A fatty acid undergoes complete β-oxidation to yield nine molecules of acetyl-CoA. No isomerization reactions occur. The structure of the original fatty acid was:

Question

A fatty acid undergoes complete β-oxidation to yield nine molecules of acetyl-CoA.  No isomerization reactions occur.  The structure of the original fatty acid was:

Quizplus · Accepted Answer

11ecba2d_0e77_0b54_a3bf_cd806aee5e9b_MD0008_00 Fatty acids are long hydrocarbon chains with a carboxylic acid group on one end. They are degraded through a process called fatty acid oxidation, or β-oxidation, which proceeds differently depending on the composition of the fatty acid being oxidized. All fatty acids are broken down into multiple two-carbon units called acetyl-CoA. Fatty acids with an even number of carbons yield half as many acetyl-CoA units as the total number of carbons in the original molecule. If the fatty acid has an odd number of carbons, the final three carbons are converted into a compound called propionyl-CoA. In other words, for odd-chain fatty acids, the number of acetyl-CoA units produced equals (total carbons − 3)/2. Unsaturated fatty acids have at least one C=C double bond, typically in the cis-conformation. For β-oxidation to proceed, the cis-bonds must be isomerized to trans-bonds, which are introduced as part of the oxidation mechanism. Saturated fatty acids do not have C=C double bonds initially and do not need to undergo isomerization during β-oxidation. The question states that the β-oxidation resulted in 9 acetyl-CoA units, so the original fatty acid must have had twice that number of carbons (18). The fact that no isomerization reactions were needed indicates that the fatty acid was saturated. Only Choice B shows a saturated fatty acid with 18 carbons. (Choice A) This option shows a saturated fatty acid with 19 carbons. Because it has an odd number of carbons, it will produce 8 acetyl-CoA units (from 16 carbons) and have 1 three-carbon unit (called propionyl-CoA) remaining. (Choices C and D) These options show unsaturated fatty acids, which require isomerization of the cis-double bond. Educational objective: Fatty acids can be oxidized to form acetyl-CoA, and those with an even number of carbons produce half as many acetyl-CoA molecules as they have carbons. Fatty acids with an odd number of carbons produce propionyl-CoA in addition to acetyl-CoA. Unsaturated fatty acids require isomerization reactions to convert cis-bonds to trans-bonds whereas saturated fatty acids do not.

A Fatty Acid Undergoes Complete β-Oxidation to Yield Nine Molecules