Question

Arrange the compounds in the order they are formed during Kreb's cycle.

• A. Fumarate
• B. Malate
• C. Succinyl-CoA
• D. Citrate
• E. Succinate

Answer 1

Every pyruvate molecule produced by glycolysis enters the Krebs cycle as Acetyl CoA. Acetyl CoA combines with the 4C-oxaloacetate to form a 6C- compound that is citric acid. Citric acid is then oxidized breaking one of the C-C bonds to form carbon dioxide and reduce NAD to NADH. Therefore citric acid loses a carbon to form the only 5C - compound in Kreb's cycle, $\alpha$-ketoglutaric acid. In this process, the intermediate compounds - cis-aconitate, isocitrate and oxalosuccinate - are formed The $\alpha$-ketoglutaric acid is similarly oxidized to form a 4C-compound, succinic acid. Succinic acid is oxidized to malate and then fumarate and $FA{D}^{+}isreducedtoFAD{H}_2$. In this reaction oxaloacetic acid is the only intermediate that is regenerated in Krebs cycle.