8 - Aerobic Respiration—Hydrocarbon Metabolism

Abstract

This chapter describes hydrocarbon metabolism of aerobic respiration. During hydrocarbon metabolism, fatty acids are formed and further metabolized. In general, fatty acids may undergo alpha, beta, or omega oxidations. The alpha oxidation of long-chain fatty acids occurs at the second position of the chain. The necessary enzyme system catalyzes the 2-hydroxylation of the particular acid. The result of this oxidative reaction, a decarboxylation of the α-hydroxy fatty acid, usually occurs, giving an odd-numbered fatty acid. However, the actual mechanism of this reaction is unknown. The beta-oxidation mechanism of fatty acid results in the continual removal of acetate (C2) units. This removal follows a sequence of reactions. The initial step in beta oxidation is the activation of a fatty acid by its transformation into the corresponding CoA-thioester, which is catalyzed by the appropriate acyl-CoA synthetizes. The most commonly known reaction is that of acetate to acetyl-CoA, which requires the enzyme acetyl-CoA synthetize. Apart from the alpha and beta oxidations, a third pathway involves conversion of the acid to the α-hydroxy compound, which may then be oxidized further to dicarboxylic acids. This system requires the participation of the electron transport system, possibly with cytochrome P-450 or thioredoxin and NADPH-cytochrome c reductase components. These oxidations predominantly occur as mixed function oxidase systems during hydrocarbon metabolism.