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Answer: X is propionyl-CoA, CH3CH2C0CoA
The structure and reaction pathway are shown in the attachments.
Explanation: In the oxidation of odd-number fatty acids, the substrate for the last pass through beta-oxidation is fatty acyl-CoA with a five-carbon fatty acid. This is oxidized and cleaved to acetyl-CoA and propionyl-CoA. Acetyl-CoA enters the citric acid cycle while the propionyl-CoA is converted in three enzymatic steps to succinyl-CoA which can then enter the citric acid cycle.
Step 1: Conversion of propionyl-CoA to D-methylmalonyl-CoA
Propionyl-CoA is carboxylated to D-methylmalonyl-CoA by the enzyme <em>propionyl-CoA carboxylase</em>, which contains the cofactor <em>biotin</em>. A molecule of ATP and Carbon (iv) oxide (in the form of hydrogen carbonate ion) is required also.
Step 2: Epimerization of D-methylmalonyl-CoA
D-methylmalonyl-CoA is epimerized by the enzyme <em>methylmalonyl-Co epimerase</em> to its L stereoisomer, <em>L-methylmalonyl-CoA.</em>
Step 3: Conversion of L-methylmalonyl-CoA to succinyl-CoA
This reaction is catalysed by <em>methylmalonyl-CoA mutase</em> which requires the <em>coenzyme B12.</em>
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