Mechanistic Studies of a Nonheme Iron Enzyme OvoA in Ovothiol Biosynthesis Using a Tyrosine Analogue, 2-Amino-3-(4-hydroxy-3-(methoxyl) phenyl) Propanoic Acid (MeOTyr)

Ovothiols are thiol-histidines that play important roles in protecting cells against oxidative stresses. Because of challenges faced in their chemical synthesis, biosynthesis provides an alternative option. In ovothiol biosynthesis, a nonheme iron enzyme (OvoA) catalyzes a four-electron oxidative coupling between L-His and L-Cys. There are debates in the literature over whether oxidative C-S bond formation or sulfur oxidation is the first half of OvoA-catalysis. In this report, by incorporating a tyrosine analogue, 2-amino-3-(4-hydroxy-3-(methoxyl) phenyl) propanoic acid (MeOTyr), via an amber-suppressor method, we modulated the rate-limiting steps of OvoA-catalysis and observed an inverse deuterium KIE for [U-H-2(5)]-His. In conjunction with the reported quantum mechanics/molecular mechanics (QM/MM) studies, our results suggest that Y417 plays redox roles in OvoA-catalysis and imply that oxidative C-S bond formation is most likely the first half of the OvoA-catalysis.