The oxidation of methane to methanol under environmentally benign conditions without over-oxidation is one of the grand challenges in chemistry. The soluble methane monooxygenase (sMMO) enzyme utilizes a dinuclear iron cluster to catalyze such a transformation in methanotroph organisms. An investigation of sMMO catalysis has provided one of the most detailed pictures of oxygen activation in metalloenzymes, with the characterization of 6 catalytic states of the diiron cluster. Chief among them is the methane-reactive diiron(IV) species termed compound Q. Apart from furnishing nature’s most powerful oxidant in Q, sMMO also displays remarkable specificity for methane, which is the smallest hydrocarbon and also possesses the strongest aliphatic C-H bond. This specificity is engendered by the formation of a protein complex between the hydroxylase protein (MMOH) containing the diiron cluster in the active site and a regulatory protein MMOB. In order to understand this process, crystal structures of the MMOH:MMOB complex from M. trichosporium OB3b have been solved in both oxidized and reduced states. Serial femtosecond crystallography (SFX) at X-ray free electron laser (XFEL) facilities has been utilized in order to avoid aberrant synchrotron radiation-mediated reduction of the iron atoms. These high-resolution (1.95 Å) crystal structures provide a detailed view of the reduced state of sMMO that is primed to bind and activate O2. They also delineate the manner by which MMOB reorganizes the diiron cluster and second-sphere residues within the active site of MMOH. These structural modifications shed light on how MMOB enforces specificity for methane by enabling the hydrogen atom abstraction reaction to occur by quantum tunneling. They even provide some clues to how MMOB enhances the reactivity of compound Q towards methane. Finally, these crystal structures show promise towards achieving the goal of obtaining a crystal structure of compound Q and the other intermediate states of the diiron cluster.

Reorganization of the diiron cluster active site of soluble methane monooxygenase by MMOB enforces specificity for methane as described by XFEL crystal structures of the sMMOH:MMOB complex

ACS Fall 2020 Virtual Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.