2.2 sMMO
25
Fig. 2.1 Protein structures of MMOH (PDB ID: 1MTY) (a) ferredoxin domain of MMOR (PDB ID:
1TVC) (b) 2Fe-2S domain of MMOR (PDB ID:1JQ4) (c) and MMOB (PDB ID: 1CKV) (d) from
Methylococcus capsulatus (Bath)
Table 2.1 Structural
properties of the sMMO
components (Methylococcus
capsulatus (Bath))
MMOH
MMOR
MMOB
Protein
quaternary
structure
Homodimer Single
peptide
Single
peptide
Subunits of
monomer
α, β, γ
–
–
Molecular
weight
α: 60,600
~39,000
~16,000
β: 45,100
γ: 19,800
Cofactor
2 Fe
[2Fe-2S]
No cofactor
FAD
These proteins are water-soluble with polar amino acid side chains distributed on
their surfaces.
MMOH (Fig. 2.1a) exists as a dimer, with a monomeric unit consisting of three
subunits (α, β, and γ). The di-nuclear iron center present in the α subunit is the
catalytic site for the oxidation of methane to methanol [10]. The iron atoms of this
center are coordinated by the nitrogen atoms of histidine and glutamic acid residues
and the oxygen atom of a water molecule.
MMOR (Fig. 2.1b) plays a role in the transfer of electrons from nicotinamide
adenine dinucleotide (NADH) to MMOH [11]. MMOR has two domains: A FADbinding domain and a ferredoxin domain, which contains a [2Fe–2S] cluster. The
former domain accepts electrons from NADH, which are subsequently transferred to
the ferredoxin domain. The ferredoxin domain donates the electrons to the di-nuclear
iron center of MMOH.
MMOB (Fig. 2.1c) is a small protein compared to MMOH and MMOR, but plays
an important role in the oxidation of methane to methanol at the catalytic site of