4.2 Methane Metabolism in Methane-Oxidizing Bacteria
79
(1) Methane oxidation to methanol
There are two forms of MMO: iron-containing soluble MMO (sMMO) located
in the cytoplasm, and copper-containing membrane-bound MMO (pMMO) located
in the cytoplasmic inner membrane [30]. The two MMOs have completely different
properties as functional proteins (see Chapter 2).
All methane-oxidizing bacteria, except those of the genera Methylocella [62]
and Methylogerula [36], produce pMMO. Type II methane-oxidizing bacteria also
produce sMMO. On the other hand, the genera Methylocella and Methylogerula
produce only sMMO.
In Type II methane-oxidizing bacteria, pMMO and sMMO are not present in
the bacterial cell under the same growth conditions; their expression is controlled
by the concentration of copper ions in the growth medium. pMMO is expressed at
copper ion concentrations above 0.85 μmol per g-dry weight of bacterial cells, while
sMMO is produced at concentrations below 0.85 μmol per g-dry weight of cells [63].
MMO expression is controlled by the bacterial gene expression regulation system,
which involves copper ions, although the corresponding mechanism is still under
investigation [64–69].
In the oxidation of methane to methanol, sMMO utilizes NADH generated from
the dehydrogenation of formaldehyde and formate as an electron donor [70, 71]. On
the other hand, the electron donor for pMMO remains controversial. As discussed in
Sect. 2.3.5, the direct electron donor for pMMO is assumed to be a quinol derivative.
Based on the structural similarities between pMMO and ammonia monooxygenase
and the phylogenetic similarities between methane-oxidizing bacteria and ammoniaoxidizing bacteria, ubiquinol 8 (coenzyme Q 8 ) has been identified as a probable
pMMO electron donor [72]. However, the ubiquinone reduction mechanism has not
been clarified.
The other possible mechanism by which electrons may be supplied to pMMO
was proposed by Higgins and co-workers, as discussed in Sect. 2.3.5. These authors
presented an alternative model for the electron transport that requires reverse electron flow; in this model, electrons from methanol are provided via not only MDH
and cytochrome c, but also by low potential respiratory chain components such as
cytochrome b and quinone. The proteins involved in this pathway have not been yet
identified biochemically. Direct electron transfer from MDH to pMMO has also been
considered based on complex formation between pMMO and MDH [73, 74].
Alternatively, the NADH produced in conjunction with the dehydrogenation
of formaldehyde and formate may be responsible for the ubiquinone reduction,
as an enhancement in methane oxidation and methanol production was observed
when exogenous formate was added [75, 76]. Hence, as shown in Chapter 2
(Fig. 2.15), pMMO is believed to use various electron sources depending on the
growth conditions [75, 76].
Bacterial cells that express pMMO oxidize methane more efficiently than sMMOcontaining cells because of their high affinity for methane and growth yield [76].
Therefore, the use of pMMO-expressing cells is considered to be more favorable for
the production of methanol from methane. On the other hand, sMMO has a wider
79
(1) Methane oxidation to methanol
There are two forms of MMO: iron-containing soluble MMO (sMMO) located
in the cytoplasm, and copper-containing membrane-bound MMO (pMMO) located
in the cytoplasmic inner membrane [30]. The two MMOs have completely different
properties as functional proteins (see Chapter 2).
All methane-oxidizing bacteria, except those of the genera Methylocella [62]
and Methylogerula [36], produce pMMO. Type II methane-oxidizing bacteria also
produce sMMO. On the other hand, the genera Methylocella and Methylogerula
produce only sMMO.
In Type II methane-oxidizing bacteria, pMMO and sMMO are not present in
the bacterial cell under the same growth conditions; their expression is controlled
by the concentration of copper ions in the growth medium. pMMO is expressed at
copper ion concentrations above 0.85 μmol per g-dry weight of bacterial cells, while
sMMO is produced at concentrations below 0.85 μmol per g-dry weight of cells [63].
MMO expression is controlled by the bacterial gene expression regulation system,
which involves copper ions, although the corresponding mechanism is still under
investigation [64–69].
In the oxidation of methane to methanol, sMMO utilizes NADH generated from
the dehydrogenation of formaldehyde and formate as an electron donor [70, 71]. On
the other hand, the electron donor for pMMO remains controversial. As discussed in
Sect. 2.3.5, the direct electron donor for pMMO is assumed to be a quinol derivative.
Based on the structural similarities between pMMO and ammonia monooxygenase
and the phylogenetic similarities between methane-oxidizing bacteria and ammoniaoxidizing bacteria, ubiquinol 8 (coenzyme Q 8 ) has been identified as a probable
pMMO electron donor [72]. However, the ubiquinone reduction mechanism has not
been clarified.
The other possible mechanism by which electrons may be supplied to pMMO
was proposed by Higgins and co-workers, as discussed in Sect. 2.3.5. These authors
presented an alternative model for the electron transport that requires reverse electron flow; in this model, electrons from methanol are provided via not only MDH
and cytochrome c, but also by low potential respiratory chain components such as
cytochrome b and quinone. The proteins involved in this pathway have not been yet
identified biochemically. Direct electron transfer from MDH to pMMO has also been
considered based on complex formation between pMMO and MDH [73, 74].
Alternatively, the NADH produced in conjunction with the dehydrogenation
of formaldehyde and formate may be responsible for the ubiquinone reduction,
as an enhancement in methane oxidation and methanol production was observed
when exogenous formate was added [75, 76]. Hence, as shown in Chapter 2
(Fig. 2.15), pMMO is believed to use various electron sources depending on the
growth conditions [75, 76].
Bacterial cells that express pMMO oxidize methane more efficiently than sMMOcontaining cells because of their high affinity for methane and growth yield [76].
Therefore, the use of pMMO-expressing cells is considered to be more favorable for
the production of methanol from methane. On the other hand, sMMO has a wider