Key NAD+-binding residues in human 15-hydroxyprostaglandin dehydrogenase

NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (15-PGDH), a member of the short-chain dehydrogenase/reductase (SDR) family, catalyzes the first step in the catabolic pathways of prostaglandins and lipoxins, and is believed to be the key enzyme responsible for the biological inactivation of these biologically potent eicosanoids. The enzyme utilizes NAD+ specifically as a coenzyme. Potential amino acid residues involved in binding NAD+ and facilitating enzyme catalysis have been partially identified. In this report, we propose that three more residues in 15-PGDH, Ile-17, Asn-91, and Val-186, are also involved in the interaction with NAD+. Site-directed mutagenesis was used to examine their roles in binding NAD+. Several mutants (I17A, I17V, I17L, I17E, I17K, N91A, N91D, N91K, V186A, V186I, V186D, and V186K) were prepared, expressed as glutathione S-transferase (GST) fusion enzymes in Escherichia coli, and purified by GSH-agarose affinity chromatography. Mutants I17E, I17K, N91L, N91K, and V186D were found to be inactive. Mutants N91A, N91D, V186A, and V186K exhibited comparable activities to the wild type enzyme. However, mutants I17A, I17V, I17L, and V186I had higher activity than the wild type. Especially, the activities of I17L and V186I were increased nearly 4- and 5-fold, respectively. The kcat/Km ratios of all active mutants for PGE2 were similar to that of the wild type enzyme. However, the kcat/Km ratios of mutants I17A and N91A for NAD+ were decreased 5- and 10-fold, respectively, whereas the kcat/Km ratios of mutants I17V, N91D, V186I, and V186K for NAD+ were comparable to that of the wild type enzyme. The kcat/Km ratios of mutants I17L and V186A for NAD+ were increased over nearly 2-fold. These results suggest that Ile-17, Asn-91, and Val-186 are involved in the interaction with NAD+ and contribute to the full catalytic activity of 15-PGDH.