Biochemistry of Methyl-Coenzyme M Reductase: The Nickel Metalloenzyme that Catalyzes the Final Step in Synthesis and the First Step in Anaerobic Oxidation of the Greenhouse Gas Methane - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 132

Figure 3 Comparison of
the Ni(II), Ni(I), and
methyl-Ni(III) states
of methyl-coenzyme M
reductase, based on X-ray
absorption spectroscopy.
Ni-ligand bond lengths are
shown for the axial and
equatorial (tetrapyrrole)
ligands. See the text for
details. Reprinted by
permission from [ 74 ];
copyright 2009 American
Chemical Society.

The inactive Ni(II) state (called MCR silent ) is the form that has been best
characterized crystallographically [ 52 ]. This state, carrying a 3d 8 metal ion, is
EPR-silent. When cells are harvested directly from the growth medium (i.e.,
without activation by H 2 or CO), this is the predominant state of the enzyme.
This also is the form that accumulates as the Ni(I) states undergo oxidation; in
fact, because the Ni(II)/(I) redox couple has such a low potential, the Ni(I) forms
decay to Ni(II) even inside anaerobic chambers maintained at

1 ppm O 2 . In the
MCR silent state, the Ni center is six-coordinate, with four nitrogen ligands from
the F 430 cofactor, one oxygen ligand from a glutamine residue from opposite alpha
subunit (Gln ʱ ' 147) and another axial ligand, e.g., the thiolate sulfur of CoMS or
one of the sulfonate oxygens of CoBS-SCoM [ 48 ].

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