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Write down the steps of the TCA (Krebs) Cycle.

GeneralClass 12AllAnswered 27 Mar 2026
Answer

The Krebs cycle proceeds through nine enzymatic steps in the mitochondrial matrix:

Step

Reactant → Product

Enzyme

Cofactor/Output

1

Acetyl CoA + OAA + H₂O → Citric acid + CoA

Citrate synthase

—

2

Citric acid → Isocitrate

Aconitase

Fe²⁺

3

Isocitrate → Oxalosuccinic acid

Isocitrate dehydrogenase

NADH + H⁺

4

Oxalosuccinic acid → α-Ketoglutaric acid

Decarboxylase

CO₂

5

α-Ketoglutaric acid → Succinyl CoA

α-Ketoglutaric dehydrogenase

NADH + H⁺, CO₂

6

Succinyl CoA → Succinic acid

Succinate thiokinase

GTP (→ ATP)

7

Succinic acid → Fumaric acid

Succinate dehydrogenase

FADH₂

8

Fumaric acid → Malic acid

Fumarase

H₂O

9

Malic acid → Oxaloacetic acid

Malate dehydrogenase

NADH + H⁺

Net output per cycle (per Acetyl CoA):

  • 3 NADH + H⁺
  • 1 FADH₂
  • 1 GTP (equivalent to ATP)
  • 2 CO₂

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