382
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
captolipoate. The cofactor is reoxidized by hypoxanthine (H 2 X) which
is converted to an as yet unidentified form (Reaction 2). The latter
stages involve the transfer of electrons to diphosphopyridine nucleotide
(Reaction 3), then to flavin adenine dinucleotide (Reaction 4), and
finally the reoxidation of reduced flavin by oxygen with the formation
of hydrogen peroxide (Reaction 5). At present, it is not known if this
exergonic process gives rise to high-energy phosphate bonds which the
organism can use for other physiological functions. From the point of
view of comparative biochemistry, it is of interest that an enzyme system similar to the one just described in mammalian liver has also been
demonstrated in the chemo-lithotrophic bacterium T. denitrificans (115).
IV. Oxidation of Molecular Hydrogen
The ability to oxidize molecular hydrogen is tied to the action of a
specific enzyme, hydrogenase, which activates the biatomic molecule of
gaseous hydrogen and reversibly catalyzes the reaction:
hydrogenase
H 2 r
- 2H+ + 2e
This enzyme is almost exclusively present in bacteria, with the exception
of a few other organisms (i.e., some algae). Among the bacteria, however, hydrogenase is very widely distributed; it is found not only in
chemo-lithotrophic organisms capable of utilizing the aerobic or anaerobic oxidation of H 2 as sole energy source but also in phototrophic
and chemo-heterotrophic organisms where hydrogenase assumes various
functions and takes part in various metabolic pathways.
The abundant literature on the different metabolic functions of
hydrogenase and the properties of microorganisms endowed with it was
reviewed in great detail by Gest (127) some years ago.
A. ORGANISMS POSSESSING HYDROGENASE
1. Chemo-lithotrophic Bacteria
a. Hydrogen Bacteria. These are autotrophs which use as energy
source the aerobic oxidation of hydrogen gas:
H 2 + 0.5 0 2 = H 2 0
(-AF° 29 8° = 57.4 kcal.)
Since this reaction synthesizes water from detonating gas, the bacteria
which can carry it out are often called Knallgasbakterien by German
authors.
The genus Hydro genomonas (61) comprises the type species H. pantotropha, the first form described by Kaserer (128) in 1906, and several
other species including H. vitrea and H. flava, described by Niklevskii
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
captolipoate. The cofactor is reoxidized by hypoxanthine (H 2 X) which
is converted to an as yet unidentified form (Reaction 2). The latter
stages involve the transfer of electrons to diphosphopyridine nucleotide
(Reaction 3), then to flavin adenine dinucleotide (Reaction 4), and
finally the reoxidation of reduced flavin by oxygen with the formation
of hydrogen peroxide (Reaction 5). At present, it is not known if this
exergonic process gives rise to high-energy phosphate bonds which the
organism can use for other physiological functions. From the point of
view of comparative biochemistry, it is of interest that an enzyme system similar to the one just described in mammalian liver has also been
demonstrated in the chemo-lithotrophic bacterium T. denitrificans (115).
IV. Oxidation of Molecular Hydrogen
The ability to oxidize molecular hydrogen is tied to the action of a
specific enzyme, hydrogenase, which activates the biatomic molecule of
gaseous hydrogen and reversibly catalyzes the reaction:
hydrogenase
H 2 r
- 2H+ + 2e
This enzyme is almost exclusively present in bacteria, with the exception
of a few other organisms (i.e., some algae). Among the bacteria, however, hydrogenase is very widely distributed; it is found not only in
chemo-lithotrophic organisms capable of utilizing the aerobic or anaerobic oxidation of H 2 as sole energy source but also in phototrophic
and chemo-heterotrophic organisms where hydrogenase assumes various
functions and takes part in various metabolic pathways.
The abundant literature on the different metabolic functions of
hydrogenase and the properties of microorganisms endowed with it was
reviewed in great detail by Gest (127) some years ago.
A. ORGANISMS POSSESSING HYDROGENASE
1. Chemo-lithotrophic Bacteria
a. Hydrogen Bacteria. These are autotrophs which use as energy
source the aerobic oxidation of hydrogen gas:
H 2 + 0.5 0 2 = H 2 0
(-AF° 29 8° = 57.4 kcal.)
Since this reaction synthesizes water from detonating gas, the bacteria
which can carry it out are often called Knallgasbakterien by German
authors.
The genus Hydro genomonas (61) comprises the type species H. pantotropha, the first form described by Kaserer (128) in 1906, and several
other species including H. vitrea and H. flava, described by Niklevskii