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Question

Identify the correct sequence of events in Kreb’s cycle.

A
Acetyl CoA → Citrate → Pyruvate → α-ketoglutarate→ Succinate → Malate → Fumarate → OAA
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B
Acetyl CoA → Citric acid → α-ketoglutaric acid→ Succinic acid → Fumaric acid → Malic acid →OAA
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C
Acetyl CoA → Citric acid → Malic acid →α-ketoglutaric acid → Succinic acid → OAA
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D
None of the above
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Solution

The correct option is B Acetyl CoA → Citric acid → α-ketoglutaric acid→ Succinic acid → Fumaric acid → Malic acid →OAA
Steps of Kreb cycle are:

Step 1: The first step is the condensation of acetyl CoA with 4-carbon compound oxaloacetate to form 6C citrate, coenzyme A is released. The reaction is catalysed by citrate synthase.

Step 2: Citrate is converted to its isomer, isocitrate. The enzyme aconitase catalyses this reaction.

Step 3: Isocitrate undergoes dehydrogenation and decarboxylation to form 5C 𝝰-ketoglutarate. A molecular form of CO2 is released. Isocitrate dehydrogenase catalyses the reaction. It is an NAD+ dependent enzyme. NAD+ is converted to NADH.

Step 4: 𝝰-ketoglutarate undergoes oxidative decarboxylation to form succinyl CoA, a 4C compound. The reaction is catalyzed by the 𝝰-ketoglutarate dehydrogenase enzyme complex. One molecule of CO2 is released and NAD+ is converted to NADH.

Step 5: Succinyl CoA forms succinate. The enzyme succinyl CoA synthetase catalyses the reaction. This is coupled with substrate-level phosphorylation of GDP to get GTP. GTP transfers its phosphate to ADP forming ATP.

Step 6: Succinate is oxidised by the enzyme succinate dehydrogenase to fumarate. In the process, FAD is converted to FADH2.

Step 7: Fumarate gets converted to malate by the addition of one H2O. The enzyme catalysing this reaction is fumarase.

Step 8: Malate is dehydrogenated to form oxaloacetate, which combines with another molecule of acetyl CoA and starts the new cycle. Hydrogens removed, get transferred to NAD+ forming NADH. Malate dehydrogenase catalyses the reaction.


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