Metabolism

Question 1

Define metabolism

Answer 1

The processing of fuels for the provision and sustainability of cellular energy (ATP) in times of feeding (prandial state) and fasting (post-prandial state).

Question 2

Describe catabolism

Answer 2

Catabolic reactions destroy reactants reducing big, complex substances into smaller molecules.

Question 3

Describe anabolism

Answer 3

Anabolic reactions reassemble the smaller, simpler molecules into new, bigger products.

Question 4

What is the function of the energy produced from catabolic processes?

Answer 4

Energy production from catabolic processes allow anabolic pathways to produce specialised proteins and complex metabolites that are essential for the maintenance of tissue structure and function.

Question 5

What is the criteria for classifying a metabolic reaction as either catabolic or anabolic?

Answer 5

It is dependant on whether the product molecules are larger or smaller in size than the reatant molecules.

A reaction is catabolic if it involves breakdown of larger molecules into smaller ones.

A reaction is anabolic if it involves the production of larger molecules from smaller reactants.

Question 6

What is the law of mass action?

Answer 6

An increase in the concentration of reactants relative to products tends to push a reaction forward, and an increase in the concentration of products relative to reactants tends to push a reaction in reverse.

Question 7

How is the energy from a catabolic reaction stored and used?

Answer 7

The energy released during catabolism needs to be captured and stored in a form that is usable. ATP is this form of energy.

Question 8

How much energy is produced from the hydrolysis of ATP?

Answer 8

A large amount of energy is released when ATP is hydrolysed to ADP + inorganic phosphate = ~ -7 to -12 kcal/mol.

Question 9

Approximately how much ATP does a resting human comsume in 24 hours?

Answer 9

~40kg

(~140g/5 min)

Question 10

How are carbohydrates used as fuels?

Answer 10

Excess dietary carbohydrate is stored as glycogen mainly in the liver (120-140g) and muscle (400-500g).

Question 11

How long do glycogen stores last?

Answer 11

~1 day in a sedentary individual and ~3-5 hours in an individual doing low-moderate exercise.

Question 12

How are fats stored and used as fuels in the body?

Answer 12

Fats from the diet or synthesised within the body are stored as triglycerides in the liver and adipose tissue.

Triglycerides are mainly synthesised in the liver and stored in the fat.

Question 13

How are proteins used as fuels in the body?

Answer 13

• Aino acids produced by the breakdown of dietary proteins can be incorporated into new proteins.
• However, amino acids not needed to make new proteins cannot be stored, so they are used as metabolic fuel.
• Some amino acids can become deaminated.

Question 14

What happens when to amino acids when they become deaminated?

Answer 14

The deamination yields NH4 and a keto acid that are intermediates of the glycolytic pathway or of the citric acid cycle.

Question 15

Describe the properties of glycolysis

Answer 15

• The conversion of glucose to pyruvate which happens in all cells.
• It is reversible in some tissues (liver).
• Generates 2NADH and 2ATP per glucose molecule.

Question 16

What are the steps involved in carrying out oxidation of glucose?

Answer 16

1. Glycolysis - takes place in the cytosol.
2. Krebs cycle - occurs in the mitochondrial matrix.
3. Oxidative phosphorylation - occurs within the inner mitochondrial membrane.

Question 17

Describe what happens during glycolysis.

Answer 17

• Each 6-carbon glucose molecule is broken down into 2 molecules of pyruvate (ionised form of pyruvic acid) containing 3 carbons apiece.
• 2 ATP molecules are consumed during this process, but 4 are produced so there is a net production of 2ATP.
• 2 molecules of NAD+ are reduced in step 6, yielding 2 molecules of NADH for every molecule of glucose.
• No oxygen is consumed and no carbon dioxide is produced.

Question 18

Which enzymes control the rate of glycolysis?

Answer 18

• Hexokinase
• Phosphofructokinase - main regulator of glycolysis
• Pyruvate kinase

Question 19

Describe allosteric regulation.

Answer 19

Certain enzyme molecules possess a binding site called the regulatory site that is specific for molecules known generally as modulators. A modulator induces a change in an enzyme’s conformation the alters the shape of the active site, causing a change in the enzyme’s activity by altering its catalytic rate, its affinity for substrate, or both.

Question 20

What happens during glycolysis under anaerobic conditions?

Answer 20

NADH is re-oxidised by reducing pyruvate to lactate.

Question 21

What is Coenzyme A and what does it do?

Answer 21

• A nucleotide that contains the vitamin pantothenic acid (= vitamin B5).
• CoA forms a high energy bond with carboxylic acids such as acetate.

Question 22

What are the major results of the Krebs cycle?

Answer 22

• By the end of each turn of the Krebs cycle, a total of 2 carbon dioxide molecules have ben generated as end-products.
• Only 1 ATP is generated directly during the Krebs cycle by substrate-level phosphorylation.
• In a single turn of the Krebs cycle, a total of 4 reduced coenzymes - 3NADH and 1 FADH2 - are produced.

Question 23

What happens during the Krebs cycle?

Answer 23

1. A 2-carbon molecule (acetyl-CoA) is condensed with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule (citrate).
2. The 6-carbon molecule is broken down in stages, finally resulting in formation of a new molecule of the original 4-carbon compound.
3. Additional products 3x NADH, 1xFADH2, 2x carbon dioxide, 1x GTP.

Question 24

Which chemical agents target the respiratory chain?

Answer 24

• Rotenone - insecticide, piscicide
• Antimycin A - antibiotic
• Cyanide and carbon monoxide block the final step by which hydrogen combines with oxygen.

Question 25

What generates a pH gradient across the inner mitochondrial membrane?

Answer 25

• Movement of electrons down the respiratory chain generates a H+ ion (pH) gradient across the inner mitochondrial membrane.

Question 26

There is a pH gradient across the inner mitochondrial membrane. What does this result in?

Answer 26

The resulting electrochemical gradient is used t drive ATP synthesis by way of ATP synthase.

Question 27

Describe how and when lipids would be used for energy.

Answer 27

• There is a limited supply of glycogen in the body; during extended periods of exercise or starvation, other fuels must be metabolised to generate ATP.
• Lipids are important because weight-for-weight, triglycerides can produce about 6x as much energy as glycogen.

Question 28

Describe the breakdown of fats during lipolysis.

Answer 28

• Fatty acids are linked to CoA in the cytosol before they are oxidised and 2-carbon fragments cleaved off (as acetyl-CoA) in the mitochondria.
• Although no ATP is generated directly by this process, hydrogen and electrons are removed during fatty acid oxidation are passed along the respiratory chain from NADH and FADH2, so indirectly producing ATP.
• Fatty acids and glycerol released by adipose tissue are mainly metabolised by the liver.

Question 29

What is transamination?

Answer 29

• Most amino acids cannot release their amino groups as NH4+.
• Instead, theyhave to pass their amino group to a keto acid (mainly α-ketoglutarate) in the process known as transamination.

Question 30

What happens to the keto acid produced by transamination?

Answer 30

It can be fed into glycolysis or the Krebs cycle while the resulting amino acid can then be transaminated / deaminated and used similarly.

Question 31

What happens to the NH4+ which is produced when proteins are used in metabolism?

Answer 31

The NH4+ produced is highly toxic, so the body rapidly converts it to urea, which is far less toxic.

Question 32

What is gluconeogenesis?

Answer 32

Formation of glucose from non-carbohydrate sources.

It is similar to glycolysis in reverse, but some enzymes are different to bypass irreversible steps.

Question 33

Where does the majority of gluconeogenesis occur?

Answer 33

Liver