Atomic resolution structures of key enzyme MCR in anaerobic methanotrophy reveal novel and extensive post-translational modifications.
Anaerobic methanotrophic archaea (ANME) are crucial to planetary carbon cycling. They oxidise methane in anoxic niches by transferring electrons directly to nitrate or metal oxides and alternatively to sulfate-reducing bacteria. Due to their physiological complexity, no ANME species have been isolated, hampering the biochemical investigation of the enzymatic processes involved in anaerobic methane oxidation. To study the methane-capturing enzyme of these microorganisms, we circumvented the isolation barrier by exploiting microbial enrichments of freshwater nitrate-reducing ANME-2d grown in bioreactors, and marine ANME-2c in syntrophy with bacterial partners. The crystal structures of their Methyl-Coenzyme M Reductases (MCRs), refined to true atomic resolution, provided the most precise image of the enzyme to date. Despite their physiological differences, these ANMEs have extremely conserved MCR structures, similar to homologs from methanogenic Methanosarcinales, rather than the phylogenetically distant MCR of ANME-1 isolated from Black Sea mats. The three studied MCRs are highly modified, with seven post-translational modifications. Among them was a novel 3(S)-methylhistidine on the {gamma}-chain of both ANME-2d MCRs. Labelling with gaseous krypton did not reveal any internal channels that would facilitate alkane diffusion to the active site as observed in the ethane-specialized enzyme. Based on our data, the methanotrophic MCRs should follow the same radical reaction mechanism proposed for the methane-generating homologues. The described pattern of post-translational modifications underscores the importance of native purification as a powerful approach to discovering intrinsic enzymatic features in uncultivated microorganisms existing in nature.