146
F. M. HUENNEKENS AND H. R. WHITELEY
glycolysis. Phosphate may also be transferred from PEP to glycerol and
other alcohols by the action of phosphatase (299); the importance of
this reaction in cellular economy is probably limited.
The formation of oxaloacetate from PEP by reversal of Reaction 57
provides an important mechanism for the fixation of carbon dioxide by
a variety of animal, plant, and microbial cells. In heterotrophs, Reaction 57 is the chief mechanism for carbon dioxide fixation (279, 302).
In plants and many autotrophic bacteria (91ir-91k, 303-306a), the carboxydismutase reaction (discussed in Vol. I, Chapter 9) appears to be
the major route of entry of carbon dioxide. However, these systems
also contain oxaloacetate carboxylase (303, 307-310), although this enzyme may function solely to provide oxaloacetate for the synthesis of
citrate or amino acids.
In autotrophs, carbon dioxide fixation is also achieved (293, 301,
311-313) via Reaction 59:
PEP + C0 2 -> Oxaloacetate + P<
(59)
This irreversible reaction is catalyzed by PEP carboxylase, and does
not require the participation of any nucleotide. The enzyme has a high
affinity for carbon dioxide (311, 313), thereby permitting Reaction 59
to take place at low tensions of carbon dioxide. It has been suggested
that the action of PEP carboxylase would account for the accumulation
of C 4 acids by plants in the dark (301, 311-313), and for the fixation of
carbon dioxide by Thiobacillus thio-oxidans (308-309a). The latter
autotroph is thus able to utilize carbon dioxide as the sole carbon
source in an acid environment, in spite of the fact that the carbon
dioxide concentration is extremely small.
It has been proposed that oxaloacetate carboxylase may play an
important role in glycogenesis through the synthesis of PEP (314).
Oxaloacetate could be formed from the di- and tricarboxylic acids of
the citric acid cycle (315-317), or could arise by reductive carboxylation of pyruvate by the "malic enzyme," followed by oxidation of malate
to oxaloacetate. "Malic enzyme," malic dehydrogenase, and oxaloacetate carboxylase have been found in tissues (284, 287, 288, 301) that
are unable to produce appreciable quantities of PEP by Reaction 54.
It has been postulated also that PEP condenses with erythrose-4phosphate to yield 2-keto-3-deoxy-7-phosphoglucoheptonic acid (318,
319), which, in turn, may be dephosphorylated and cyclized to 5-dehydroshikimic acid (320). The latter compound is an intermediate in
the synthesis of tyrosine, tryptophan, and other aromatic compounds.
The production of 2-keto-3-deoxy-8-phosphooctonic acid (a precursor
of certain cell wall constituents) from PEP and arabinose-5-phosphate
has been demonstrated with a bacterial system (320a).
F. M. HUENNEKENS AND H. R. WHITELEY
glycolysis. Phosphate may also be transferred from PEP to glycerol and
other alcohols by the action of phosphatase (299); the importance of
this reaction in cellular economy is probably limited.
The formation of oxaloacetate from PEP by reversal of Reaction 57
provides an important mechanism for the fixation of carbon dioxide by
a variety of animal, plant, and microbial cells. In heterotrophs, Reaction 57 is the chief mechanism for carbon dioxide fixation (279, 302).
In plants and many autotrophic bacteria (91ir-91k, 303-306a), the carboxydismutase reaction (discussed in Vol. I, Chapter 9) appears to be
the major route of entry of carbon dioxide. However, these systems
also contain oxaloacetate carboxylase (303, 307-310), although this enzyme may function solely to provide oxaloacetate for the synthesis of
citrate or amino acids.
In autotrophs, carbon dioxide fixation is also achieved (293, 301,
311-313) via Reaction 59:
PEP + C0 2 -> Oxaloacetate + P<
(59)
This irreversible reaction is catalyzed by PEP carboxylase, and does
not require the participation of any nucleotide. The enzyme has a high
affinity for carbon dioxide (311, 313), thereby permitting Reaction 59
to take place at low tensions of carbon dioxide. It has been suggested
that the action of PEP carboxylase would account for the accumulation
of C 4 acids by plants in the dark (301, 311-313), and for the fixation of
carbon dioxide by Thiobacillus thio-oxidans (308-309a). The latter
autotroph is thus able to utilize carbon dioxide as the sole carbon
source in an acid environment, in spite of the fact that the carbon
dioxide concentration is extremely small.
It has been proposed that oxaloacetate carboxylase may play an
important role in glycogenesis through the synthesis of PEP (314).
Oxaloacetate could be formed from the di- and tricarboxylic acids of
the citric acid cycle (315-317), or could arise by reductive carboxylation of pyruvate by the "malic enzyme," followed by oxidation of malate
to oxaloacetate. "Malic enzyme," malic dehydrogenase, and oxaloacetate carboxylase have been found in tissues (284, 287, 288, 301) that
are unable to produce appreciable quantities of PEP by Reaction 54.
It has been postulated also that PEP condenses with erythrose-4phosphate to yield 2-keto-3-deoxy-7-phosphoglucoheptonic acid (318,
319), which, in turn, may be dephosphorylated and cyclized to 5-dehydroshikimic acid (320). The latter compound is an intermediate in
the synthesis of tyrosine, tryptophan, and other aromatic compounds.
The production of 2-keto-3-deoxy-8-phosphooctonic acid (a precursor
of certain cell wall constituents) from PEP and arabinose-5-phosphate
has been demonstrated with a bacterial system (320a).