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 137

tetrahedral (sp 3 ) to trigonal planar (sp 2 ) upon reaching the transition state of the
rate-limiting step (cleavage of the C-S bond of methyl-SCoM) [ 91 ].
In the context of reverse methanogenesis, the anaerobic oxidation of methane
catalyzed by MCR [ 103 ], according to mechanism 1, methane oxidation would
involve an electrophilic attack of Ni(II)F 430 on methane, resulting in release of a
proton that is captured by CoMS • radical. Considering the p K a of methane to be
around 50, this scenario is unlikely because Ni(II) of the F 430 cofactor is not
considered to be electrophilic enough to perform such an attack. Similarly, mech-
anism 2 requires, in the initial step, a hydrogen atom abstraction from methane by a
CoBS • thiyl radical. The dissociation energy of the C-H bond in methane
(439 kJ/mol) is 74 kJ/mol higher than that of the S-H bond, thus making such a
process thermodynamically unfavorable.
Regardless of the mechanism followed by MCR, the structural and spectroscopic
evidence exists to support coupling of the two active sites of MCR. The crystal
structure revealed that the two active sites are interconnected through the '

subunits, thus, any conformational change in one of the active sites would likely be
transmitted to the other site. Evidence for the conformational change induced by
the substrate binding is based on the crystal structure analysis of Ni(II)-MCR [ 52 ],
EPR studies [ 56 ], and transient kinetic data [ 104 ]. Additionally, it was found that at
most 50% of the enzyme can be converted from the MCR red1 active state into the
MCR red2 state upon addition of CoMSH and CoBSH [ 105 ], suggesting that only
half of the sites can be used in the catalysis at any given time. Thus, coupling of the
endergonic and the exergonic steps of the catalytic cycle could be envisioned as a
strategy employed by MCR to lower the activation energy of the rate-limiting step.

ʱ and ʱ

5 Summary and Prospects for Future Science
and Technology

Methanogens are masters of CO 2 reduction. They conserve energy by coupling the
reduction of CO 2 to CH 4 , the primary constituent of natural gas, which accounts for
22 percent of the U.S. energy consumption. Uncovering the mechanistic and
molecular details of how methane is formed is critical since it is an important
fuel and the second most prevalent greenhouse gas.
Methane is considered a clean fuel because it emits less sulfur, carbon,
and nitrogen than coal or oil, and leaves little ash. It is the simplest organic
compound and has the highest energy content of any carbon-based fuel. Widely
mined and used as a fuel for heating and cooking, methane also is used by the
chemical industry to produce synthesis gas, to generate electricity, and to serve as a
vehicle fuel in the form of compressed or liquid natural gas. Methanogenesis is also
a key component of the global carbon cycle, serving as a hydrogen sink and energy
source for methanotrophic organisms.