Biochemistry - The Chemical Reactions of Living Cells Volume 2 - D. Metzler 1980

Organization of Metabolism: Catabolic Pathways
Fatty Acid Oxidation
Alpha- and Omega-Oxidation

In animal Tissues, the degradation of straight-chain Fatty acids such as palmitic, stearic, and oleic acids occurs almost exclusively via ß-Oxidation [5]. In plant Cells, however, Fatty acid oxidation frequently proceeds through the sequential removal of single-carbon fragments. Although the precise details of this process remain to be fully elucidated, it is well established that the initial step typically involves hydroxylation of the a-carbon atom [equation (9-4)], yielding either a D- or L-2-hydroxy acid [6–8]. The L-hydroxy acids are subsequently oxidized at a rapid rate—presumably via dehydrogenation to form an a-keto acid [stage b in equation (9-4)]—followed by oxidative decarboxylation [potentially involving H₂O₂; equation (8-67)]. D-hydroxy acids tend to accumulate and are commonly found in green leaves; nevertheless, they are also subject to oxidation while retaining the a-hydrogen, as denoted by the asterisk in equation (9-4) (stage d). This points to the existence of an alternative dehydrogenation pathway coupled with simultaneous decarboxylation [compare with equation (7-75)].

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a-Oxidation also takes place to some extent in animal tissues, particularly within the Brain [9]. When ß-oxidation is hindered by the presence of side-chain methyl groups, a-oxidation may serve as a bypass mechanism (see Box 9-B for a Structure/133.html">Discussion of Refsum disease). In other instances, hydroxylation occurs at the opposite end of the chain (ω-oxidation), yielding a dicarboxylic acid. For example, 3,6-dimethyloctanoic acid is metabolized in The Human Body predominantly via ω-oxidation.