44
2 Selective Production of Methanol …
pool can donate electrons to the PmoB copper center. One possibility is that copper
ions other than those in the PmoB copper center mediate electron transfer from the
quinone pool to the PmoB copper center [110].
(2) NADH
At least three models of electron donation to pMMO have been proposed.
It is generally accepted that NADH can be used in vitro as an electron source for
pMMO, although in an early study, Ribbons suggested that NADH may not be the
immediate electron donor for pMMO [111]. Electron transfer proteins in methaneoxidizing bacterial cells are presumed to be involved in electron transfer from NADH
to the protein, as shown in Fig. 2.15 [88, 107, 112, 113], although a site that accepts
electrons directly from NADH has also been proposed [82].
NADH donates electrons to pMMO via other membrane components within the
bacterial membrane, such as NADH dehydrogenase and the quinone pool [88, 112,
113]. Therefore, in most cases, no NADH-driven activity was observed when pMMO
was isolated from the bacterial membrane.
It was subsequently shown that exogenous quinols can act as electron donors
for pMMO, and that the resulting quinones can be reduced in vitro via an
NADH-dependent reaction involving a type 2 NADH-quinone reductase [113].
The latter enzyme has been isolated and purified from M. capsulatus (Bath) as a
36.6 kDa monomer with a single non-covalently associated FAD cofactor. However,
experiments with inhibitors of the enzyme yielded contradictory results [113].
(3) Succinate
Cornish et al. proposed another electron donation pathway in which succinate provides electrons to pMMO by way of membrane-associated succinate dehydrogenase
(SDH, succinate:ubiquinone oxidoreductase) [114]. SDH is able to use hydroquinone
derivatives in the intracytoplasmic membrane as its electron acceptor, but not able
to use NAD(P)
+ . Mitochondria-like electron transfer systems may present in the
intracytoplasmic membrane of the bacterial. However, it is not physiologically relevant because succinate plays a signifficant role as the intermediate for providing
precurosors for biosynthesis via TCA cycle.
(4) Methanol
A third possible way in which electrons might be provided to pMMO was proposed by Higgins and co-workers, and involves electron transport from methanol via
methanol dehydrogenase (MDH) located in the periplasm [115]. These researchers
observed not only the NADH-driven pMMO activity reported by others, but also
methanol-driven pMMO activity in particulate preparations; this activity correlated
with the concentration of a CO-binding cytochrome c. Hence, this cytochrome c
may act as a mediator between MDH and pMMO. Theoretical analysis of experimental growth yield data from cultures of M. capsulatus (Bath) by Leak and Dalton
supported direct electron coupling between MDH and pMMO, and also suggested
2 Selective Production of Methanol …
pool can donate electrons to the PmoB copper center. One possibility is that copper
ions other than those in the PmoB copper center mediate electron transfer from the
quinone pool to the PmoB copper center [110].
(2) NADH
At least three models of electron donation to pMMO have been proposed.
It is generally accepted that NADH can be used in vitro as an electron source for
pMMO, although in an early study, Ribbons suggested that NADH may not be the
immediate electron donor for pMMO [111]. Electron transfer proteins in methaneoxidizing bacterial cells are presumed to be involved in electron transfer from NADH
to the protein, as shown in Fig. 2.15 [88, 107, 112, 113], although a site that accepts
electrons directly from NADH has also been proposed [82].
NADH donates electrons to pMMO via other membrane components within the
bacterial membrane, such as NADH dehydrogenase and the quinone pool [88, 112,
113]. Therefore, in most cases, no NADH-driven activity was observed when pMMO
was isolated from the bacterial membrane.
It was subsequently shown that exogenous quinols can act as electron donors
for pMMO, and that the resulting quinones can be reduced in vitro via an
NADH-dependent reaction involving a type 2 NADH-quinone reductase [113].
The latter enzyme has been isolated and purified from M. capsulatus (Bath) as a
36.6 kDa monomer with a single non-covalently associated FAD cofactor. However,
experiments with inhibitors of the enzyme yielded contradictory results [113].
(3) Succinate
Cornish et al. proposed another electron donation pathway in which succinate provides electrons to pMMO by way of membrane-associated succinate dehydrogenase
(SDH, succinate:ubiquinone oxidoreductase) [114]. SDH is able to use hydroquinone
derivatives in the intracytoplasmic membrane as its electron acceptor, but not able
to use NAD(P)
+ . Mitochondria-like electron transfer systems may present in the
intracytoplasmic membrane of the bacterial. However, it is not physiologically relevant because succinate plays a signifficant role as the intermediate for providing
precurosors for biosynthesis via TCA cycle.
(4) Methanol
A third possible way in which electrons might be provided to pMMO was proposed by Higgins and co-workers, and involves electron transport from methanol via
methanol dehydrogenase (MDH) located in the periplasm [115]. These researchers
observed not only the NADH-driven pMMO activity reported by others, but also
methanol-driven pMMO activity in particulate preparations; this activity correlated
with the concentration of a CO-binding cytochrome c. Hence, this cytochrome c
may act as a mediator between MDH and pMMO. Theoretical analysis of experimental growth yield data from cultures of M. capsulatus (Bath) by Leak and Dalton
supported direct electron coupling between MDH and pMMO, and also suggested