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<jats:title>ABSTRACT</jats:title> <jats:p> The phylogenetically diverse family of methanogenic archaea universally use methyl coenzyme M reductase (MCR) for catalyzing the final methane-forming reaction step of the methanogenic energy metabolism. Some methanogens of the orders <jats:named-content content-type="genus-species">Methanobacteriales</jats:named-content> and <jats:named-content content-type="genus-species">Methanococcales</jats:named-content> contain two isoenzymes. Comprehensive phylogenetic analyses on the basis of all three subunits grouped MCRs from <jats:named-content content-type="genus-species">Methanobacteriales</jats:named-content> and <jats:named-content content-type="genus-species">Methanococcales</jats:named-content> into three distinct types: (i) MCRs from <jats:named-content content-type="genus-species">Methanobacteriales</jats:named-content> , (ii) MCRs from <jats:named-content content-type="genus-species">Methanobacteriales</jats:named-content> and <jats:named-content content-type="genus-species">Methanococcales</jats:named-content> , and (iii) MCRs from <jats:named-content content-type="genus-species">Methanococcales</jats:named-content> . The first and second types contain MCR isoenzymes I and II from <jats:named-content content-type="genus-species">Methanothermobacter marburgensis</jats:named-content> , respectively; therefore, they were designated MCR type I and type II and accordingly; the third one was designated MCR type III. For comparison with the known MCR type I and type II structures, we determined the structure of MCR type III from <jats:named-content content-type="genus-species">Methanotorris formicicus</jats:named-content> and <jats:named-content content-type="genus-species">Methanothermococcus thermolithotrophicus</jats:named-content> . As predicted, the three MCR types revealed highly similar overall structures and virtually identical active site architectures reflecting the chemically challenging mechanism of methane formation. Pronounced differences were found at the protein surface with respect to loop geometries and electrostatic properties, which also involve the entrance of the active-site funnel. In addition, the C-terminal end of the γ-subunit is prolonged by an extra helix after helix γ8 in MCR type II and type III, which is, however, differently arranged in the two MCR types. MCR types I, II, and III share most of the posttranslational modifications which appear to fine-tune the enzymatic catalysis. Interestingly, MCR type III lacks the methyl-cysteine but possesses in subunit α of <jats:named-content content-type="genus-species">M. formicicus</jats:named-content> a 6-hydroxy-tryptophan, which thus far has been found only in the α-amanitin toxin peptide but not in proteins. </jats:p> <jats:p> <jats:bold>IMPORTANCE</jats:bold> Methyl coenzyme M reductase (MCR) represents a prime target for the mitigation of methane releases. Phylogenetic analyses of MCRs suggested several distinct sequence clusters; those from <jats:named-content content-type="genus-species">Methanobacteriales</jats:named-content> and <jats:named-content content-type="genus-species">Methanococcales</jats:named-content> were subdivided into three types: MCR type I from <jats:named-content content-type="genus-species">Methanobacteriales</jats:named-content> , MCR type II from <jats:named-content content-type="genus-species">Methanobacteriales</jats:named-content> and <jats:named-content content-type="genus-species">Methanococcales</jats:named-content> , and the newly designated MCR type III exclusively from <jats:named-content content-type="genus-species">Methanococcales</jats:named-content> . We determined the first X-ray structures for an MCR type III. Detailed analyses revealed substantial differences between the three types only in the peripheral region. The subtle modifications identified and electrostatic profiles suggested enhanced substrate binding for MCR type III. In addition, MCR type III from <jats:named-content content-type="genus-species">Methanotorris formicicus</jats:named-content> contains 6-hydroxy-tryptophan, a new posttranslational modification that thus far has been found only in the α-amanitin toxin. </jats:p>
- 出版日期2017-8
