Citric Acid Cycle Steps

Step 1

Enzyme: Citrate synthase

Substrate: Oxaloacetate and Acetyl-CoA

Product: Citrate

What it Does: Condensation

Other: There is sequential binding of oxaloacetate and acetyl-CoA, which prevents premature hydrolysis of acetyl-CoA. Spontaneous. Irreversible reaction with a regulated enzyme.

Step 2

Enzyme: Aconitase

Substrate: Citrate

Product: Isocitrate

What it Does: Isomerization of citrate into isocitrate, not a direct isomerization, does it by Dehydration—hydration. Moving OH from the center carbon to carbon 2.

Other: Has a cis-Aconitate intermediate; Is in equilibrium, no real regulation on the enzyme. Uses an Fe-S cluster to do the reaction.

Step 3

Enzyme: Isocitrate dehydrogenase

Substrate: Isocitrate

Intermediate: Oxalosuccinate

2nd Product: alpha-Ketoglutarate

What it Does: Oxidative decarboxylation

Other: Also produces an NADH and releases a CO2

Step 4

Enzyme: alpha-Ketoglutarate dehydrogenase complex (Has 3 enzyme subunits)

Substrate: alpha-Ketoglutarate

Product: Succinyl-CoA

What it Does: Oxidative decarboxylation

Other: Also produces an NADH and releases a CO2; highly regulated enzyme

Step 5

Enzyme: Succinyl-CoA synthetase

Substrate: Succinyl-CoA

Product: Succinate

What it Does: Hydrolysis and substrate level phosphorylation

Other: Also produces 1 GTP (Nucleoside diphosphate kinase is what converts GTP to ATP — GTP + ADP ←→ GDP + ATP)

Step 6

Enzyme: Succinate dehydrogenase

Substrate: Succinate

Product: Fumarate

What it Does: Oxidation

Other: Produces 1 FADH2. Also a part of the electron transport chain (FAD is a coenzyme)

Step 7

Enzyme: Fumarase

Substrate: Fumarate

Product: Malate 

What it Does: Hydration (has chiral center, stereospecific, L-malate)

Other: Has a carbanion intermediate, which is set up to only make the L-isomer

Step 8

Enzyme: Malate dehydrogenase

Substrate: Malate

Product: Oxaloacetate

What it Does: Oxidation

Other: Produces 1 NADH; Have a large positive delta-G → it would be considered non-spontaneous, this makes it a good last reaction in the Citric acid cycle