CELL Energy I: Metabolism, ATP, Glycolysis Flashcards
Why do we eat?
what does this help us do? (4)
We eat because we need energy to carry out the processes of life including: • The synthesis of new molecules • Establishing/Maintaining ion gradients • Mechanical work • Keeping warm
Why study metabolism
- To understand the metabolic basis of disease e.g. Diabetes
- The diseased state changes the way body uses food e.g. Cancer
- How body uses nutrients is important when understanding disease
- Can use changes in metabolites to aid diagnosis & to follow treatment
Explain the difference between anabolism and catabolism
Catabolism is the breakdown of complex molecules to release energy or carry out mechanical work (e.g. intake of carb, fat, protein leads to generation of energy +CO2/heat, or can be precursors to new molecules)
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, movements etc)
Describe the importance of glucose as a source of energy & importance of ATP
what can atp act as?
how much can the body have at rest? hence what must happen?
where?
- It can act as both an acceptor & donator (Short term energy)
- To meet demands the body must re-synthesise ATP from ADP for the body only has 100g of ATP even though at rest, about 40kg of ATP us used per 24hrs.
- Done in mitochondria
major oxidative pathways (4)
The major oxidative pathways include:
- Glycolysis
- Citric acid cycle /TCA/Krebs cycle
- Electron transport chain/oxidative phosphorylation: where most ATP generation occurs
- Fatty acid oxidation
Cofactors central to metabolism
what are these? (2)
what are the oxidised and reduced forms?
NAD and FAD are activated carriers of electrons used for oxidation/reduction reactions
FAD+ -> FADH2
oxidised form reduced form
NAD+ -> NADH + H+
oxidised reduced
Glycolysis
what is broken down and into what? (net effect)
what must first happen to glucose? why does this happen?
what happes to this new molecule? and again, before it is broken down?
what does each broken down molecule undergo? (3)
what are we left with at the end of glycolysis?
Glycolysis will break down 6C glucose into two 3C molecules of pyruvate.
Steps:
1. Glucose gets phosphorylated -> consuming energy to become glucose-6-phosphate (G-6-P)
• Remember this maintains concentration gradient across membrane
- Then the G6P undergoes a conformational change to become Fructose-6-phosphate
- Fructose-6-phosphate phosphorylated to form Fructose 1,6 bisphosphate (C6)
- F6BP gets split into two 3C units, each then undergoes the following process.
- It generates NADH from NAD+
- It generates ATP from ADP
- 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
- 1 glucose -> 2 pyruvates
- 2 NAD+ -> 2 NADH
- 2 ADP -> 2 ATP (net gain)
- 2Pi
What regulates Glycolysis?
potential sites for regulation?
3 ways regulation can take place?
Enzymes that catalyse irreversible reactions are potential sites for regulation
The activity of such enzymes can be regulated by:
• Reversible binding of allosteric effectors
o The binding of molecules to sites other than the active site, commonly the product of that pathway
• Covalent modification (e.g. phosphorylation)
• Transcription (basically by the amount of enzyme present)
o Long term regulation change synthesis of enzyme itself, takes a long time.
This regulation can be measured in terms of milliseconds, seconds and hours.
regulation steps of glycolysis
what are the 3 points where regulation takes place? key feature for these points?
what are the 3 conversions and enzymes involved?
which is the most important?
There are three steps where regulation occurs in glycolysis and these are at the points where energy is involved.
o The first two are where energy is consumed and the last is where ATP is produced.
- The first point is when glucose is converted to G6P, this is under action of hexokinase.
- The middle point and most important is the conversion of Fructose-6-phosphate to fructose 1,6 bisphosphate, the enzyme phosphofructokinase catalyses this. —-»»» MOST IMPORTANT
- The last point is where phosphoenol pyruvate is converted to pyruvate, with release of ATP, and this is under action of pyruvate kinase.
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).
what does ATP negatovely regulate? what else negatively regulates this too? where are they produced?
if you inhibit phosphofructokinase, what will happen? effect of this? where will this effect not happen? why? when does this work? why does this work well with its function?
what is positive regulation of phosphofructokinase? (2)
when do you get those molecules?
what is pyruvate kinase inhibited by? where does this occur?
o ATP negatively regulates phosphofructokinase, so this enzyme will be turned down if the cell has lots of ATP and thus glycolysis pathway will be turned down.
o Citrate produced from TCA cycle and H+ also inhibit phosphofructokinase, which makes sense as they are also indicators of the amount of glucose going through this glycolytic pathway.
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.
o In the liver we have hexokinase, but we also have glucokinase which is not affected by G6P build up.
o 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.
There is also positive regulation of phosphofructokinase by Fructose 2,6 bisphosphate and AMP, which is a product of the conversion of ATP -> 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. (liver only)
Regulation of glycolysis in the liver reflects its diverse functions
what inhibits PFK? (2)
what stimulates PFK?
what is inhibited by G6P? how does the liver work around this?
Regulation is more complex
High concentrations of ATP inhibit PFK
PFK is inhibited by citrate
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 muscle glycolysis is regulated to meet the need for ATP
what is atp used in muscles for?
in the short term how can it recover atp using adp?
what is the enzyme used and the equation for this?
so what will build up when the cell is short of energy?
what is the most important control point? how is it regulated? how does this happen? what else inhibits it and relevance of this?
what does inhibition of this lead to?
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
It is for this reason that the cell can use ratio of ATP to AMP as a good indicator of energy levels.
- So, AMP will build up when the cell is short of energy
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
Inhibition of PFK ultimately leads to inhibition of hexokinase as explained earlier
Glycolysis in exercising muscles
under normal circumstances what happens to the pyruvate synthesised? converted to? undergo what? where?
when exercising when need for atp exceeds capacity of mitochondria, what happens? what maintains this process? what does this allow?
how is this different to oxidative phosphorylation? drawback?
Under normal circumstances the pyruvate synthesized in glycolysis is converted to acetyl CoA and will undergo oxidative phosphorylation a process that takes place in the mitochondria.
In exercising muscle when the need for ATP exceeds the capacity of the of the mitochondria the pyruvate is converted to lactic acid. To maintain this process NADH synthesized in glycolysis is consumed the NAD produced can then be used to generate more NADH allowing glycolysis to continue.
Unlike oxidative phosphorylation this process does not require oxygen however the build up of lactate leads to pain and fatigue.
What do exercising muscles and tumours have in common?
how is the glycolytic pathway different to TCA and ETC?
how does glycolysis work well for muscles and tumours?
so when glucose turns to pyruvate, why does it only occur for a little time? what is depleted and what is built up which inhbits the pathway?
how do muslces work around this?
why is this very important with the C3 molecules?
what is the problem with this? where is it remedied?
The glycolytic pathway is oxygen independent, whereas the TCA and ETC 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 hardworking 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 in particular 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?
how is tumour different to muscle?
what does a reduction in o2 activate? what does this activate?
The reason is that fast growing tumour tissue will often grow faster than the blood supply can form around it, so the rapid metabolism of the tumour exceeds the ability of the blood supply to provide O2, so the tumour cell metabolism switches to glycolytic processes.
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.
