Chapter

Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases

Volume 15 of the series Metal Ions in Life Sciences pp 205-256

Date:

Methane Monooxygenase: Functionalizing Methane at Iron and Copper

  • Matthew H. SazinskyAffiliated withDepartment of Chemistry, Pomona College
  • , Stephen J. LippardAffiliated withDepartment of Chemistry, Massachusetts Institute of Technology Email author 

Abstract

Methane monooxygenases (MMOs) catalyze the conversion of methane to methanol as the first committed step in the assimilation of this hydrocarbon into biomass and energy by methanotrophs, thus playing a significant role in the biogeochemistry of this potent greenhouse gas. Two distinct enzymes, a copper-dependent membrane protein, particulate methane monooxygenase (pMMO), and an iron-dependent cytosolic protein, soluble methane monooxygenase (sMMO), carry out this transformation using large protein scaffolds that help to facilitate the timely transport of hydrocarbon, O2, proton, and electron substrates to buried dimetallic active sites. For both enzymes, reaction of the reduced metal centers with O2 leads to intermediates that activate the relatively inert C–H bonds of hydrocarbons to yield oxidized products. Among synthetic and biological catalysts, MMOs are unique because they are the only ones known to hydroxylate methane at ambient temperatures. As a need for new industrial catalysts and green chemical transformations increases, understanding how the different MMO metal centers efficiently accomplish this challenging chemistry has become the focus of intense study. This chapter examines current understanding of the sMMO and pMMO protein structures, their methods for substrate channeling, and mechanisms for the dimetallic activation of O2 and C–H bonds.

Keywords

bacterial multicomponent monooxygenase dicopper diiron mechanism particulate methane monooxygenase protein complexes soluble methane monooxygenase