Krebs cycle · reaction 1 of 8

Acetyl-CoA + Oxaloacetate + Water → Citrate + Coenzyme A

Citrate synthase EC 2.3.3.1

Welds the two carbons of acetyl-CoA onto oxaloacetate, which has four carbons, and produces citrate, which has six. This is where carbon enters the cycle.

exergonic
irreversible
Mitochondrial matrix

in depth

ΔG°′: −32.2 kJ/mol
Inhibited by: NADH
Inhibited by: Succinyl-CoA
Location: Mitochondrial matrix

Fun fact

The enzyme changes shape when it binds oxaloacetate: only then does the site where acetyl-CoA fits appear. That way it avoids wasting acetyl-CoA when there is nothing to combine it with.

Nelson, D. L. & Cox, M. M. (2021). Lehninger. Principios de bioquímica, 8.ª ed., ch. 16

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