S2: Energy I: Metabolism, ATP and Glycolysis

Question 1

Why do we need energy?

Answer 1

We need energy to carry out processes of life (metabolism):

• The synthesis of new molecules
• Establishing/maintaining ion gradient
• Mechanical work
• Keeping warm

Question 2

What is catabolism?

Answer 2

This is the breakdown of complex molecules to release energy or carry out mechanical work

e.g. intake of carb, fat, protein leads to the generation of energy + CO2/heat which can become precursors to new molecules

Question 3

What is anabolism?

Answer 3

Anabolism is the synthesis of new molecules from less complex components

e.g. biosynthesis of amino acids, nucleotides, sugars, fats from precursors, for growth, repair, movement etc

Question 4

Why study metabolism?

Answer 4

• To understand the metabolic basis of disease, for example diabetes, atherosclerosis and gall stones (most diseases have a metabolic origin)
  
  • The diseased state changes the way the body uses food for instance cancer
  • To understand a disease we may need to know how the body normally deals with nutrients
  • We can use changes in metabolites to aid diagnosis and to follow treatment

Metabolism should be viewed as an integrated whole where one product of one pathway is often the substrate of another.

Some metabolic pathways require rapid generation (secs) like exercise while others require longer (minutes, hours), these are generally involving storing molecules (can take months/days).

Question 5

What is ATP?

Answer 5

ATP is the biggest energy provider in us. It is central to a cell and can act as both an acceptor and donator of energy (is an intermediate of energy) acting as a short term reservoir of energy. it contains high energy phosphate bonds.
ATP is recycled again and again from ADP. This is done through oxidative phosphorlylation in the mitochondria.

Question 6

List some major oxidative pathways (important in generating ATP)

Answer 6

• Glycolysis
  
  • Citric acid cycle (TCA/Krebs cycle)
  • Electron transport chain/oxidative phosphorylation (where most ATP generation occurs)
    
    • Fatty acid oxidation

Question 7

Describe glycolysis

Answer 7

Glycolysis will break down 6C glucose into two 3C molecules of pyruvate.

1. Glucose gets taken up into cell by glucose transporters
2. Glucose gets phosphorylated using ATP to become glucose-6-phosphate (G-6-P) - this helps maintains conc gradient of glucose as G-6-P remains trapped in cell
3. G-6-P undergoes conformational change to become Fructose-6-phosphate
4. Fructose-6-phosphate is then phosphorylated using ATP to form Fructose 1,6 bisphosphate (F6BP)
5. F6BP is split into 2 3C untis
6. 3C is converted to phosphophenol pyruvate . In this process, it generates NADH from NAD+ (reduction) and ATP from ADP in reactions)
7. Phosphophenol is converted to pyruvate generating ATP from ADP

Question 8

What regulates glycolysis?

Answer 8

Enzymes that catalyse irreversible reactions are potential sites for regulation

The activity of such enzymes can be regulated by:

- Reversible binding of allosteric effectors (very common) – the binding of molecules to sites other than the active site, commonly the product of that particular pathway
- Covalent modification (e.g. phosphorylation)
   - Transcription (long term regulation change synthesis of enzyme itself, takes a long time)

There are 3 steps where regulation occurs (2 where energy is consumed and 1 where ATP is produced)

Question 9

What enzymes are involved in the steps of glycolysis?

Answer 9

Hexokinase = converts glucose to G6P

Phosphofructokinase= Converts fructose-6-phosphate to fructose 1,6 bisphosphate

Pyruvate kinase converts phosphophenol pyruvate to pyruvate

Question 10

What regulates phosphofructokinase?

Answer 10

ATP negatively regulates it so the enzyme will have lower affinity for Fructose 6 phosphate and if the cell has lots of ATP and glycolysis will stop.

Citrate (from TCA) and H+ (from lactate) will also inhibit

If you have an inhibition of phosphofructokinase you will ultimately have a build up of G6P, if this builds up then it will inhibit hexokinase.

There is also positive regulation of phosphofructokinase by Fructose 1,6 bisphosphate and AMP (which is a product of the conversion of ATP -> ADP, which gives an indication of energy levels of the cell)

Question 11

What regulates hexokinase?

Answer 11

G6P inhibits hexokinase.

Question 12

Where is glucokinase found?

Answer 12

In the liver

Hexokinase is also found but glucokinase is not affected by G6P build up.

Question 13

What is glucokinase function?

Answer 13

it converts glucsoe to G6P (like hexokinase)

Glucokinase has a much lower affinity for glucose so is active at much higher concentrations of glucose and it is not affected by G6P build up.

One of the livers functions in terms of glucose metabolism is to get excess glucose and store it, so the fact the liver has this glucokinase enzyme allows it to do this (in order to generate G6P so can be stored)

Question 14

What regulates pyruvate kinase?

Answer 14

Pyruvate kinase is inhibited by ATP which makes sense as it is regulating a pathway preventing too much ATP from being produced.

Question 15

How is AMP formed?

Answer 15

ATP is used for energy generating ADP. In short term, 2 ADP molecules can be combined to make another ATP molecule and AMP. This allows muscle to contract for a little longer.

ADP + ADP –> ATP + AMP
The enzyme Adenylate kinase is used.

Question 16

Why is AMP important?

Answer 16

The ratio of ATP to AMP is an good indicator of energy levels.

AMP will build up when the cell is short of energy and more ATP synthesis is required.

Question 17

What is the most important control point in glycolysis?

Answer 17

Phosphofructokinase (PFK)

Question 18

What do excercising muscles and tumours have in common?

Answer 18

Both these tissues rely on anaerobic respiration as a large part of their metabolism.

Question 19

How do muscle cells get around depletion of NAD+?

Problem?

Answer 19

They generate lactate from pyruvate which is an oxidation reaction converting NADH to NAD+.

This is very important for the pathway to work and this is also the step that then enables ATP to be produced.

The problem with this pathway in muscle is that it can’t cope with large amount of lactate, it is exported via blood to liver. But without this use of lactate, glycolysis would only be able to continue for a short amount of time as pyruvate builds up and there is a lack of substrate (NAD+).

Question 20

Which cycles in respiration require oxygen and don’t?

Answer 20

The glycolytic pathway is oxygen independent, whereas the TCA and ETC (oxidative phosphorylation) reactions require oxygen.
Therefore the glycolytic pathway is an anaerobic pathway that will continue in the absence of ideal oxygen levels, allowing us to still generate ATP.

Question 21

Why is glycolysis limited (stops)?

Answer 21

In our hard working cell we have glucose -> pyruvate which will occur for a limited time only because it will deplete the cell of certain important factors that are required for the pathway to continue (e.g. NAD+) as well as their being build up of molecules which would inhibit the pathway (e.g. ATP).

Question 22

Why do tumours use glycolysis?

Answer 22

Fast growing tumour tissue will often grow faster than the blood supply can form around it. Rapid metabolism of tumour exceeds the ability of the blood supply to provide O2 so the tumour cell metabolism switches to glycolytic pathway. Tumour cells are not as sensitive to lactate.

A reduction in O2 also leads to the activation of the transcription factor HIF-1α which activates certain genes for enzymes in the glycolytic pathway, hence HIF-1 α moves metabolism more to the glycolytic pathway.