Question

Question:A fatty acid (a long straight-chain carboxylic acid with an even number of carbons) is metabolized to acetyl- CoA, which can then enter the citric acid cycle to be further metabolized (Section 24.9). A fatty acid with an odd number of carbons is metabolized to acetyl-CoA and one equivalent of propionyl-CoA. Propionyl-CoA cannot enter the citric acid cycle. Two coenzyme-requiring enzymes are needed to convert it to succinyl-CoA, a compound that can enter the citric acid cycle. Write the two enzyme-catalyzed reactions and include the names

of the required coenzymes.

Short answer

Propionyl-CoA converts to succinyl-CoA with the help of two enzymes propionyl-CoA carboxylase and methylmalonyl-CoA mutase.

Step-by-step solution

Citric acid cycle

Catabolism of amino acid produces acetyl-CoA molecule. The acetyl group of each molecule of acetyl-CoA that enters the citric acid cycle is converted to two molecules of CO₂.

Conversion of propionyl-CoA to succinyl-CoA

Propionyl-CoA is derived from the catabolism of amino acid is converted to succinyl-CoA using the enzyme propionyl-CoA carboxylase. This enzyme gives D-methylmalonyl-CoA, which is then racemized into L-methylmalonyl-CoA and isomerized into succinyl-CoA. The last product enters the citric acid cycle with the help of oxaloacetate and pyruvate. The two conversions are shown below.

1) Propionyl-CoA to D-methylmalonyl-CoA

2) D-methylmalonyl to succinyl-CoA

Thus, the conversion of Propionyl-CoA to succinyl-CoA explained above.