8. REACTIONS OF INORGANIC SUBSTANCES
359
Escherichia coli and other members of the colon-aerogenes group possess a hydrogenlyase system, one of whose constituents is the enzyme
hydrogenase (cf. Section IV,A,3). This system carries out the reversible
synthesis of formate from C0 2 and H 2 :
C0 2 + H 2 ^± HCOOH
The strict anaerobe Methanobacterium omelianskii reduces C0 2 to
methane by a process which was studied in detail by Barker et al. (21);
carbon dioxide acts as the final hydrogen acceptor for the anaerobic
oxidation of various organic substrates (DH 2 ):
C0 2 + 4Z)H 2 -> CH 4 + 4D + 2H 2 0
It is noteworthy that these processes differ with those in which C0 2
is reduced by autotrophic organisms in that their physiological significance is purely catabolic. Barker showed that in M. omelianskii carbon
dioxide is reduced to methane without being incorporated even transitorily in more complex carbon compounds. With regard to hydrogenlyase, it is also unlikely that the C ± compound produced in this system
can be incorporated into cell constituents and serve as the starting point
of biosyntheses. Woods and Lascelles (22) stressed that the reaction
in which formate is synthesized from C0 2 and H 2 is not very exergonic and cannot provide much free energy for biosynthesis from this
starting point nor for autotrophic development of the organism under
anaerobiosis. It seems that in heterotrophic bacteria the hydrogenlyase
system operates physiologically in reverse, enabling the organism to
oxidize formate anaerobically without the intervention of any exogenous
hydrogen acceptor.
Furthermore, heterotrophic bacteria, yeasts, and higher animals
possess biochemical processes in which carbon dioxide is incorporated
into cell constituents by carboxylation of a C 2 -C 5 compound. These reactions have been studied extensively in the last few years by Utter
and Wood (23), Werkman (24), and Ochoa (25). Table VII lists some
examples; from the point of view of comparative biochemistry, the
most interesting are the modes of formation of oxalacetate by ßcarboxylation of a C 3 -acid. One of these reactions is the carboxylation
of pyruvate by the malic enzyme of Ochoa et al. (26):
C0 2 + CH 3 · CO ■ COOH ^± COOH · CH 2 · CO · COOH
This enzyme, which is found in Lactohacillus arabinosus and in pigeon
liver, takes its name from the fact that in the presence of Coenzyme I
(diphosphopyridine nucleotide, DPN) it also catalyzes the formation
of malate from lactate:
+DPN
C0 2 + CH 3 · CHOH · COOH ;
COOH · CH 2 · CHOH · COOH
359
Escherichia coli and other members of the colon-aerogenes group possess a hydrogenlyase system, one of whose constituents is the enzyme
hydrogenase (cf. Section IV,A,3). This system carries out the reversible
synthesis of formate from C0 2 and H 2 :
C0 2 + H 2 ^± HCOOH
The strict anaerobe Methanobacterium omelianskii reduces C0 2 to
methane by a process which was studied in detail by Barker et al. (21);
carbon dioxide acts as the final hydrogen acceptor for the anaerobic
oxidation of various organic substrates (DH 2 ):
C0 2 + 4Z)H 2 -> CH 4 + 4D + 2H 2 0
It is noteworthy that these processes differ with those in which C0 2
is reduced by autotrophic organisms in that their physiological significance is purely catabolic. Barker showed that in M. omelianskii carbon
dioxide is reduced to methane without being incorporated even transitorily in more complex carbon compounds. With regard to hydrogenlyase, it is also unlikely that the C ± compound produced in this system
can be incorporated into cell constituents and serve as the starting point
of biosyntheses. Woods and Lascelles (22) stressed that the reaction
in which formate is synthesized from C0 2 and H 2 is not very exergonic and cannot provide much free energy for biosynthesis from this
starting point nor for autotrophic development of the organism under
anaerobiosis. It seems that in heterotrophic bacteria the hydrogenlyase
system operates physiologically in reverse, enabling the organism to
oxidize formate anaerobically without the intervention of any exogenous
hydrogen acceptor.
Furthermore, heterotrophic bacteria, yeasts, and higher animals
possess biochemical processes in which carbon dioxide is incorporated
into cell constituents by carboxylation of a C 2 -C 5 compound. These reactions have been studied extensively in the last few years by Utter
and Wood (23), Werkman (24), and Ochoa (25). Table VII lists some
examples; from the point of view of comparative biochemistry, the
most interesting are the modes of formation of oxalacetate by ßcarboxylation of a C 3 -acid. One of these reactions is the carboxylation
of pyruvate by the malic enzyme of Ochoa et al. (26):
C0 2 + CH 3 · CO ■ COOH ^± COOH · CH 2 · CO · COOH
This enzyme, which is found in Lactohacillus arabinosus and in pigeon
liver, takes its name from the fact that in the presence of Coenzyme I
(diphosphopyridine nucleotide, DPN) it also catalyzes the formation
of malate from lactate:
+DPN
C0 2 + CH 3 · CHOH · COOH ;
COOH · CH 2 · CHOH · COOH