2.3 pMMO
45
multiple pathways of electron transfer between these two enzymes [116]. These
authors presented an alternative model for electron transport in which electrons from
methanol are transferred to pMMO not only via MDH and cytochrome c, but also
via low-potential respiratory chain components such as cytochrome b and quinone
along a pathway that requires reverse electron flow. However, the proteins involved
in this pathway have not been identified biochemically. Although in vivo studies
have revealed the effect of methanol on methane oxidation in methanotrophs [117,
118], the electron transfer pathway from methanol to the hydroxylase component
of pMMO (pMMO-H) in methanotrophic bacteria remains controversial because of
issues with the reproducibility of methanol-driven pMMO activity in vitro.
(5) Hydrogen
Two dehydrogenases are present in the bacterial membrane of M. capsulatus Bath,
and hydrogen-driven methane hydroxylation has been observed [119].
2.3.6 Reaction Control
Regarding the control of the enzymatic reaction of pMMO, a pMMO–methanol
dehydrogenase complex that is believed to be involved in reaction control has been
isolated. This enzyme shows higher activity than pMMO alone and highly stable
enzyme activity. Low-temperature electron microscope image analysis indicated that
methanol dehydrogenase is bound to the hydrophilic domain in which the active site
is located [91]. On the other hand, since pMMO is a membrane protein, interaction
with the electron transfer proteins of the cell membrane would also seem to be
involved in its control [113, 120].
2.4 Summary
In this chapter, the protein structure of methane monooxygenase, an enzyme that
selectively synthesizes methanol from methane using oxygen molecules as oxidants under atmospheric temperature/pressure, and the structural changes and protein interactions involved in the oxidation of methane to methanol were reviewed.
The molecular mechanism of catalysis by methane monooxygenase is extremely
complex, and mimicking all its functions with inorganic materials and complex
organic molecules would seem to be difficult. Therefore, catalysts for the oxidation of
methane to methanol should not necessarily imitate the molecular structure and function of methane monooxygenase. In the next chapter, we will review the development
of heterogeneous and homogeneous catalysts that enable the methane-to-methanol
oxidation reaction.
45
multiple pathways of electron transfer between these two enzymes [116]. These
authors presented an alternative model for electron transport in which electrons from
methanol are transferred to pMMO not only via MDH and cytochrome c, but also
via low-potential respiratory chain components such as cytochrome b and quinone
along a pathway that requires reverse electron flow. However, the proteins involved
in this pathway have not been identified biochemically. Although in vivo studies
have revealed the effect of methanol on methane oxidation in methanotrophs [117,
118], the electron transfer pathway from methanol to the hydroxylase component
of pMMO (pMMO-H) in methanotrophic bacteria remains controversial because of
issues with the reproducibility of methanol-driven pMMO activity in vitro.
(5) Hydrogen
Two dehydrogenases are present in the bacterial membrane of M. capsulatus Bath,
and hydrogen-driven methane hydroxylation has been observed [119].
2.3.6 Reaction Control
Regarding the control of the enzymatic reaction of pMMO, a pMMO–methanol
dehydrogenase complex that is believed to be involved in reaction control has been
isolated. This enzyme shows higher activity than pMMO alone and highly stable
enzyme activity. Low-temperature electron microscope image analysis indicated that
methanol dehydrogenase is bound to the hydrophilic domain in which the active site
is located [91]. On the other hand, since pMMO is a membrane protein, interaction
with the electron transfer proteins of the cell membrane would also seem to be
involved in its control [113, 120].
2.4 Summary
In this chapter, the protein structure of methane monooxygenase, an enzyme that
selectively synthesizes methanol from methane using oxygen molecules as oxidants under atmospheric temperature/pressure, and the structural changes and protein interactions involved in the oxidation of methane to methanol were reviewed.
The molecular mechanism of catalysis by methane monooxygenase is extremely
complex, and mimicking all its functions with inorganic materials and complex
organic molecules would seem to be difficult. Therefore, catalysts for the oxidation of
methane to methanol should not necessarily imitate the molecular structure and function of methane monooxygenase. In the next chapter, we will review the development
of heterogeneous and homogeneous catalysts that enable the methane-to-methanol
oxidation reaction.