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 126

to measure the relative proportions of “marshy air” and regular air that produce the
most energy (often measured as the loudness of the bangs upon ignition) and to
determine which catalysts (e.g., iron filings in sulfuric acid) best promote the
explosions.
At the turn of the 20th century it was shown that the “combustible air” was
methane and that methane is cyclically produced and utilized by microbes as energy
and carbon sources [ 27 , 28 ]. Methods were developed to isolate and culture these
anaerobic microbes in pure culture [ 27 - 31 ] and to work with cell-free extracts and
purify enzymes from methanogens [ 32 ]. A recent Methods in Enzymology volume
describes current methods used to study the microbiology, biochemistry, ecology,
and molecular genetics of methanogens [ 33 ]. By following those methods, growing
anaerobic microbes and working with anaerobic enzymes really is not that difficult.
During the last three decades, the individual steps in the pathway of methane
formation have been elucidated and shown to involve many novel enzymes and
cofactors [ 34 ]. The first purified enzyme clearly defined to be involved in
methanogenesis was MCR, the topic of this review, which catalyzes the chemical
step of methane synthesis via reaction (1). As described below, the structure and
mechanism of MCR has been examined from various angles, though a consensus
about the individual steps in its reaction mechanism has not been reached.

2 Structure and Properties of Methyl-Coenzyme M
Reductase and Its Bound Coenzyme F 430

2.1 Structure, Properties, and Reactivity of Coenzyme F 430

The activity of MCR requires a bright yellow nickel cofactor called F 430 , which has
two major absorption bands with maxima at 430 nm ( 23,100 M 1 cm 1 ) and ʵ 430 ¼

20,000 M 1 cm 1 )[ 35 - 37 ] (Figure 1 ). Actually it is only yellow in
its oxidized Ni(II) or Ni(III) state; in the active Ni(I) state the cofactor is green.
All methanogens appear to contain F 430 [ 38 ], which apparently is only the cofactor
for this enzyme. Because the structure, biosynthesis, and redox properties of F 430
have been extensively reviewed elsewhere [ 34 , 39 - 41 ], I will only briefly describe
its properties.
In 1980, this cofactor was isolated and shown to contain Ni and to be a tetrapyrrole
[ 35 , 37 ]. NMR and X-ray crystallographic methods revealed this cofactor to be a
hydrocorphin, containing only five double bonds (of these only four are conjugated),
thus, earning it the distinction of being themost reduced tetrapyrrole in nature [ 42 - 44 ].
Recognition that F 430 is a component of MCR came when Wolfe and coworkers
released and isolated 63 Ni-F 430 from the purified enzyme [ 45 ]. The redox potential of
the F 430 -Ni(II)/F 430 -Ni(I) pair is between

274 nm ( ʵ 274 ¼

600 and 700 mV ( versus the normal

hydrogen electrode, NHE) [ 46 , 47 ].