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 130

3 Redox and Coordination Properties of the Nickel Center
in Methyl-Coenzyme M Reductase

The F 430 cofactor in MCR can assume the Ni(I), Ni(II), and Ni(III) oxidation states
and square planar four-, square pyramidal five-, and octahedral six-coordinate states.
Both the free and bound cofactors have been studied. These states have been
identified and characterized by UV-visible, X-ray absorption, resonance Raman,
MCD, and EPR and related methods, including electron spin echo envelope modu-
lation (ESEEM), electron nuclear double resonance (ENDOR) and HYSCORE.

3.1 Coordination and Oxidation States of the Free F 430
Cofactor and Its Pentamethyl Ester Derivative

One protocol for obtaining the free F 430 cofactor is to denature MCR with
perchloric acid or trichloroacetic acid [ 40 ]. F 430 is then released with Ni in the 2+
oxidation state. The isolated cofactor is a pentaacid, which is often derivatized to
the pentamethyl ester, F 430M , for analysis because of its higher stability, easier
purification, and solubility in non-coordinating organic solvents [ 60 ].
Ni(II)-F 430M has an absorption maximum at 420 nm. The Ni(II)-F 430M state is
EPR-silent with either spin states S

1 depending on the presence and
types of axial ligands. Using X-ray absorption (EXAFS) studies, Hamilton and
coworkers showed that both enzyme-bound and isolated F 430 in aqueous solution
contain 6-coordinate pseudo-octahedral Ni(II) at low temperature (

¼ 0orS ¼

250K) that
undergoes conversion at higher temperatures to a 4-coordinate, square-planar
form [ 61 ]. There was no evidence for a 5-coordinate form of free F 430 [ 40 , 61 ].
Ni(II)-F 430M can be efficiently reduced with sodium amalgam in tetrahydrofuran
to generate Ni(I)-F 430M [ 47 ], which is analogous to the catalytically active form of
F 430 in MCR. F 430 itself can be reduced by Ti(III) citrate in aqueous solution at pH
10.0, indicating that the Ni(II)/(I) couple has a redox potential between

<

600

and 700 mV [ 3 ]. Similar to F 430 , Ni(I)-F 430M has absorption peaks at 383 nm

and 759 nm [ 47 ]. Like Cu(II), Ni(I) is a 3d 9 metal ion; thus Ni(I)-F 430M is
paramagnetic with an S spin state and an EPR spectrum ( g values of 2.065,

2.074 and 2.250) that exhibits hyperfine splitting from coupling to the tetrapyrrole
nitrogen atoms ( 14 N nuclear spin, I, equals 1). Of significance to the MCR mech-
anism, Ni(I)-F 430M can react with methyl iodide, methyl tosylate and methyl
sulfonium salts, in which the methyl group is activated, to generate methane
formation via a methyl-Ni intermediate [ 40 , 62 - 64 ]. Similar reactions have been
performed with the enzyme, as described below.
The Ni(III) state of F 430M can also be generated electrochemically to give an
EPR spectrum with g values of 2.020 and 2.211, characteristic of a tetragonally-
distorted S

¼ ½

¼ ½ system with a nickel d z 2 ground state [ 40 ].