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 127

Figure 1 Properties of coenzyme F 430 .(a) Photograph of methyl-coenzyme M reductase (MCR)
crystals in the MCR red1 and MCR Me states. (b) UV-visible spectra of bound F 430 in its three
accessible oxidation states. (c) The structure of free F 430 .(d) Structure of bound F 430 (PDB ID
1hbu [ 52 ]).

2.2 Structure, Properties, and Reactivity
of Methyl-Coenzyme M Reductase

The first structure of the enzyme was of MCR isoenzyme I from Methanother-
mobacter marburgensis [ 48 ]. Highly refined crystal structures of various
MCR-substrate and MCR-product complexes have since been obtained. All of
these studies reveal the quarternary structure of MCR as a 270 kDa dimeric
association of three different subunits in an (

ʱʲʳ ) 2 arrangement with F 430 sitting

at the base of a narrow channel (~50 ) that accommodates the substrates/product.

Structures have been determined for the MCRs from M. marburgensis [ 48 ],
Methanosarcina barkeri, and Methanopyrus kandleri [ 49 ] and from an anaerobic
methane oxidizer (ANME-1) [ 50 ]. All but one (that of the methyl-Ni(III) enzyme,
PDB code 3pot [ 51 ]) contain nickel in its inactive Ni(II) state and but one contain
coenzyme B (CoBSH, mercapto-heptanoylthreonine phosphate) and coenzyme M
(CoMSH, mercaptoethanesulfonate) in the active site (PDB codes 1hbn, 1hbo, 1hbu,
1e6y, 1e6v) [ 18 , 49 , 52 ]. Another crystal structure has the bound heterodisulfide

Å