4. ENERGY-RICH COMPOUNDS
145
Wood (279)], the fixation reaction involved carbon dioxide and free
pyruvate (Reaction 58).
CO2 + Pyruvate ;=± Oxaloacetate
(58)
In some bacteria, notably Micrococcus lysodeikticus (280-283), Reaction 58 is the major route for carbon dioxide fixation. In most bacteria,
however, and in animal and some plant tissues, Reaction 57 in reverse
accounts for the uptake of carbon dioxide (284, 285).
The nucleotide in Reaction 57 was first thought to be ATP (284,
285), but was later shown to be ITP (286). The effectiveness of ATP
in earlier experiments (273, 286-289) could be attributed to its contamination by ITP and to the presence of transphosphorylating enzymes (52, 290) in the oxaloacetate carboxylase preparation. An absolute requirement for ITP was found with an enzyme obtained from
lamb liver (291), whereas an enzyme purified from avian liver had a
higher activity with GTP than with ITP (292). In contrast, a purified
yeast enzyme utilizes only ATP (292a). It has been suggested
(293, 294) that a phosphorylated derivative of oxaloacetate may be
formed in Reaction 57, but conclusive evidence on this point is still
lacking (295).
The biosynthesis of PEP may also be achieved via the pyruvate
phosphokinase (296-298) reaction (cf. Eq. 54). Studies on the synthesis
of pyruvate from PEP showed that ADP can be replaced by IDP
and GDP (299). A comparison of Eqs. 54 and 57 reveals that
PEP may participate with a nucleoside diphosphate in two similar reactions, both yielding a nucleoside triphosphate. Thus, an unequivocal
demonstration (287, 300) of oxaloacetate carboxylase can be attained
only after the preparation has been freed from pyruvate phosphokinase.
An alternate method of following Reaction 57 makes use of the nucleotide-dependent exchange of C
14 0 2 into oxaloacetate.
The equilibrium position of Reaction 54 lies far to the left; thus,
only small amounts of PEP can be formed by the action of pyruvate
phosphokinase, unless the products are removed by coupled reactions.
However, the synthesis of PEP from pyruvate can easily take place by
the following sequence of reactions (301): (a) reductive carboxylation
of pyruvate to malate by the "malic enzyme" (300); (b) oxidation of
malate to oxaloacetate; and (c) decarboxylation of oxaloacetate via
Reaction 57.
c. Function. The transphosphorylation of nucleotides from PEP via
Reaction 54 constitutes one of the key reactions in glycolysis. The
equilibrium of Reaction 54 favors the formation of the triphosphate
and provides one of the two net "energy-rich" bonds generated in
145
Wood (279)], the fixation reaction involved carbon dioxide and free
pyruvate (Reaction 58).
CO2 + Pyruvate ;=± Oxaloacetate
(58)
In some bacteria, notably Micrococcus lysodeikticus (280-283), Reaction 58 is the major route for carbon dioxide fixation. In most bacteria,
however, and in animal and some plant tissues, Reaction 57 in reverse
accounts for the uptake of carbon dioxide (284, 285).
The nucleotide in Reaction 57 was first thought to be ATP (284,
285), but was later shown to be ITP (286). The effectiveness of ATP
in earlier experiments (273, 286-289) could be attributed to its contamination by ITP and to the presence of transphosphorylating enzymes (52, 290) in the oxaloacetate carboxylase preparation. An absolute requirement for ITP was found with an enzyme obtained from
lamb liver (291), whereas an enzyme purified from avian liver had a
higher activity with GTP than with ITP (292). In contrast, a purified
yeast enzyme utilizes only ATP (292a). It has been suggested
(293, 294) that a phosphorylated derivative of oxaloacetate may be
formed in Reaction 57, but conclusive evidence on this point is still
lacking (295).
The biosynthesis of PEP may also be achieved via the pyruvate
phosphokinase (296-298) reaction (cf. Eq. 54). Studies on the synthesis
of pyruvate from PEP showed that ADP can be replaced by IDP
and GDP (299). A comparison of Eqs. 54 and 57 reveals that
PEP may participate with a nucleoside diphosphate in two similar reactions, both yielding a nucleoside triphosphate. Thus, an unequivocal
demonstration (287, 300) of oxaloacetate carboxylase can be attained
only after the preparation has been freed from pyruvate phosphokinase.
An alternate method of following Reaction 57 makes use of the nucleotide-dependent exchange of C
14 0 2 into oxaloacetate.
The equilibrium position of Reaction 54 lies far to the left; thus,
only small amounts of PEP can be formed by the action of pyruvate
phosphokinase, unless the products are removed by coupled reactions.
However, the synthesis of PEP from pyruvate can easily take place by
the following sequence of reactions (301): (a) reductive carboxylation
of pyruvate to malate by the "malic enzyme" (300); (b) oxidation of
malate to oxaloacetate; and (c) decarboxylation of oxaloacetate via
Reaction 57.
c. Function. The transphosphorylation of nucleotides from PEP via
Reaction 54 constitutes one of the key reactions in glycolysis. The
equilibrium of Reaction 54 favors the formation of the triphosphate
and provides one of the two net "energy-rich" bonds generated in