lecture 24 - TCA cycle and fermentation Flashcards
What is the first step of the TCA cycle?
Oxaloacetate + acetyl CoA –> citryl CoA –> citrate
This aldol condensation is catalysed by citrate synthase. Hydrolysis of the high energy thioester bond drives the reaction.
What are steps 2-3?
- Citrate –> cis-aconitate –> isocitrate (aconitase)
- isocitrate –> oxalosuccinate (first NADH generated) –> a-ketoglutarate (first CO2 released) (isocitrate dehydrogenase)
reactions where CO2 released irreverisble - important for regulating cycle
What is step 4?
a-ketoglutarate + NAD+ + CoA –> succinyl CoA + CO2 + NADH (second NADH and CO2) (a-ketoglutarate dehydrogenase)
Very similar to the reaction catalyzed by pyruvate decarboxylase:
pyruvate + NAD+ + CoA –> acetyl CoA + CO2 + NADH + H+
a-ketoglutarate dehydrogenase is a multienzyme complex. It is homologous to PDH, and has the same mechanism as well.
What is step 5?
succinyl CoA synthase
Pi + Succinyl CoA –> Succinyl phosphate + CoASH
The C-S bond is directly attacked by inorganic phosphate creating succinyl-phosphate, which can ..
.. donate Pi onto a histidine sidechain of the enzyme leaving free succinate behind.
The 3-phospho-His residue has sufficient energy to phosphorylate GDP to GTP.
So far:
lost 2 carbons
gained 2 NADH
gained 1 GTP
What happens in the remainder of the cycle?
The rest of the cycle converts the CH2 group in succinate into the C=O group in oxaloacetate
succinate converted to fumarate by succinate dehdrogenase, FADH2 generated
fumarate converted to malate by fumarase
malate converted to oxaloacetate by malate dehydrogenase
How are the TCA cycle and oxidative phosphorylation linked?
The electron carriers feed electrons to the oxidative enzymes of the mitochondrial membrane
The TCA cycle enzyme succinate dehydrogenase is at the same time a component of the oxidative phosphorylation chain. This allows it to have FAD covalently linked.
How is the TCA cycle regulated?
Regulation of the TCA cycle is coupled to the energy needs of the cell and the availability of the substrates – pyruvate and acetyl-CoA.
Regulation is maintained at the three TCA steps functioning far from equilibrium.
citrate synthase
isocitrate dehydrogenase
a-ketoglutarate dehydrogenase
The connection between glycolysis and the TCA cycle is regulated through pyruvate dehydrogenase.
Summarise the TCA cycle
The TCA cycle produces reducing intermediates – 3 NADH and 1 FADH2 per acetyl CoA – that can be used by the oxidative phosphorylation chain.
Entry into the cycle is through acetyl-CoA, which is produced by the multi-enzyme complex pyruvate dehydrogenase, which generates another NADH molecule as well. The acetate entering the cycle is completely ‘burned’ – converted into CO2.
The first half of the TCA cycle burns the two carbons, the second half regenerates oxaloacetate to be able to receive another acetate.
The TCA cycle intermediates are only needed in catalytic amounts, since only the acetate, which enters from Ac-CoA is used up.
Since acetate is converted to CO2, the cycle can be used together with glycolysis to convert glucose to CO2. This produces in total 10 NADH, 2 FADH2, 2 ATP and 2 GTP.
What is homolactic fermentation?
Pyruvate + NADH __> lactate + NAD+ catalysed by lactate dehydrogenase
Used for metabolising pyruvate in animals during anaerobic conditions, such as during strenuous exercise in the muscle.
Describe the energy balance of fermentative glycolysis in animals
Glucose + 2 ADP + 2 Pi –> 2 Lactate + 2 ATP
The main purpose of the lactate dehydrogenase reaction is to regenerate NAD+ when oxidative phosphorylation is inhibited due to a lack of oxygen.
Describe alcoholic fermentation
pyruvate converted to acetaldehyde by pyruvate decarboxylase, CO2 released
acetaldehyde converted to ethanol by alcohol dehydrogenase, NAD+ regenerated
Used by yeast and bacteria under anaerobic conditions. Pyruvate decarboxylase (PDC) is absent in animals.
In contrast to pyruvate dehydrogenase, PDC does not oxidize its product, supplying the acetaldehyde substrate for alcohol dehydrogenase.
This fermentation is also used to replenish NAD+ levels, similarly to homolactic fermentation
