Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Carbohydrate Biosynthesis in Animal Tissues
The gluconeogenesis pathway comprises seven steps shared with glycolysis.

Just as The conversion of glucose to Pyruvate is the central pathway in Carbohydrate Catabolism, the conversion of pyruvate to glucose is the central pathway of Gluconeogenesis. These pathways are not identical, although they share A number of common steps (Fig. 20-2). Seven ENZYMATIC REACTIONS OF Glycolysis are also common to gluconeogenesis; all of them are readily reversible.

However, glycolysis features three virtually irreversible steps that, for this reason, cannot be utilized in gluconeogenesis. To bypass these steps, gluconeogenesis employs alternative reactions with different stoichiometry, catalyzed by different Enzymes; these are exclusive to gluconeogenesis and do not operate in glycolysis (Fig. 20-2). These bypass reactions (which we will examine below) are also irreversible, but they proceed in the direction of glucose synthesis. Consequently, both glycolysis and gluconeogenesis are essentially irreversible processes within Cells. Furthermore, as we will see, these pathways are regulated independently of one another—regulation is exerted precisely at those enzymatic steps that are not shared by the two pathways.

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Fig. 20-2. Oppositely directed pathways of glycolysis and gluconeogenesis in rat Liver (the gluconeogenesis pathway is shown in red).

In some species, phosphoenolpyruvate is formed in the Cytosol without mitochondrial involvement. The scheme also indicates the two main regulatory checkpoints of gluconeogenesis. G6P stands for glucose-6-phosphate; G1P - glucose-1-phosphate; F6P - fructose-6-phosphate; FDP - fructose biphosphate; G3P - glyceraldehyde-3-phosphate; DHAP - dihydroxyacetone phosphate; PGP - 3-phosphoglyceryl phosphate; 3PG - 3-phosphoglycerate; 2PG - 2-phosphoglycerate.