Tryptophan 500 and Arginine 707 Define Product and Substrate Active Site Binding in Soybean Lipoxygenase-1

There is much debate whether the fatty acid substrate of lipoxygenase binds "carboxylate-end first" or "methyl-end first" in the active site of soybean lipoxygenase-1 (sLO-1). To address this issue, we investigated the sLO-1 mutants Trp500Leu, Trp500Phe, Lys260Leu, and Arg707Leu with steady-state and stopped-flow kinetics. Our data indicate that the substrates (linoleic acid (LA), arachidonic acid (AA)), and the products (13-(S)-hydroperoxy-9,11-(Z,E)-octadecadienoic acid (HPOD) and 15-(S)-hydroperoxyeicosatetraeonic acid (15-(S)-HPETE)) interact with the aromatic residue Trp500 (possibly pi-pi interaction) and with the positively charged amino acid residue Arg707 (charge-charge interaction). Residue Lys260 of soybean lipoxygenase-1 had little effect on either the activation or steady-state kinetics, indicating that both the substrates and products bind "carboxylate-end first" with sLO-1 and not "methyl-end first" as has been proposed for human 15-lipoxygenase.