The Citric Acid Cycle Flashcards
What is the citric acid cycle (CAC)?
The citric acid cycle/krebs cycle occurs in the mitochondrial matrix except for one enzyme
CAC starts and finishes with the same molecule
Two carbons enter the CAC coming in from acetyl-CoA
What are the two parts to the CAC?
In the first part of the CAC the 2-carbons in acetyl-CoA are added to 4C oxaloacetate
In the second part of CAC we rearrange the 4 carbon molecule to end up with the 4 carbon molecule we started with, 4C oxaloacetate
What is the key reaction in the CAC?
Acelty-CoA 2 carbons join onto oxalocaetate which is a condensation reaction - product is citrate and a free CoA
The addition of carbons requires energy, in this reaction energy comes from the removal of CoA from acetyl-CoA hence this is an energetically favourable reaction
What is the isomerisation of citrate?
Before removing the carbons we need to rearrange citrate to isocitrate so it is susceptible to decarboxylation - removing carbons
This reaction is catalysed by aconitase
What is the first step of the CAC?
Once we have rearranged molecule to isocitrate in oxidative decarboxylation
Isocitrate is oxidised to oxalosuccinate, NAD+ is reduced to NADH H+ providing oxidising power
Oxalosuccinate is decarboxylased into a-ketoglutarate which results in losing a carbon molecule and producing a 5C molecule
What is the second step in the CAC?
a-ketoglutarate goes through a oxidative decarboxylation reaction producing succinyl-CoA
NAD+ being reduced to NADH H+ provides oxidising power for this to occur
Energy is captured in NADH, some is released and some is used to add the CoA back to the molecule
Carbon dioxide is removed from a-ketoglutarate producing 4C molecule succinyl-CoA
What is the third step in CAC?
Succinyl CoA goes through a SLP and succinate is produced
CoA is cleaved which releases energy to drive SLP, enzymes use the energy to phosphorylate GDP to GTP (GDP + phosphate -> GTP)
What is the final step in the CAC?
Succinate is converted to oxaloacetate
Succinate goes through an oxidation reaction to produce Fumarase, Fumarase is hydrolysed to Malate and Malate is oxidised to oxaloacetate
In the first oxidation reaction, FAD is reduced to FADH2 which drives the reaction of succinate to fumarate
In the hydrolysis reaction, H2O is split over the double bond resulting in a OH group being added
In the second oxidation reaction, NAD is reduced to NADH H+ which drives the reaction of malate to oxaloacetate
What is important about the succinate -> fumarate reaction?
SDH enzyme sits in the inner membrane of the mitochondria hence reaction of succinate and fumarate can still occur in the matrix. FAD is associated with the enzyme SDH hence this reaction occurs close to the SDH enzyme. When next succinate comes along we need to reverse FADH2 to FAD, this occurs in the electron transport chain which is located in the inner mitochondrial membrane
What is the overall reaction for CAC?
Acetyl CoA + 3NAD+ + FAD + 2H2O + GDP + Pi -> 2CO2 + CoASH + 3NADH + 3H+ + FADH2 + GTP
∆G˚ is -44.3kJ/mol hence it is energetically favourable
