Biohydrogenation of Unsaturated Fatty Acids

Three parameters involved in the binding of substrate to linoleate isomerase of Butyrivibrio fibrisolvens have been identified from studies of inhibition by fatty acid isomers and analogues. They are (a) the π system of the substrate double bond, (b) hydrophobic interaction, and (c) hydrogen bonding of the substrate carboxyl group. Almost every unsaturated fatty acid tested inhibited the enzyme, regardless of configuration or double bond position between carbons 3 and 12. Superimposed on this contribution of unsaturation was the correlation of inhibition with chain length and with the presence of an active hydrogen on a C-1 substituent. Inhibition by unsaturated fatty acids or their derivatives in all cases examined was competitive. More restrictive requirements for isomerization were revealed by incubation of the enzyme with positional and configurational isomers and carboxyl derivatives of the substrate. An absolute requirement for a cis-9, cis-12-diene system and a free C-1 carboxyl group was shown. Two chelators, o-phenanthroline and EDTA, were identified as reversible inhibitors of the enzyme. Inhibition by o-phenanthroline was noncompetitive. The isomerase was also inhibited by the sulfhydryl reagents, p-hydroxymercuribenzoate, iodoacetamide, and N-ethylmaleimide.