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 134

Ni(III)-propyl sulfonate in resonance with a Ni(II)-propyl sulfonate radical [ 85 , 86 ].
MCR PS can be converted back to the MCR red1 state by reaction with the low
potential reductant, Ti(III) citrate, or with various thiolates, including analogs of
CoMSH and CoBSH with methylene group(s) longer than the native substrate
(CoB 8 SH or CoB 9 SH), but not with CoBSH itself [ 84 , 87 ].
4 The Catalytic Mechanism of Methyl-Coenzyme M
Reductase
The mechanism of the MCR-catalyzed conversion of methyl-SCoM to methane,
reaction (1), requires CoBSH as the two-electron [ 88 ] and possibly also as
the proton donor [ 9 ]. Two leading mechanisms have been proposed (Figure 4 );
however, because none of the intermediates shown in this figure have been isolated,
one cannot yet conclusively state which mechanism is correct. Various experimen-
tal measurements set ground-rules for the correct mechanism. The first mechanistic
constraint is that initiation of the MCR reaction requires the F 430 cofactor to be in
the Ni(I) oxidation state [ 71 , 72 ]. Rule 2 is that net inversion of stereoconfiguration
occurs during the reduction of methyl-CoM [ 89 ]. The third mechanistic constraint
is that catalysis involves a ternary complex mechanism and has an absolute
requirement for CoBSH for even a single catalytic turnover [ 55 ]. Fourth, the carbon
kinetic isotope effect from 12 CH 3 -SCoM versus 13 CH 3 -SCoM is 1.04, indicating
that the rate-limiting step is breakage of the C-S bond of CH 3 -SCoM [ 90 ].
The secondary kinetic isotope effect is 1.19 in the methyl group of CD 3 -SCoM,
indicating that the methyl group goes from tetrahedral to trigonal planar upon
reaching the transition state of the rate-limiting step [ 91 ]. The two guiding mech-
anisms described below follow those four rules. Now, what is needed is to trap the
intermediates in the mechanism.
The two major proposals for the MCR catalytic mechanism differ in the nature of
the first intermediate. Mechanism 1 involves a methyl-Ni(III) intermediate [ 48 ],
while mechanism 2 proposes a methyl radical and a CoMS-Ni(II) complex [ 92 ].
Additionally, it is hypothesized that substrate binding initiates a conformational
change that triggers the catalytic cycle. This concept is supported by 19 F-ENDOR
studies [ 56 ] and transient kinetic studies [ 93 ], and by the observation that inacti-
vation of MCR red1 by bromoethanesulfonate (BES) requires CoBSH [ 94 ].
In mechanism 1, MCR red1 Ni(I) attacks the methyl group of methyl-SCoM in an
S N 2 nucleophilic substitution reaction to form a methyl-Ni(III) intermediate and
CoMS (Figure 4 ). Concurrently, a proton is transferred from CoBSH to the
CoMS leaving group. Methyl-Ni(III) accepts one electron from CoMSH to form a
radical cation (CoMS • H + ) and methyl-Ni(II). Proton transfer from CoMS • H + to the
methyl-Ni(II) species is proposed to follow, thus generating methane and Ni(II).
The CoBS thiolate then attacks the CoMS • H + thiyl radical cation to give a disulfide
radical anion (CoBS • SCoM ) with a two-center three-electron bond, which subse-
quently reduces Ni(II) back to Ni(I), leaving the neutral heterodisulfide CoBS-SCoM
as the final product.