MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 108

sulfur ligand with the group were discussed in (Ogliaro et al., 2000). As a result
of theoretical calculations, two conclusions emerged: 1) the interactions in the protein
pocket strengthens to the Fe-S bond and 2) the hydrogen bonding of the thiolate ligand
stabilizes resonance structure of a compound I species (SR) Fe(IV)O].
300-picosecond molecular dynamics simulation of Compound I of cytochrome P450
with fifth ligand and methane as a substrate revealed that the methyl radical is a
product of the hydrogen abstraction (Hata et al., 2001). In another recent molecular
dynamics calculation (Yoshizawa et al., 2001) the dynamic aspect of ethane
hydroxylation mediated by Compound I was considered. The calculations have also
supported the rebound mechanisms and added important details concerning the hydrogen
atom abstraction transition state. These indicate that the molecular vibrations of the C-H
bond being dissociated and the O-H bond being formed are significantly activated before
and after the transition state, respectively. The porphyrin ring vibrational modes and
are involved in Fe-N stretching motion and in energy transfer during the enzymatic
process.
Thus, it is becoming increasingly evident that the rebound mechanism is the most
probable mechanism of the hydroxylation. Nevertheless, direct proof of occurrence of
the ferryl active intermediate is as yet incomplete. Above mentioned the masked radical
rebound mechanism can not be excluded.
3.3. Methane Monooxigenase
3.3.1 OVERVIEW
In microorganisms utilizing methane, a methane-hydroxylating system, metan
monooxigenase, (MMO) has been detected, which catalyzed the reaction:
The electron donor is NADH, which can be replaced by NADPH, and ascorbic
acid (Wallar and Lipscomb, 1996). Methane is produced as a primary product of
anaerobic metabolism by methanogenis bacteria and is assimilated as biomass, the
energy source, by the methanothrophus. Rapid and specific hydroxylation of such an
inert molecule as methane has attracted the special interest of biochemists, chemists and
physico-chemists. The soluble preparations of MMO have been isolated from a number
microorganisms. For the last two decades, the enzymes from Methylococcus capsulatus
(Bath) and Methylosynus trichosporium OB3b have been intensively investigated with
the entire arsenal of biochemical, kinetic and physical methods (Belova et al., 1976;
Gvozdev et al., 1982; Rozenzwieg et al., 1993; DeRose et al., 1996; Waller and
Lipscomb, 1996; Willems et al., 1998; Jin and Lipscomb. 2000; Austin et al., 2000; Stahl
et al., 2001; Merkx et al., 2001; Guallar et al., 2002.).
The enzyme consists of three protein components: a 245 kDa hydroxylase (MOH), a
15 kDa protein (component B), and a 40 kD reductase (MMOR). The hydroxylase is a