43766-G3
Production and Anaerobic Oxidation of Methane by Methanogenic Archea
Eduardus C. Duin, Auburn University
Methyl-coenzyme M
reductase (MCR) is the key enzyme in two important biological processes, the
production of methane in methanogenesis and the activation of methane in the
anaerobic methane oxidation:
CH3-S-CoM
+ HS-CoB ↔ CH4 + CoM-S-S-CoB
(CH3-S-CoM,
methyl coenzyme M; HS-CoB, coenzyme B; CoM-S-S-CoB, heterodisulfide of coenzyme
M and coenzyme B)
It has been
proposed that anaerobic methane oxidation is a complete reversal of
methanogenesis. To overcome the positive free-energy change the ‘reverse' pathway
is coupled to sulfate reduction. Present in the active site of MCR is a
nickel-containing tetrapyrrole (factor 430) that plays a central role in the
catalysis.
The goal of our
research is to elucidate the reversible reaction mechanism of MCR and the ways
the activity of the enzyme is regulated in the cell. This will provide valuable
information about the one-step activation of methane at ambient temperature and
pressure that will be used to design a nickel-based catalyst that can perform
this function. Secondly, the data should result in the development of
inhibitors that prevent the production of methane in, for example, livestock
and rice fields, which are the main contributors in the increase of atmospheric
methane levels that in turn contributes to the greenhouse effect.
The PRF funding allowed us to investigate the interaction
of several substrate analogs and inhibitors with MCR isolated from Methanothermobacter marburgensis. A
highly exciting result was the discovery that incubation of MCR with bromomethane
resulted in the formation of an organometallic methyl-nickel species in the active
site of MCR. This finding does not prove, but is in line with hypothetical mechanisms
that include such a species. The proposed mechanisms that are available, however,
do not incorporate the relative recent discovery that MCR is involved in both
methane production and methane uptake. Together with the group of Dr. Michael
McKee we proposed a DFT-based reversible reaction mechanism that includes a nickel-methyl
intermediate. We proposed that the reaction cycle begins with the protonation
of factor 430, either on Ni or on the C-ring nitrogen of the tetrapyrrole ring,
both of which are nearly equally favorable. Although this proposal was purely
hypothetical, subsequent ENDOR investigations showed that a nickel hydrate
species can indeed be detected in MCR.
We would like to
thank the PRF for their support. The Department of Chemistry and Biochemistry
provided the salaries for the two students working on this project, Ms. Na
Yang, and Mr. Mi Wang. The PRF grant was used to buy the necessary supplies and
chemicals.
