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positions distal to the active site pocket. The remaining ligands in the two hydroxylases, MMOH and ToMOH, are
quite different from those in the other five. In the resting state, Fe1 is coordinated by a monodentate Glu and
a water molecule and Fe2 by two monodentate Glu, with bridging hydroxide ions completing the octahedral
geometry around the iron atoms. In RNR-R2,
9
desaturase, bacterioferritin, and rubrerythrin, the flanking
carboxyl ligands on the opposite side of the di-iron centre are all quite different. The very varied chemistry carried
out by these proteins, no doubt, is reflected in the active site geometry, but we are as yet unable to predict what
changes in ligands might have what consequences for biological activity.
We consider briefly the structure and mechanism of action of soluble MMO (sMMO), the best characterized of
the BMMs (bacterial methane monooxygenases) (
Figure 13.27
), which is able to activate the inert C
D
e
H bond of
FIGURE 13.27
Structures of sMMOH components and proposed reaction cycle. (a) MMOH; (b) the MMOR FAD and ferredoxin (Fd)
domains; and (c) MMOB. In MMOH, the
a
,
b
, and
g
subunits are coloured blue, green, and purple, respectively. Iron, sulfur, and FAD are
coloured orange, yellow, and red, respectively, and are depicted as spheres. The MMO reaction cycle is shown on the right., with atoms
coloured [Fe (black), C (grey), O (red), and N (blue)].
(From
Sazinsky & Lippard, 2006
.
Copyright (2006) American Chemical Society.)
methane and catalyse its transformation to methanol. sMMO contains three protein components, the hydroxylase,
MMOH, which contains the carboxylate-bridged di-iron centre, a regulatory protein MMOB, and a [2Fe
2S]- and
FAD-containing reductase (MMOR) which shuttles electrons from NADH to the di-iron centre. The hydroxylase
component (MMOH) is composed of an
e
a
2
b
2
g
2
-heterodimer, with the di-iron centre located within a character-
istic four-helix bundle made up of helices B, C, E, and F of the
-subunit. Helices E and F are on the surface of the
hydroxylase, forming part of the rim of a cleft, with the di-iron centre some 12
˚
beneath the rim. In the proposed
reaction cycle for MMOH, the resting enzyme, with both iron atoms in the ferric state, is reduced by the MMOHR
to the di-iron(II) form. The bridging hydroxyls are expelled and Glu 243 shifts to become a bridging ligand while
remaining bound to Fe2, while a water molecule coordinates weakly to Fe1. The Fe
a
Fe distance lengthens, and
the open coordination position which forms on Fe2 facing the active site pocket can now bind dioxygen, forming
an intermediate designated as H
superoxo
. This rearranges to a peroxo-intermediate designated H
peroxo
, which can
itself carry out oxygen insertion reactions with some substrates. However, the key intermediate in MMOHs is Q,
e
