ω-Oxidation impairs oxidizability of polyenoic fatty acids by 15-lipoxygenases: consequences for substrate orientation at the active site

During oxygenation by 15-lipoxygenases, polyenoic fatty acids are bound at the active site in such a way that the ω-terminus of the fatty acids penetrates into the substrate binding pocket. In contrast, for arachidonic acid 5-lipoxygenation, an inverse head to tail orientation has been suggested. However, an inverse orientation may be hindered by the large energy barrier associated with burying the charged carboxylate group in the hydrophobic environment of the substrate binding cleft. We studied the oxygenation kinetics of ω-modified fatty acids by 15-lipoxygenases and found that ω-hydroxylation strongly impaired substrate affinity (higher K_m), but only moderately altered V_max. In contrast, ω-carboxylation completely prevented the lipoxygenase reaction; however, methylation of the additional carboxylate group restored the activity. Arg⁴⁰³ of the human 15-lipoxygenase has been implicated in fatty acid binding by forming a salt bridge with the carboxylate group, and thus mutation of this amino acid to an uncharged residue was supposed to favour an inverse substrate orientation. The prepared Arg⁴⁰³ → Leu mutant of the rabbit 15-lipoxygenase was found to be a less effective catalyst of linoleic acid oxygenation. However, the oxygenation rate of ω-hydroxyarachidonic acid was similar when the wild-type and mutant enzyme were compared, and the patterns of oxygenation products were identical for both enzyme species. These data suggest that introduction of a polar, or even charged residue, at the ω-terminus of substrate fatty acids in connection with mutation of Arg⁴⁰³ may not alter substrate alignment at the active site of 15-lipoxygenases.