394
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
less rapidly than the reduction of methylene blue. Based on these findings and on the activation of the hydrogenases of M. lactilyticus and C.
butylicum by Fe
2+ , Peck and associates describe hydrogenase as a ferromolybdofiavoprotein with two distinct prosthetic groups whose activities can be dissociated. One of these groups would carry out specifically
the transfer of hydrogen to natural or artificial acceptors with two electrons while the other, which would involve both FAD and molybdenum,
would transfer hydrogen specifically to acceptors with one electron,
e.g., cytochrome c or the viologens. This second group was likewise
implicated in the hydrogen-deuterium exchange reaction and the liberation of gaseous hydrogen.
The hypothesis which states that flavometalloenzymes may be multifunctional and that molybdenum may be involved only in hydrogen
transport to acceptors with a single electron was also advanced by
Shug et al. (172) for the hydrogenase of C. pasteurianum and by Mackler et al. (181) for xanthine oxidase.
5. Natural Hydrogen Carriers Coupled Functionally with Hydrogenase
The number and nature of the natural acceptors which are coupled
with hydrogenase vary with the organism. Korkes (182) demonstrated a
system coupling hydrogenase with the reduction of pyridine nucleotides
in C. kluyveri. Analogous systems were observed by Peck and Gest
(183) in C. pasteurianum, by Packer and Vishniac (184) in H. ruhlandii
and by Wittenberger and Repaske (185) in H. eutropha. However, functional coupling between hydrogenase and pyridine nucleotides was not
encountered in cells or extracts of E. colt (138, 186) or D. desulfuricans
(187).
Korkes (182) showed that coupling with pyridine nucleotides requires a thermostable cofactor; this was recently studied exhaustively by
Kinsky (188). His investigation established that the complete coupling
system in C. kluyveri comprises a DPN reductase and the Korkes cofactor which most probably is a flavin but does not appear to be flavin
mononucleotide (FMN) or flavin adenine dinucleotide (FAD). This
cofactor can be replaced by artificial carriers such as benzyl viologen
or safranine O or, in part, by FAD. The hydrogenase of C. kluyveri
reduces FAD and the dyes directly, without the participation of any
intermediary carrier, while in H. ruMandii (184) catalytic amounts of
DPN must be added to the extracts to permit the reduction of methylene
blue or benzyl viologen to proceed. Since FAD is reduced rapidly by
several hydrogenases, it is likely that in certain organisms the oxidation
of H 2 is coupled to oxygen consumption by a flavin respiratory chain,
but experimental support for this hypothesis is still lacking.
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
less rapidly than the reduction of methylene blue. Based on these findings and on the activation of the hydrogenases of M. lactilyticus and C.
butylicum by Fe
2+ , Peck and associates describe hydrogenase as a ferromolybdofiavoprotein with two distinct prosthetic groups whose activities can be dissociated. One of these groups would carry out specifically
the transfer of hydrogen to natural or artificial acceptors with two electrons while the other, which would involve both FAD and molybdenum,
would transfer hydrogen specifically to acceptors with one electron,
e.g., cytochrome c or the viologens. This second group was likewise
implicated in the hydrogen-deuterium exchange reaction and the liberation of gaseous hydrogen.
The hypothesis which states that flavometalloenzymes may be multifunctional and that molybdenum may be involved only in hydrogen
transport to acceptors with a single electron was also advanced by
Shug et al. (172) for the hydrogenase of C. pasteurianum and by Mackler et al. (181) for xanthine oxidase.
5. Natural Hydrogen Carriers Coupled Functionally with Hydrogenase
The number and nature of the natural acceptors which are coupled
with hydrogenase vary with the organism. Korkes (182) demonstrated a
system coupling hydrogenase with the reduction of pyridine nucleotides
in C. kluyveri. Analogous systems were observed by Peck and Gest
(183) in C. pasteurianum, by Packer and Vishniac (184) in H. ruhlandii
and by Wittenberger and Repaske (185) in H. eutropha. However, functional coupling between hydrogenase and pyridine nucleotides was not
encountered in cells or extracts of E. colt (138, 186) or D. desulfuricans
(187).
Korkes (182) showed that coupling with pyridine nucleotides requires a thermostable cofactor; this was recently studied exhaustively by
Kinsky (188). His investigation established that the complete coupling
system in C. kluyveri comprises a DPN reductase and the Korkes cofactor which most probably is a flavin but does not appear to be flavin
mononucleotide (FMN) or flavin adenine dinucleotide (FAD). This
cofactor can be replaced by artificial carriers such as benzyl viologen
or safranine O or, in part, by FAD. The hydrogenase of C. kluyveri
reduces FAD and the dyes directly, without the participation of any
intermediary carrier, while in H. ruMandii (184) catalytic amounts of
DPN must be added to the extracts to permit the reduction of methylene
blue or benzyl viologen to proceed. Since FAD is reduced rapidly by
several hydrogenases, it is likely that in certain organisms the oxidation
of H 2 is coupled to oxygen consumption by a flavin respiratory chain,
but experimental support for this hypothesis is still lacking.