388
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
This concept has been criticized because some strains of E. coli or
Aerobacter aerogenes have hydrogenlyase activity but appear to lack
formic dehydrogenase or hydrogenase. It was shown later by Ordal and
Halvorson (153) that the absence of these enzymatic activities is due not
to the absence of the enzymes themselves but rather to the lack of
transport systems required to permit demonstration of hydrogenase or
dehydrogenase activities by standard techniques. Peck and Gest (154)
made a detailed study of the enzyme constitution of the formic hydrogenlyase system of E. coli and an atypical strain of KlebsieUa. They
concluded that the complete system involves four distinct components,
formic dehydrogenase, hydrogenase, and two transport agents called
Xi and X 2 .
Hydrogenase has also been demonstrated in other chemo-heterotrophic bacteria which neither consume nor liberate H 2 in their normal
metabolism, particularly in the nitrogen-fixing strict aerobe Azotobacter
vinelandii (155), in Acetobacter peroxydans (156), Bacillus delbruckii
(157), and Streptococcus faecalis (158),
This brief analysis of the reactions carried out by heterotrophic bacteria with the consumption or production of H 2 allows us to isolate the
major function of hydrogenase in most of these organisms. It appears
that the physiological role of the enzyme is not the activation of the
molecule of H 2 for oxidation to yield an energy source; on the contrary,
it seems to function in the inverse sense, enabling the bacterium to eliminate in molecular form the hydrogen derived from the oxidation of
organic substrates. By this mechanism, hydrogenase gives to strict or
facultative anaerobes a means of oxidizing their substrates anaerobically
in the absence of any exogenous hydrogen acceptor. As Gest (127) has
pointed out, this physiological role is comparable to that of the respiratory chain, i.e., cytochromes-cytochrome oxidase, in aerobic organisms.
B. PROPERTIES AND CONSTITUTION OF HYDROGENASE
1. Methods of Investigation
Stephenson and Stickland (159) were the first to demonstrate the
existence, in suspensions of E. coli and other bacteria, of the enzyme
which catalyzes activation of H 2 specifically and which they called
hydrogenase. They found that in an H 2 atmosphere this enzyme reduces a variety of acceptors, particularly methylene blue (MB) which
is converted stoichiometrically to a colorless leuco-derivative (MBH 2 ):
hydrogenase
H 2 + MB
> MBH 2
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
This concept has been criticized because some strains of E. coli or
Aerobacter aerogenes have hydrogenlyase activity but appear to lack
formic dehydrogenase or hydrogenase. It was shown later by Ordal and
Halvorson (153) that the absence of these enzymatic activities is due not
to the absence of the enzymes themselves but rather to the lack of
transport systems required to permit demonstration of hydrogenase or
dehydrogenase activities by standard techniques. Peck and Gest (154)
made a detailed study of the enzyme constitution of the formic hydrogenlyase system of E. coli and an atypical strain of KlebsieUa. They
concluded that the complete system involves four distinct components,
formic dehydrogenase, hydrogenase, and two transport agents called
Xi and X 2 .
Hydrogenase has also been demonstrated in other chemo-heterotrophic bacteria which neither consume nor liberate H 2 in their normal
metabolism, particularly in the nitrogen-fixing strict aerobe Azotobacter
vinelandii (155), in Acetobacter peroxydans (156), Bacillus delbruckii
(157), and Streptococcus faecalis (158),
This brief analysis of the reactions carried out by heterotrophic bacteria with the consumption or production of H 2 allows us to isolate the
major function of hydrogenase in most of these organisms. It appears
that the physiological role of the enzyme is not the activation of the
molecule of H 2 for oxidation to yield an energy source; on the contrary,
it seems to function in the inverse sense, enabling the bacterium to eliminate in molecular form the hydrogen derived from the oxidation of
organic substrates. By this mechanism, hydrogenase gives to strict or
facultative anaerobes a means of oxidizing their substrates anaerobically
in the absence of any exogenous hydrogen acceptor. As Gest (127) has
pointed out, this physiological role is comparable to that of the respiratory chain, i.e., cytochromes-cytochrome oxidase, in aerobic organisms.
B. PROPERTIES AND CONSTITUTION OF HYDROGENASE
1. Methods of Investigation
Stephenson and Stickland (159) were the first to demonstrate the
existence, in suspensions of E. coli and other bacteria, of the enzyme
which catalyzes activation of H 2 specifically and which they called
hydrogenase. They found that in an H 2 atmosphere this enzyme reduces a variety of acceptors, particularly methylene blue (MB) which
is converted stoichiometrically to a colorless leuco-derivative (MBH 2 ):
hydrogenase
H 2 + MB
> MBH 2