Lecture 27 - The critic acid cycle Flashcards
Where does the citric acid cycle occur in the cell
Mitochondrial matrix
What molecule enters the CAC and where does it come from
Acetyl-CoA enters the CAC and comes from glycolysis and B oxidation
Two main parts of CAC
- Release of 2 carbon by CO2
- Regeneration of the starting molecule Oxaloacetate
What is the first step of the citric acid cycle
The first step is the condensations reaction of Acetyl-coA (2C) with oxaloacetate (4C) to form citrate (6C), catalysed by citrate synthase. Releasing CoA.
- These two carbon need to be removed
Where does the energy for the citrate formation come from
The energy comes from hydrolysis of CoA from Acetyl-CoA
What happens to citrate in the second step of the citric acid cycle
Isomerisation of Citrate. citrate is isomerised into isocitrate through a two step process catalysed by aconitase. This makes the molecule susceptible to decarboxylation
What happens in the first oxidative decarboxylation step of the CAC
Isocitrate (6C) is oxidised and decarboxylated by Isocitrate dehydrogenase to form a-Katoglutarate (5C), capturing energy as NADH.
- An oxidative decarboxylation
- NADH and Carbon release as CO2
What is Fluoroacetate
Fluoroacetate is a toxic compound found in plants
How does fluoroacetate affect the citric acid cycle
Fluroacetate is metabolised into flurocitrate, which inhibits the enzyme aconitase, blocking the CAC
What happens when aconitase is inhibited by flurorocitrate
When aconitase is inhibited, citrate accumulates, and the CAC is disrupted, leading to a stop in the ATP production.
- Increased acetyl CoA (build up)
- Decreased reduced coenzymes
- Less ATP
What happens in the second oxidative decarboxylation step of CAC
a-Ketoglutarate (5C) is oxidised and decarboxylated by a-ketoglutarate dehydrogenase to form succinyl-CoA (4C), capturing energy as NADH. Carbon removed as CO2
what is the significance of the succinct-CoA to succinate conversion
Succinly-CoA is converted to succinate, driving a substrate level phosphorylation to produce GTP which is equivalent to ATP
What is substrate level phosphorylation
The direct use of energy from substrate molecule to drive ATP synthesis.
e.g. Succinyl-CoA to succinate
- The P does not have to come from the substrate
what enzymes catalyses the conversion of succinate to fumarate
Succinate dehydrogenase catalysis the conversion of succinate to fumarate, capturing energy as FADH2
- Oxidation reaction similar to B oxidation
Succinate to Oxaloacetate reaction
The reactions used to convert succinate ti Oxaloacetate are very similar to B oxidation
1. Oxidation - FAD - FADH2
2. Hydration - H2O
3. Oxidation - NAD+ - NADH
What reaction occurs between fumarate and malate
Fumarate is hydrated to malate by fumarase
What is the final reaction of CAC
Malate is oxidised to regenerate Oxaloacetate by malate dehydrogenase, capturing energy as NADH
Overall reaction for the CAC
Acetyl-CoA + 3NAD + FAD + 2H2O + GDP + P -> 2CO2 + 3NADH +3H + FADH2 + GTP
what is the main purpose of CAC
Is to oxidise Acetyl-CoA to CO2 while capturing energy in the form of NADH, FADH2 and ATP/GTP
Why is succinate dehydrogenase considered a shared enzyme
Succinate dehydrogenase is part of CAC where FAD is reduced, so needs to also be in the ETC to oxidise FADH2 back to FAD
- Needed in CAC and ETC
