N-linoleoylamino acids as chiral probes of substrate binding by soybean lipoxygenase-1

Lipoxygenases catalyze the oxygenation of polyunsaturated fatty acids and their derivatives to produce conjugated diene hydroperoxides. Soybean lipoxygenase-1 (SBLO-1) has been the subject of intensive structural and mechanistic study, but the manner in which this enzyme binds substrates is uncertain. Previous studies suggest that the fatty acyl group of the substrate binds in an internal cavity near the catalytic iron with the polar end at the surface of the protein or perhaps external to the protein. To test this model, we have investigated two pairs of enantiomeric N-linoleoylamino acids as substrates for SBLO-1. If the amino acid moiety binds external to the protein, the kinetics and product distribution should show little or no sensitivity to the stereochemical configuration of the amino acid moiety. Consistent with this expectation, N-linoleoyl-l-valine (LLV) and N-linoleoyl-d-valine (LDV) are both good substrates with k_cat/K_m values that are equal within error and about 40% higher than k_cat/K_m for linoleic acid. Experiments with N-linoleoyl-l-tryptophan (LLT) and N-linoleoyl-d-tryptophan (LDT) were complicated by the low critical micelle concentrations (CMC = 6-8 μM) of these substances. Below the CMC, LDT is a better substrate by a factor of 2.7. The rates of oxygenation of LDT and LLT continue to rise above the CMC, with modest stereoselectivity in favor of the d enantiomer. With all of the substrates tested, the major product is the 13(S)-hydroperoxide, and the distribution of minor products is not appreciably affected by the configuration of the amino acid moiety. The absence of stereoselectivity with LLV and LDV, the modest magnitude of the stereoselectivity with LLT and LDT, and the ability micellar forms of LLT and LDT to increase the concentration of available substrate are all consistent with the hypothesis that the amino acid moiety binds largely external to SBLO-1 and interacts with it only weakly.