Other important fatty-acid catabolic pathways include a-oxidation and w-oxidation. a-Oxidation is required for degradation of the dietary fatty acid phytanic acid (3,7,11,15-tetramethyl-16:0). This fatty acid cannot be degraded by ^-oxidation owing to the methyl group on carbon-3. In the human diet, phytanic acid is obtained from the consumption of ruminant meats, fats, and dairy products. Rumen bacteria hydrolyze chlorophyll, releasing the phytol side chain; phytol is oxidized to phytanic acid and incorporated into triacylglycerol and phospholipids by the animal. Humans typically ingest 50-100 mg of phytanic acid per day. The current view of the a-oxidation pathway, which is found in peroxisomes, is shown in Figure 5. After activation to its CoA derivative, phytanoyl-CoA is hydroxylated on the 2-carbon. The next reaction catalyzes the removal of a one-carbon CoA derivative as formyl-CoA. The other product of this reaction is an aldehyde, prista-nal, that can be oxidized to form pristanic acid (2,6,10,14-tetramethyl-15:0). This chain-shortening reaction effectively shifts the position of the first methyl group from carbon-3 (in phytanic acid) to carbon-2 (in pristanic acid). The 2-methyl-branched chain fatty acids can then be degraded further via peroxisomal ^-oxidation.
Another mechanism for degradation of fatty acids that cannot undergo ^-oxidation is known as !-oxidation. In this process, the terminal methyl group (referred to as the w-end) of a fatty-acid chain is oxidized to a carboxylic acid via cyto-chrome P450 isozymes, particularly the CYP52A family, in the endoplasmic reticulum. The resulting
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