Energy 1: Metabolism, ATP and glycolysis Flashcards
Define Catabolism
Catabolism - breakdown of molecules to release energy (ATP) (Intake of carb, fat, protein = lead to generation of energy +CO2/heat, can be precursors to new molecules).
Define Anabolism
Anabolism - build up of molecules which requires energy (ATP) (Biosynthesis of amino acids, nucleotides, sugars, fats from precursors, for growth, repair, movement etc.).
Why do we study metabolism?
- 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
Give an overview of the flow diagram
Stick on wall
Describe energy provision
- It is the bodies energy provision and can act as both an acceptor and donator of energy (is an intermediate of energy) acting as a short-term reservoir of energy. ATP contains high energy phosphate bonds
- Total amount of energy available from the hydrolysis of ATP is 65kj/mole.
- How much do we use at rest? 40Kg/24hour.
- What about during exercise? 0.5Kg/minute.
- However, our body only actually contains 100g of ATP, so to meet the demands of the body it must re-synthesise ATP from ADP (i.e. that ATP is being recycled again and again very quickly). This is largely done through oxidative phosphorylation and takes place in the mitochondria.
What are the 4 main oxidative pathways?
- Citric acid cycle (Krebs/TCA Cycle)
- Electron transport coupled to oxidative phosphorylation
- Fatty acid oxidation
- Glycolysis
What happens in glycolysis?
Glycolysis
• Glycolysis will break down 6C glucose into two 3C molecules of pyruvate.
• Firstly, glucose gets phosphorylated (consuming energy) to become glucose-6-phosphate (G-6-P) (remember this maintains conc. gradient across membrane).
• Then the G6P undergoes a conformational change to become Fructose-6-phosphate.
• Fructose-6-phosphate is then phosphorylated to form Fructose 1,6 bisphosphate (C6).
• F6BP gets split into two 3C units, each then undergoes the following process.
• Firstly, it generates NADH from NAD+ and ATP from ADP in its reactions ultimately generating phosphoenol pyruvate which is then converted to pyruvate during which ATP is synthesised again.
• And as this occurs with both 3C we get double
• So, at the end of glycolysis we are left with pyruvate
Glycolysis
• The first stage in both aerobic and anaerobic respiration - a 10 stage metabolic pathway occurring in the cytoplasm of all living organisms, converting glucose to pyruvate.
• NAD (nicotinamide adenine dinucleotide) is a non-protein molecule helping dehydrogenase enzymes carry out oxidation by accepting (up to 2) hydrogen atoms to become reduced NAD. In this state, protons and electrons are carried to the cristae to be used in oxidative phosphorylation to produce ATP.
• When NAD releases protons which it accepted, it becomes reoxidised NAD - can accept more protons.
• Glucose is a hexose sugar and its molecules are stable, so needs to be activated before splitting into two three-carbon compounds.
• 1. ATP → ADP + Pi (hydrolysis). Phosphoryl group added (C1) to glucose → hexose monophosphate.
• 2. ATP → ADP + Pi . Phosphoryl group added (C6) to hexose monophosphate → hexose bisphosphate.
• This forms glucose-6-phosphate (G-6-P). Then the G6P undergoes a conformational change to become Fructose-6-phosphate.
• 3. Each Fructose-6-phosphate is split into two three-carbon compounds called triose phosphate.
• Each molecule of TP contains one of the phosphate groups. 4. Dehydrogenase enzymes (aided by coenzyme NAD) remove hydrogen (oxidation) from triose phosphate to form phosphoenol pyruvate then forming two molecules of pyruvate.
• 4 molecules of ATP produced (4ADP + 4Pi → 4ATP), and 2 molecules of NAD reduced.
• Net formation: 2 molecules of red. NAD, 2 molecules of pyruvate, 2 molecules of ATP (as 2 used).
What are the methods in which glycolysis can be regulated?
- 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).
- This regulation can be measured in terms of milliseconds, seconds and hours.
- There are three steps where regulation occurs in glycolysis and these are at the points where energy is involved. The first two are where energy is consumed and the last is where ATP is produced.
What enzymes are targeted in glycolysis in regulation?
- The first point is when glucose is converted to G6P, this is under action of hexokinase.
- The last point is where phosphoenol pyruvate is converted to pyruvate (with release of ATP) and this is under action of pyruvate kinase.
- The middle point (and most important) is the conversion of Fructose-6-phosphate to fructose 1,6 bisphosphate, the enzyme phosphofructokinase (PFK) catalyses this.
Describe regulation in the liver
- Certain molecules/substances will regulate the activity of these enzymes, these are often the end products (e.g. ATP, as purpose of reaction is to generate ATP).
- Regulation of glycolysis in the liver reflects its diverse functions. High concentrations of ATP inhibit PFK. PFK is inhibited by citrate (from TCA cycle) and H+ which makes sense as they are also indicators of the amount of glucose going through the glycolytic pathway. PFK is stimulated indirectly by a build-up of F6P. Hexokinase is inhibited by G6P. But the liver also has glucokinase which is not inhibited by G6P.
- In the liver we have hexokinase, but we also have glucokinase which is not affected by G6P build up. Glucokinase has a much lower affinity for glucose so is active at much higher concentrations of glucose.
- 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).
- There is also positive regulation of phosphofructokinase by Fructose 2,6 bisphosphate and AMP (which is a product of the conversion of ATP to ADP, which gives an indication of energy levels of the cell).
- Pyruvate kinase is inhibited by ATP which makes sense as it is regulating a pathway preventing too much ATP from being produced.
How can glycolysis be regulated in muscles?
- In muscle, glycolysis is regulated to meet the need for ATP
- In muscle ATP is used to produce energy for contraction, short term it can take the ADP generated from this reaction and combine two ADP molecules to give another ATP molecule and AMP. Allowing the muscle to contract for that little bit longer.
- ADP + ADP = ATP + AMP - Adenylate cyclase
- AMP regulator of ATP synthesis in muscle
- Good indicator of energy status in muscle. Why?
- ADP and ATP already present in muscle cell and not a good indicator of activity
- But AMP will be active when sufficient ADP present to require re-synthesis into ATP
- AMP rises as ADP:ATP ratio falls;; high cellular ratio of AMP:ATP
- Signal that the energy status of the cell is compromised
- PFK is the most important control point and is regulated by the amount of ATP present. The way it does this is that high concentrations of ATP inhibit PFK by lowering the affinity for fructose 6 phosphate = It is also inhibited by low pH produced by lactate build up in muscle.
Explain why anaerobic respiration occurs
- Energy is required for tumours and exercising muscle are met through anaerobic respiration.
- 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.
- This allows muscles to work under fast burst exertions like sprint etc. Tumours tend to be working under conditions of low oxygen also.
- So, 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).
- Muscle cells have a way of getting around this by generating lactate which allows the pathway to continue.
- One of the reasons is that if we look at the flowchart above we can see that the C3 undergoes a reaction where NAD+ is reduced to NADH and this is very important for the pathway to work and this is also the step that then enables ATP to be produced.
- In order to allow this reaction to continue, pyruvate is converted to lactate and in doing this NADH is oxidised to NAD+, this NAD+ can then be fed back to earlier reaction to enable more molecules to flow through this part of glycolysis!
- 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.
Why do tumours use glycolysis?
When tumours outgrow their blood supply oxygen delivery is reduced, tumour cell metabolism reverts to glycolysis
A reduction in O2 leads the
activation of the
transcription factor HIF-1α
HIF-1α regulates the
expression of a number
enzymes in the glycolytic
pathway
