Although most components of the diet contain fatty acids with unbranched chains,

Question

Although most components of the diet contain fatty acids with unbranched chains, some dietary plants have fatty acids with methyl groups at odd numbered carbons in the fatty acyl chain. Such branched molecules cannot be metabolized by the ß-oxidation pathway.

a) Which step in ß-oxidation is blocked by the presence of the methyl group?
(Explain your reasoning)

b) Some tissues can carry out the a-oxidation of fatty acids on a limited basis. This
pathway involves the use of molecular oxygen and NAD+ to oxidatively decarboxylate a Cn fatty acid to a Cn–1 aldehyde, which is in turn oxidized by NAD+ to a carboxylic acid containing one fewer carbon than the original substrate. Utilizing palmitate a (16:0 fatty acid) with a methyl branch at C3 as a model compound, show how a round of a-oxidation can bypass the block imposed by the presence of the methyl group on the ß-oxidation pathway.

Explanation / Answer

A) When propionyl-CoA is formed, the beta oxidation can't continue, because the compound is not a suitable substrate for the TCA cycle or similar pathways. Normally it produces two acetyl-CoA instead of one of each which is why it can't continue. B) Attach CoA, oxidize by phytanoyl-CoA dioxygenase, cleave by 2-hydroxyphytanoyl-CoA lyase, oxidize by oxidized by aldehyde dehydrogenase, return to beta oxidation. Alternatively oxidation by molecular oxygen can remove the methyl group.

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