Ligand bound structure of the alkanesulfonate monooxygenase MsuD provides insight into catalysis

Abstract

The production of sulfite is essential for an abundance of organisms, including many bacteria. During sulfate starvation of certain bacteria, the assimilation and conversion of sulfur from organosulfur compounds is done through usage of two-component flavin-dependent monooxygenases to cleave stable C–S bonds. The monooxygenase MsuD converts methanesulfonate to sulfite, however its mechanism is as yet unresolved. In order to elucidate the alkanesulfonate monooxygenase mechanism, the crystal structure of MsuD from Pseudomonas fluorescens was obtained with flavin and methanesulfonate bound. As a result, the first structure of an alkanesulfonate monooxygenase with bound flavin and alkanesulfonate reveal the structural changes necessary for ligand binding to position flavin and methanesulfonate for catalysis. An intricate network of interactions is observed between the protein, flavin, and methanesulfonate binding sites. Furthermore, the ternary MsuD structure has enabled us to revisit and update the C–S cleavage mechanism of two-component flavin-dependent alkanesulfonate monooxygenases, and biochemical assays spurred by these discoveries have allowed us to redefine MsuD as a small to medium chain alkanesulfonate monooxygenase.