396
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
to possess a hydrogenase and which these authors showed to fix N 2 .
The hydrogenase activity of R. rubrum, like that of A. vinelandii, is much
larger if the cells have developed by fixation of gaseous nitrogen. Moreover, in this organism the interaction between the hydrogenase and the
nitrogen-fixing system is further emphasized by the fact that the photoevolution of H 2 by resting cells from pyruvate and C 4 -dicarboxylic acids
is completely inhibited by N 2 , showing that gaseous nitrogen can combine with the enzymatic system which carries out the photoevolution
and which comprises the hydrogenase as one of its constituents.
Recently, Le Gall et al. (196) demonstrated that D. desulfuricans, a
strict anaerobe with a very high hydrogenase activity, can fix nitrogen.
It continues to do so at relatively high partial pressures of hydrogen but
cannot grow chemo-lithotrophically in an H 2 + N 2 atmosphere on a
medium devoid of combined nitrogen.
Shug et al. (172) performed an experiment that seemed to demonstrate directly the interaction between the hydrogenase of C. pasteurianum and the "nitrogenase," i.e., the nitrogen-fixing system, of the same
organism. They took the difference spectrum of purified hydrogenase
in pure H 2 and in the presence of a mixture of H 2 and N 2 . Under these
conditions, they obtained spectra with two absorption peaks which
were interpreted as those of the first intermediate compound of nitrogen
during its fixation. Analogous findings were made by Hamilton et al.
(197) with extracts of Azotobacter.
Shug and associates assign to hydrogenase a role in nitrogen fixation
similar to that predicted by Winfield (178) on theoretical grounds.
"Nitrogenase" is considered a metalloflavoprotein whose active site involves two atoms of iron or molybdenum suitably spaced, on which the
N 2 molecule would be fixed by simple or double covalent bonds. Hydrogenase, itself a metalloflavoprotein, would transfer hydrogen atoms
to the two atoms of nitrogen thus fixed and activated, as shown in Fig.
3; these successive hydrogenations would finally lead to ammonia. In
this theory, the inhibition of nitrogen fixation by hydrogen is explained
by competition between nitrogen and hydrogen for the active sites
of the nitrogenase.
Gest et al. (195) feel that in R. rubrum nitrogen fixation is also a reduction process and that in this photosynthetic organism the principal
source of hydrogen or electrons is the photolysis of water. Active hydrogen derived from the photochemical reaction, in addition to that
formed by hydrogenase when this enzyme disposes of suitable donors,
would accumulate in a sort of pool of reducing power with a very low
oxidation-reduction potential. The electrons would be directed from this
pool either toward reduction and assimilation of N 2 or toward other
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
to possess a hydrogenase and which these authors showed to fix N 2 .
The hydrogenase activity of R. rubrum, like that of A. vinelandii, is much
larger if the cells have developed by fixation of gaseous nitrogen. Moreover, in this organism the interaction between the hydrogenase and the
nitrogen-fixing system is further emphasized by the fact that the photoevolution of H 2 by resting cells from pyruvate and C 4 -dicarboxylic acids
is completely inhibited by N 2 , showing that gaseous nitrogen can combine with the enzymatic system which carries out the photoevolution
and which comprises the hydrogenase as one of its constituents.
Recently, Le Gall et al. (196) demonstrated that D. desulfuricans, a
strict anaerobe with a very high hydrogenase activity, can fix nitrogen.
It continues to do so at relatively high partial pressures of hydrogen but
cannot grow chemo-lithotrophically in an H 2 + N 2 atmosphere on a
medium devoid of combined nitrogen.
Shug et al. (172) performed an experiment that seemed to demonstrate directly the interaction between the hydrogenase of C. pasteurianum and the "nitrogenase," i.e., the nitrogen-fixing system, of the same
organism. They took the difference spectrum of purified hydrogenase
in pure H 2 and in the presence of a mixture of H 2 and N 2 . Under these
conditions, they obtained spectra with two absorption peaks which
were interpreted as those of the first intermediate compound of nitrogen
during its fixation. Analogous findings were made by Hamilton et al.
(197) with extracts of Azotobacter.
Shug and associates assign to hydrogenase a role in nitrogen fixation
similar to that predicted by Winfield (178) on theoretical grounds.
"Nitrogenase" is considered a metalloflavoprotein whose active site involves two atoms of iron or molybdenum suitably spaced, on which the
N 2 molecule would be fixed by simple or double covalent bonds. Hydrogenase, itself a metalloflavoprotein, would transfer hydrogen atoms
to the two atoms of nitrogen thus fixed and activated, as shown in Fig.
3; these successive hydrogenations would finally lead to ammonia. In
this theory, the inhibition of nitrogen fixation by hydrogen is explained
by competition between nitrogen and hydrogen for the active sites
of the nitrogenase.
Gest et al. (195) feel that in R. rubrum nitrogen fixation is also a reduction process and that in this photosynthetic organism the principal
source of hydrogen or electrons is the photolysis of water. Active hydrogen derived from the photochemical reaction, in addition to that
formed by hydrogenase when this enzyme disposes of suitable donors,
would accumulate in a sort of pool of reducing power with a very low
oxidation-reduction potential. The electrons would be directed from this
pool either toward reduction and assimilation of N 2 or toward other