9. COMPARATIVE BIOCHEMISTRY OF GLYCOLYSIS
417
III. Glycolysis in Specialized Tissues and Cells
A. BACTERIA
The occurrence of the Embden-Meyerhof scheme of phosphorylating
glycolysis has been observed in numerous bacteria. However, in many
of them this is not the only mechanism of carbohydrate utilization. At
least two other pathways have been demonstrated.
1. The Pentose Phosphate Pathway (51, 52)
Following the dehydrogenation of glucose-6-phosphate (53, 54), the
latter is oxidized to pentosephosphate (53, 55-61) and hexosephosphate
is regenerated through the actions of transketolase (51, 62-65) and of
transaldolase (66-68). Transketolase catalyzes the conversion of 2 moles
of pentosephosphate to 1 mole of sedoheptulose-7-phosphate and 1 mole
of glyceraldehyde-3-phosphate. These products yield fructoses-phosphate and a tetrosephosphate, probably erythrose-4-phosphate (transaldolase reaction). This series of reactions also occurs in liver (64, 69)
and in plants (64).
2. The 2-Keto-3-desoxy-6-phosphogluconate
Pathway
Certain pseudomonads and aerobacters oxidize glucose to gluconic
acid without prior phosphorylation (70-75). This is brought about by
the action of a glucose oxidase (76-77). Two pathways of gluconate
metabolism have been found in these organisms, (a) Gluconate is
phosphorylated in the presence of ATP by gluconokinase (78, 79) to
6-phosphogluconate which undergoes enzymatic degradation and rearrangement to 2-keto-3-desoxy-6-phosphogruconic acid (80-82). This
compound, through an aldolase type of reaction is degraded to glyceraldehyde-3-phosphate and to pyruvate (80, 81, 83, 84). Alternatively,
phosphogluconate may be metabolized via the pentose phosphate route
(85, 86) (see above), (b) Gluconate is oxidized to 2-ketogluconate
(74, 75, 79, 87-89) which is phosphorylated to 2-keto-6-phosphogluconate (79, 88) and then reduced by TPNH to 6-phosphogluconate (9092). This in turn is metabolized either via the pentose phosphate or the
2-keto-3-desoxy-6-phosphogluconate pathways.
Attempts have been made to ascertain the relative contribution of
each of these pathways by the use of isotopically labeled glucose followed by the determination of the isotope distribution of various
metabolic products. The limitations of this approach have been analyzed
(93) and it is recognized that at best only semiquantitative information
about this problem can be obtained in this manner (94). Among bac-
417
III. Glycolysis in Specialized Tissues and Cells
A. BACTERIA
The occurrence of the Embden-Meyerhof scheme of phosphorylating
glycolysis has been observed in numerous bacteria. However, in many
of them this is not the only mechanism of carbohydrate utilization. At
least two other pathways have been demonstrated.
1. The Pentose Phosphate Pathway (51, 52)
Following the dehydrogenation of glucose-6-phosphate (53, 54), the
latter is oxidized to pentosephosphate (53, 55-61) and hexosephosphate
is regenerated through the actions of transketolase (51, 62-65) and of
transaldolase (66-68). Transketolase catalyzes the conversion of 2 moles
of pentosephosphate to 1 mole of sedoheptulose-7-phosphate and 1 mole
of glyceraldehyde-3-phosphate. These products yield fructoses-phosphate and a tetrosephosphate, probably erythrose-4-phosphate (transaldolase reaction). This series of reactions also occurs in liver (64, 69)
and in plants (64).
2. The 2-Keto-3-desoxy-6-phosphogluconate
Pathway
Certain pseudomonads and aerobacters oxidize glucose to gluconic
acid without prior phosphorylation (70-75). This is brought about by
the action of a glucose oxidase (76-77). Two pathways of gluconate
metabolism have been found in these organisms, (a) Gluconate is
phosphorylated in the presence of ATP by gluconokinase (78, 79) to
6-phosphogluconate which undergoes enzymatic degradation and rearrangement to 2-keto-3-desoxy-6-phosphogruconic acid (80-82). This
compound, through an aldolase type of reaction is degraded to glyceraldehyde-3-phosphate and to pyruvate (80, 81, 83, 84). Alternatively,
phosphogluconate may be metabolized via the pentose phosphate route
(85, 86) (see above), (b) Gluconate is oxidized to 2-ketogluconate
(74, 75, 79, 87-89) which is phosphorylated to 2-keto-6-phosphogluconate (79, 88) and then reduced by TPNH to 6-phosphogluconate (9092). This in turn is metabolized either via the pentose phosphate or the
2-keto-3-desoxy-6-phosphogluconate pathways.
Attempts have been made to ascertain the relative contribution of
each of these pathways by the use of isotopically labeled glucose followed by the determination of the isotope distribution of various
metabolic products. The limitations of this approach have been analyzed
(93) and it is recognized that at best only semiquantitative information
about this problem can be obtained in this manner (94). Among bac-