Exposing the Protein-Protein Interaction Sites and Probing the Flavin Transfer Mechanism in the Alkanesulfonate Monooxygenase System from Escherichia coli.

Abstract

Sulfur is an essential element for all living organisms. Bacteria acquire sulfur through a sulfur assimilation pathway, but under sulfur limiting conditions bacteria must acquire sulfur from alternative sources. These alternative sources of sulfur are sulfonates and sulfonate esters, which are naturally occurring compounds. The ssu operon is induced under sulfur limiting conditions and is comprised of genes that encode for a flavin reductase (SsuE), alkanesulfonate monooxygenase (SsuD), and alkanesulfonate transporter proteins (SsuA, SsuB and SsuC). The SsuE enzyme reduces FMN with NADPH and reduced flavin is then transferred to SsuD. The SsuD uses the reduced flavin and dioxygen to catalyze the desulfonation of diverse alkanesulfonates producing the corresponding aldehyde and sulfite. The reduced flavin is a highly unstable molecule and it readily reacts with dioxygen to form hydrogen peroxide and superoxide radicals. The two mechanisms for the transfer of reduced flavin in two-component system are free diffusion and direct channeling. Previous studies have identified stable protein-protein interactions between SsuE and SsuD that are thought to play a role in flavin transfer events. Protein-protein interactions may also influence the oligomeric state of SsuE and SsuD. Evaluating the influence of protein-protein interactions over the dynamic changes in oligomeric states of proteins and its role in transfer of reduced flavin molecule from SsuE to SsuD will help us in understanding the desulfonation mechanism carried out by this system.