80
4 Application of Biocatalysts for the Production of Methanol …
substrate range than pMMO. This characteristic makes bacterial cells expressing
sMMO attractive for producing several chemicals for biotechnological applications.
(2) Oxidation of methanol to formaldehyde
The methanol produced in the MMO-catalyzed oxidation of methane is dehydrogenated to formaldehyde via catalysis by methanol dehydrogenase (MDH). MDH
is located in the periplasm (the space between outer membrane and intracytoplasmic membrane of methane-oxidizing bacteria) [30], and contains pyrroloquinoline
quinone (PQQ) as a cofactor. The reduction of PQQ is coupled with the dehydrogenation of methanol. The reduced form of PQQ (PPQH 2 ) is oxidized and transfers
two electrons to the terminal oxidase by way of cytochromes c and other carriers as
intermediates, or to reduce the electron donor for pMMO [77].
(3) Formaldehyde conversion
As described above, formaldehyde plays a vital role as a metabolic intermediate in
the methane metabolism system. Some of the produced formaldehyde is oxidized to
CO 2 for energy generation, while the rest is incorporated into the carbon assimilation
pathway for cell replication.
Two systems for the dehydrogenation of formaldehyde into formate have been
proposed. The first is dehydrogenation catalyzed by formaldehyde dehydrogenase
(FalDH). Two types of FalDH enzymes, a PQQ-containing and cytochrome-linked
type and a NAD-linked type, have been reported [78, 79]. PQQ-containing FalDH was
found to be the major formaldehyde oxidizing enzyme in pMMO-expressing Methylococcus capsulatus cells [80]. The second system is the tetrahydromethanopterin
(H 4 MPT)-linked formaldehyde dehydrogenation pathway (Fig. 4.2). In the H 4 MPT
pathway, methylene tetrahydromethanopterin dehydrogenase (MtdB) reduces NAD
+
to produce NADH [75, 81]. In addition to these two systems, several other formaldehyde dehydrogenation systems have been found in methanol-oxidizing bacteria.
These formaldehyde dehydrogenation systems may also exist in methane-oxidizing
bacteria [78].
4 Application of Biocatalysts for the Production of Methanol …
substrate range than pMMO. This characteristic makes bacterial cells expressing
sMMO attractive for producing several chemicals for biotechnological applications.
(2) Oxidation of methanol to formaldehyde
The methanol produced in the MMO-catalyzed oxidation of methane is dehydrogenated to formaldehyde via catalysis by methanol dehydrogenase (MDH). MDH
is located in the periplasm (the space between outer membrane and intracytoplasmic membrane of methane-oxidizing bacteria) [30], and contains pyrroloquinoline
quinone (PQQ) as a cofactor. The reduction of PQQ is coupled with the dehydrogenation of methanol. The reduced form of PQQ (PPQH 2 ) is oxidized and transfers
two electrons to the terminal oxidase by way of cytochromes c and other carriers as
intermediates, or to reduce the electron donor for pMMO [77].
(3) Formaldehyde conversion
As described above, formaldehyde plays a vital role as a metabolic intermediate in
the methane metabolism system. Some of the produced formaldehyde is oxidized to
CO 2 for energy generation, while the rest is incorporated into the carbon assimilation
pathway for cell replication.
Two systems for the dehydrogenation of formaldehyde into formate have been
proposed. The first is dehydrogenation catalyzed by formaldehyde dehydrogenase
(FalDH). Two types of FalDH enzymes, a PQQ-containing and cytochrome-linked
type and a NAD-linked type, have been reported [78, 79]. PQQ-containing FalDH was
found to be the major formaldehyde oxidizing enzyme in pMMO-expressing Methylococcus capsulatus cells [80]. The second system is the tetrahydromethanopterin
(H 4 MPT)-linked formaldehyde dehydrogenation pathway (Fig. 4.2). In the H 4 MPT
pathway, methylene tetrahydromethanopterin dehydrogenase (MtdB) reduces NAD
+
to produce NADH [75, 81]. In addition to these two systems, several other formaldehyde dehydrogenation systems have been found in methanol-oxidizing bacteria.
These formaldehyde dehydrogenation systems may also exist in methane-oxidizing
bacteria [78].