment and side effects even if the drug is physiologically effective. The trial was described as ‘pragmatic’.

Question 27

Outline the three components of an attitude.

Answer 27

• affective component, which is how you feel about something and your emotions.
• behavioural component is your actions and intentions.
• cognitive component is your thoughts, feelings and beliefs towards something.

Question 28

Suggest factors which may influence attitude change.

Answer 28

• people around you
• experiences
• response to attitudes.

Question 29

Explain phenomena of cognitive dissonance, stereotyping, prejudice and stigma.

Answer 29

cognitive dissonance: when thoughts, beliefs, or feelings don’t match a behaviour or action

stereotyping: Some stereotypes are positive, but others are negative, and when repeated can lead to people thinking that they are true. When this happens, (known as stereotype threat) people stop trying to overturn the idea.
prejudice: an unfavorable opinion or feeling formed beforehand or without knowledge, thought, or reason
stigma: Attitudes can develop into stigma. A stigma is a cluster of negative attitudes and beliefs that motivates the general public to fear, reject, avoid and discriminate against a particular group based on an identifying feature.

Question 30

Discuss the implications of doctor’s attitudes for medical practice.

Answer 30

The attitude of doctors influences career choice and how we deal with patients.

Question 31

Define a systematic review and explain its purpose

Answer 31

“An overview of all primary studies that contain explicit statements of objectives, materials, and methods and has been conducted according to explicit and reproducible methodology”. A systematic review attempts to synthesis the results of two studies that are investigating the same thing, usually using the same methodology.

Question 32

Outline the stages in carrying out a systematic review

Answer 32

stage 1 - define focused, answerable question, using PICO(S) analysis
stage 2 - identify relevant literature
stage 3 - identify quality of literature: Questions to consider are if the study was randomised, if there was allocation concealment, if there was evidence of blinding, how many patients were lost to follow-up and if the treatment and the placebo were indistinguishable.
stage 4 and 5 - summarise and interpret the evidence qualitatively, and answer what proportion of studies found significant results.

Question 33

Define a meta-analysis and explain its purpose

Answer 33

Meta-analysis can pool data from different studies to produce a single summary estimate called a pooled estimate. It is not a simple average of the results from all studies, instead the results are weighted.

Question 34

Interpret a forest plot

Answer 34

The central square of each result represents the measure of effect of risk ratios, and the lines at either side represent the confidence interval.

Question 35

Describe the problems with systematic reviews and meta-analyses, including heterogeneity and publication bias

Answer 35

Statistical heterogeneity (or simply heterogeneity) is where there is a variability in the intervention effects being evaluated. Observed intervention effects are more different from each other than one would expect due to random error (chance) alone.

A fixed effect model assumes a single underlying effect size of all studies is the same. The random effect model assumes that the underlying studies are estimating similar, but not the same, effect sizes. It allows 95% confidence intervals to be wider to allow for the extra variation between studies.

Publication bias occurs when research that is published is not representative of all completed studies, for example, due to non-publication of studies with negative or inconclusive results. This applies particularly to smaller studies and will distort results and conclusions drawn from a meta-analysis.

Question 36

Describe the enzyme activities of pyruvate dehydrogenase and the factors regulating its activity.

Answer 36

This is a multi enzyme complex, with a molecular mass of 4 to 10 million daltons. It consists of 3 enzyme complexes and 5 coenzymes.

• These coenzymes are thiamine pyrophosphate (TPP), from thiamine and vitamin B1.
• Lipoamide is a lipoic acid which is a 10 carbon fatty acid with sulfhydryl groups on carbon 8 and carbon 10.
• CoA is a pantothenic acid with vitamin B5.
• FAD+ is riboflavin with a vitamin B2 prosthetic group. - NAD+ is niacin and vitamin B3.

PDH is active when dephosphorylated. It can be inactivated by kinase with ATP, NADH and acetylCoA and, where it is phosphorylated. This can then be activated on the addiction of phosphates, insulin and Ca2+.

Question 37

Outline the intermediates and enzyme activities of the TCA cycle.

Answer 37

• In the citric acid cycle all the intermediates (e.g. citrate, iso-citrate, alpha-ketoglutarate, succinate, fumarate, malate, and oxaloacetate) are regenerated during each turn of the cycle.

Question 38

State which steps of the TCA cycle produce reduced coenzymes.

Answer 38

The citric acid cycle provides a series of intermediate compounds that donate protons and electrons to the electron transport chain by way of the reduced coenzymes NADH and FADH2. The electron transport chain then generates additional ATPs by oxidative phosphorylation.

Question 39

Explain how the TCA cycle is regulated.

Answer 39

The citric acid cycle is regulated primarily by the concentration of ATP and NADH. The key control points are the enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. Isocitrate dehydrogenase is allosterically stimulated by ADP, which enhances the enzyme’s affinity for substrates.

Question 40

Describe the physiological states in which gluconeogenesis is active.

Answer 40

this occurs when glucose concentration is low, and ATP levels are high. glucose is made from non-carbohydrate stores.

Question 41

State in which tissues gluconeogenesis occurs.

Answer 41

liver (90%), kidney, small intestine

Question 42

Describe the key reactions of gluconeogenesis and how they are regulated.

Answer 42

1. Pyruvate carboxylase converts pyruvate to oxaloacetate in the mitochondrion.
2. Oxaloacetate is converted to malate or aspartate, which travels to the cytosol and is reconverted to oxaloacetate.
3. Phosphoenolpyruvate carboxykinase converts oxaloacetate to phosphoenolpyruvate.
4. Phosphoenolpyruvate forms fructose 1,6-bisphosphate by reversal of the steps of glycolysis.
5. Fructose 1,6-bisphosphatase converts fructose 1,6-bisphosphate to fructose-6-phosphate, which is converted to glucose-6-phosphate.
6. Glucose-6-phosphatase converts glucose-6-phosphate to free glucose, which is released into the blood.

There is a reciprocal regulation of gluconeogenesis and glycolysis by allosteric effectors.

Question 43

Explain the reciprocal regulation between glycolysis and gluconeogenesis.

Answer 43

both of these pathways cannot occur simultaneously as they use the same enzymes and substrates therefore, when one pathway is active, the other is not. this is regulated reciprocally.

Question 44

Define the terms anabolism and catabolism.

Answer 44

Anabolic pathways synthesise larger molecules, such as biosynthetic and fuel storage pathways, and catabolic pathways break down larger molecules, such as in fuel oxidation pathways.

Question 45

Describe the broad effects of hormones, particularly insulin, glucagon and adrenaline on metabolic processes.

Answer 45

Insulin acts on the live, muscle and adipose tissue. It promote glycogen formation in the liver and muscle, the conversion of glucose to triacylglycerols in the liver, protein synthesis (e.g. albumin) in the liver, the storage of triacylglycerols in the adipose tissue, increases glucose uptake by muscle and adipose tissues, and amino acid uptake and protein synthesis in the skeletal muscle.

Glucagon acts to maintain fuel availability. It acts principally in the liver and adipose tissue, but not in muscle as there are a lack of receptors here. Glucagon promotes increased glycogenolysis, reduced glycogen synthesis in the liver, stimulates gluconeogenesis and ketogenesis, and mobilises fatty acids from adipose triacylglycerols.

Question 46

Describe the involvement of the major body tissues

Describe the coordination of metabolism in liver, skeletal muscle, adipose tissue, and brain in the fed state.brain) in the generation and use of chemical energy.

Answer 46

liver: Increased glucose is taken up by hepatocytes., increased phosphorylation of glucose, and glucokinase creates glucose-6-phosphate. Excess glucose is converted to triacylglycerol and packaged into very low density lipoproteins. Increased glycogenesis occurs and glycogen synthase is activated. The liver is the primary site of fatty acid synthesis. This is increased during absorptive state due to acetyl CoA and NADPH availability.

adipose tissue: increased glucose transport, GLUT-4 is recruited by insulin and glucose is phosphorylated. Glycolysis supplies glycerol 3-phosphate for TAG synthesis. Fatty acids and glycerol is released by lipoprotein lipase (LPL) in the capillary walls.

skeletal: Glucose is used to replenish glycogen stores depleted by exercise. Amino acids are taken up for restorative synthesis. Glucose is the primary source of energy, and fatty acids are of secondary importance.
brain: The brain is dependent on blood glucose.

Question 47

Describe the coordination of metabolism in skeletal muscle, adipose tissue, liver and brain in the starved state.

Answer 47

liver: The main role of the liver is to produce glucose by glycogenolysis and gluconeogenesis. Ketone bodies are also produced for non-glucose dependent tissues. These rates of production will drop as usage by the rest of the tissue also drops during prolonged starvation.
adipose: Glucose transport is depressed as GLUT-4 is insulin sensitive. There is reduced glycolysis and TAG synthesis. Adipose triacylglycerols are mobilized by lipolysis.
skeletal: During fasting, resting muscle moves further from glucose to fatty acids and ketone bodies. For contraction as glycogen is depleted, fatty acids are mobilized from TAG in the adipose tissue and become the dominant energy source.
brain: At the start of fasting, the brain uses glucose. In prolonged fasting (beyond 2-3 weeks) plasma ketone levels rise significantly, and replace glucose, but some glucose is needed for neurotransmitters.

Question 48

Compare metabolism in starvation and diabetes mellitus.

Answer 48

type 1 - failure to respond to glucose, increased moblisation of fatty acids and increased gluconeogenesis

type 2 - insluin resistance. Lipoprotein lipase is low, so VLDL and chylomicrons are elevated

Question 49

Describe the transport of electrons through the respiratory chain.

Answer 49

CI - accepts electron from NADH, gives to CoQ via Fe-S protein
CII - accepts elecront from FADH2, gives to CoQ via Fe-S protein
CoQ - transfers electrons to CIII
CIII - transfers electrons to cytochrome c
cytochrome c - transfers electrons to CIV
CIV - accepts electrons from cytochrome c and transfers them to 1/2O2 which reduces to H2O

Question 50

Recognise inhibitors of the electron transport chain and oxidative phosphorylation.

Answer 50

ATPase inhibitors, site specific inhibitors, uncouplers

cyanide, rotenone

UCP1 and UCP3

Question 51

Explain how ATP is synthesised through chemiosmotic coupling.

Answer 51

ATP synthase pumps protons to the cytosolic side of the mitochondrial membrane, which re-enter the matrix by passion through the F0 proton channel. As protons pass down the channel, they drive the rotation of the C ring of F0 which causes conformational change in the beta subunit of the F1 domain (which binds ADP and Pi ). ATP is then formed and released.

Question 52

Explain the significance of the P/O ratio.

Answer 52

Number of ATP molecules formed per oxygen atom.
NADH - P/O ratio of 3 (2.5)
FADH2 - P/O ratio of 2 (1.5)

Question 53

Describe the physiological role of uncoupling protein.

Answer 53

Uncouplers uncouple electron transport from ATP synthesis and destroy the proton gradient.