S-2-Octenoyl coenzyme A is an intermediate metabolite of fatty acid metabolism. Mitochondrial beta-oxidation of saturated acyl-CoA esters proceeds by a repeated cycle of four concerted reactions: flavoprotein-linked dehydrogenation, hydration, NAD-linked dehydrogenation and thiolysis. The three chain-length-specific acyl-CoA dehydrogenases which catalyse the first dehydrogenation step are linked to the respiratory chain by the electron-transferring flavoprotein (ETF) and ETF: ubiquinone oxidoreductase (ETF: QO). The second dehydrogenation step is catalysed by two chain-length-specific NAD+-dependent 3-hydroxyacyl-CoA dehydrogenases. The control of beta-oxidation in the mitochondrial matrix occurs at several steps and depends on the redox state and the rate of recycling of CoA. The rate is lowered with reduced states, since high NAD+/NADH ratios impair the activity of the hydroxyacyl-CoA dehydrogenase and increase the formation of ETF semiquinone (ETFSq), which is a potent inhibitor of the acyl-CoA dehydrogenases. These changes affect the steady-state concentrations of acyl-CoA intermediates, which in turn may change the control strength of other enzymes of the pathway. In liver mitochondria, acetyl-CoA produced by each cycle of beta-oxidation has four major routes of disposal: ketogenesis, oxidation by the citrate cycle, conversion into acetylcarnitine or hydrolysis to acetate; each of these reactions generates free CoA. During maximum flux through beta-oxidation, up to 95 % of the mitochondrial CoA pool is acylated, and thus the rate of recycling of CoA may partly control beta-oxidation. Increased steady-state concentrations of some acyl-CoA esters may also occur when one or more of the enzymes of beta-oxidation is inhibited, as in hypoglycin poisoning, or where one or more of the enzymes of the pathway is absent. Such inborn errors of beta-oxidation are being increasingly recognized as important causes of disease, especially in children, and deficiencies of long-chain-acyl-CoA dehydrogenase, medium-chain-acyl-CoA dehydrogenase, short-chain-acyl-CoA dehydrogenase, ETF, ETF: QO and acetoacetyl-CoA thiolase have been described. (PMID: 2818568).