Biochemistry and Molecular Biology - Belyasova, N. A. 2002

Metabolism. Processes Requiring Energy Input
Features of Biosynthesis. Carbohydrate Biosynthesis
Patterns of Monosaccharide Metabolism and Biosynthesis

Glucose-6-phosphate can be considered a key intermediate in Carbohydrate METABOLISM. This compound can be formed from exogenous glucose via its phosphorylation during membrane transport or immediately afterward. In addition, glucose-6-phosphate is produced in the Cells of autotrophic organisms capable of CO₂ fixation, as well as through Gluconeogenesis from glyceraldehyde-3-phosphate (Fig. 13.5). Other sources of glucose-6-phosphate include Pyruvate (generated by most carbohydrate Catabolic pathways), several tricarboxylic acid (TCA) cycle intermediates, carbon skeletons of glycogenic Amino Acids, lactate (the product of Lactic acid Fermentation), and glycerol (produced during lipid breakdown). All of these substances can enter gluconeogenesis—the Synthesis of glucose from non-carbohydrate precursors. Furthermore, in PLANT CELLS AND many microorganisms, glucose can be formed from acetyl-CoA, a breakdown product of Fatty acids. This pathway utilizes two specific Enzymes of The Glyoxylate cycle that are absent in higher animals. These enzymes (isocitrate lyase and malate synthase) participate in The conversion of acetyl-CoA into succinate, which is subsequently converted into oxaloacetate (Fig. 11.7) and enters gluconeogenesis. Finally, Reserve Polysaccharides serve as yet another source of glucose and, consequently, glucose-6-phosphate.

Glucose-6-phosphate is utilized within cells in the following ways. First, it acts as a substrate for various catabolic pathways that store energy, generate reducing equivalents, and provide building blocks (Chapter 9). Second, glucose-6-phosphate can undergo isomerization to glucose-1-phosphate, serving as a substrate for The Biosynthesis of Glycogen or other storage and structural polysaccharides. Glucose-6-phosphate can also be dephosphorylated to yield glucose, which can then be transformed within cells into other Monosaccharides, such as galactose, mannose, fructose, and glucuronic acid. These compounds participate in The formation of complex Cell-wall polysaccharides and other cellular structures.

Most of the aforementioned pathways for monosaccharide synthesis and utilization have already been discussed in previous sections. Here, we will focus on one of the central pathways for producing this key carbohydrate intermediate—glucose-6-phosphate—namely, gluconeogenesis.

Gluconeogenesis. This process occurs in representatives of all domains of life, but it is of paramount importance for higher animal cells. Embryonic Tissues, the Brain, Testes, and erythrocytes rely exclusively on D-glucose as a carbon source. During carbohydrate deprivation, glycogen breakdown is stimulated in the Liver, but even this source may prove insufficient (the human brain consumes over 120 g of glucose per day). Under such conditions, glucose is synthesized within the Organism from non-carbohydrate precursors via gluconeogenesis. In animals, gluconeogenesis is most active in liver and Kidney cells.

The reactions of gluconeogenesis are largely the reverse of Glycolysis, and many of them are catalyzed by the same enzymes involved in The Glycolytic Pathway (Fig. 14.1).

Thus, glycolysis contains three virtually irreversible reactions, which are bypassed in gluconeogenesis by specific detour pathways. The first bypass involves the conversion of pyruvate into phosphoenolpyruvate. The energy released from ATP Hydrolysis is insufficient to directly convert pyruvate to phosphoenolpyruvate; therefore, this step proceeds through a series of reactions. First, pyruvate—generated primarily in the Cytoplasm (from lactate, amino acids, and glycolysis)—is transported into the Mitochondria and carboxylated to oxaloacetate (Fig. 14.2). This reaction is catalyzed by pyruvate carboxylase, which utilizes biotin as a cofactor. This anaplerotic reaction and The properties of the enzyme are described in Section 11.2.

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Fig. 14.1. Reaction sequences of glycolysis (dashed arrows) and gluconeogenesis (solid arrows). Enzymes common to both pathways are labeled. The steps where the two processes diverge are designated as bypass pathways in gluconeogenesis.

Within the mitochondria, oxaloacetate is reduced to malate by mitochondrial malate dehydrogenase, and malate is then transported into the cytoplasm via specific carriers. In the cytoplasm, malate is re-oxidized to oxaloacetate by cytoplasmic malate dehydrogenase, which is subsequently decarboxylated to phosphoenolpyruvate (PEP) by a GTP-dependent phosphoenolpyruvate carboxykinase.

The second bypass in gluconeogenesis is the conversion of fructose-1,6-bisphosphate into fructose-6-phosphate (Fig. 14.2). In glycolysis, the Phosphofructokinase reaction, accompanied by ATP hydrolysis, is irreversible. Gluconeogenesis utilizes a different enzyme, fructose-1,6-bisphosphatase, which catalyzes the practically irreversible removal of the phosphate group from the first carbon atom. Like pyruvate carboxylase, fructose-1,6-bisphosphatase is an allosteric enzyme; its activity is inhibited by AMP and activated by ATP.

The third bypass—the dephosphorylation of glucose-6-phosphate—cannot be achieved by simply reversing the hexokinase reaction. Instead, this reaction is catalyzed by glucose-6-phosphate phosphatase, which is localized on the inner surface of the smooth Endoplasmic reticulum (ER) membrane. Consequently, for this reaction to take place, glucose-6-phosphate must be transported into the ER, where it is dephosphorylated to free glucose (Fig. 14.2). It is worth noting that glucose-6-phosphate phosphatase is absent in tissues such as Skeletal Muscle and the brain; therefore, these tissues cannot supply free glucose to the bloodstream.

The overall equation for gluconeogenesis is as follows:

2Pyruvate + 4ATP + 2GTP + 2NADH + 2H⁺ + 4H₂O →

→ Glucose + 2NAD⁺ + 4ADP + 2GDP + 6 P_i

As this balance indicates, the synthesis of a single glucose molecule via gluconeogenesis consumes six high-energy phosphate bonds and two molecules of NADH. Notably, The rate of glucose synthesis in this pathway is regulated by enzymes that do not participate in glycolysis. Furthermore, gluconeogenesis proceeds most intensively under conditions where cellular fuel molecules, particularly acetyl-CoA, and ATP are abundant.

Glycerol enters the gluconeogenic pathway at the level of dihydroxyacetone phosphate, into which it is converted following phosphorylation (mediated by ATP) and dehydrogenation.

Amino acids enter the pathway via metabolites such as pyruvate and oxaloacetate, which are formed through the rearrangement of their carbon skeletons. Before entering gluconeogenesis, lactate must first be oxidized to pyruvate.

Fig. 14.2. Reactions of the bypass pathways in gluconeogenesis.