2.2 sMMO
27
molecule to complete the di-iron coordination sphere. Two water molecules are coordinated in another conformation of reduced MMOH, in which one water molecule
bridges to iron ion. The pentacoordinate geometry of both iron ions was supported
by magnetic circular dichroism (MCD) spectroscopy of MMOH [17, 18]. On the
other hand, in the crystal structure of oxidized MMOH (Fig. 2.2b) [13, 16], both iron
atoms exhibit hexacoordinate geometry and are bridged by a μ-1,3 carboxylate from
Glu144, by three or by one hydroxo and one water molecule.
2.2.3 Catalytic Cycle of the Oxidation of Methane
to Methanol
The catalytic cycle of oxygen activation followed by methane hydroxylation via
MMOH is shown in Fig. 2.3. Several intermediates formed during sMMO catalysis
Fe
II
Fe
II
O
O
O
O
O
H 2 O
H 2
MMOH red
MMOH ox
Step 1
Step 2
Step 3
O 2
Fe
II
Fe
II
O
O
O
O
O
H 2
Intermediate O
Intermediate P*
Fe
III
Fe
III
O
O
O
O
O
O
Intermediate P
Fe
IV
Fe
IV
O
O
O
O
O
O
Intermediate Q
Step 4
Step 5
Fe
III
Fe
III
O
O
O
O
O
Intermediate T
MMOB + O 2
CH 4
CH 3 OH
2H 2 O
MMOR red
MMOB + H 2 O
H 2 O
MMOR ox
Fe
III
Fe
III
O
O
O
O
O
H 2 O
H
H
O
Fig. 2.3 Catalytic cycle for the oxidation of methane to methanol at the di-iron site of MMOH
using molecular oxygen or hydrogen peroxide as the oxidant