Crystal structure of a key enzyme for anaerobic ethane activation

When released from ocean floor seeps, small hydrocarbons are rapidly consumed by micro-organisms. Methane is highly abundant and is both produced and consumed by microbes through well understood biochemical pathways. Less well understood is how ethane, also a major natural component of gaseous hydrocarbons, is metabolized. To understand how microbes take advantage of this energy and carbon source, Hahn et al. solved the x-ray crystal structures of an enzyme they call ethyl coenzyme-M reductase, which converts ethane into the thioether ethyl-coenzyme M as the entry point for catabolism. They found an expanded active site and, using a xenon gas derivatization experiment, a distinctive tunnel through the protein that is proposed to permit access of the gaseous substrate. Science , abg1765, this issue p. 118 Crystal structures reveal key features in a metalloenzyme from ocean floor archaea that activates ethane. Ethane, the second most abundant hydrocarbon gas in the seafloor, is efficiently oxidized by anaerobic archaea in syntrophy with sulfate-reducing bacteria. Here, we report the 0.99-angstrom-resolution structure of the proposed ethane-activating enzyme and describe the specific traits that distinguish it from methane-generating and -consuming methyl-coenzyme M reductases. The widened catalytic chamber, harboring a dimethylated nickel-containing F 430 cofactor, would adapt the chemistry of methyl-coenzyme M reductases for a two-carbon substrate. A sulfur from methionine replaces the oxygen from a canonical glutamine as the nickel lower-axial ligand, a feature conserved in thermophilic ethanotrophs. Specific loop extensions, a four-helix bundle dilatation, and posttranslational methylations result in the formation of a 33-angstrom-long hydrophobic tunnel, which guides the ethane to the buried active site as confirmed with xenon pressurization experiments.