MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 111

3.3.3. MECHANISM OF HYDROXYLATION CATALYZED BY THE MMO
COMPLEX

Methane and other substrate hydroxylation by dioxygen occurs with the participation of
all three components of the enzyme: MMOH, MMOR and MMOB (Feig and Lippard,
1994; Wallar and Lipscomb, 1996; and references therein). The redox potential of the
transition is in the MMOH resting state
and changes into -0.084, +0.097 and +0.100 V after the addition of MMOB, MMOR and
(MMOB + MMOR), respectively (Waller and Lipscomb, 1996).
X-ray structural analysis indicates that the M. capsulatus MMOH reduction is
accompanied by a shift of the E243 carboxylate ligand position. This finding has been
confirmed by a density functional study (Torren et al., 2000), which indicated the
flexibility of carboxylate ligands in MMO. The complete MMO reacts very quickly with
dioxygen forming compound O (Fig. 3.13). The oxygen kinetic isotope effect
in the oxidation of substrate analog by MMO from M.
capsulatus is similar to KIE for reversible dioxygen binding with myoglobin,
hemoglobin and hemerythrin (Stahl et al., 2000). Compound O consequently converts at
4°C to compound P (presumably peroxoadduct) with the rate constant about and,

after protonation and water elimination, to compound Q (Fig. 3.13).

The latter compound attracts special interest because it forms more rapidly in the
absence of substrates than it autodecays and, therefore, can be
directly investigated by physicochemical methods. The Mössbauer spectrum of
compound Q from M. trichosporium indicates that the diiron center consists of two high-
spin antiferromagnetically-coupled iron atoms, each in the Fe(IV) state bridged by
oxygen atom. Compound Q reacts very quickly with methane and other substrates with
the formation of compound T. The latter releases a product and is transformed to diferric
MMOH.
It has been suggested that the oxidizing reactivity of compound Q is similar to the
reactivity of Compound I in peroxidase, catalase and cytochrome P450 (Wallar and
Lipscomb, 1996; and references therein). This suggestion is supported by experiments
with the use of radical clocks and chiral substrates. Similar to cytochrome P450
reactions, the MMO system from M. trichosporium (Ruzicka et. al., 1990) catalyzes the
oxidation of a range of radical-clock reagents (norboran and cyclohexane derivatives)
and produces a rearranged product corresponding to a mechanism based on hydrogen
atom abstraction followed by recombination with the diiron cluster radical species and