Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Glycolysis: The Central Pathway of Glucose Catabolism
The entry of glucose residues into the glycolytic pathway is regulated

The rates of the main catabolic reactions responsible for glucose breakdown and the generation of chemical energy in the form of ATP are regulated at any given moment According to the cellular demand for ATP, regardless of how that ATP will subsequently be utilized—whether in biosynthetic reactions, Active Transport, or mechanical work in contractile structures. Because glucose degradation products play a vital role both as precursors and as intermediates in other metabolic processes, the regulatory Enzymes of Carbohydrate Catabolism also recognize and respond to corresponding signals from other metabolic pathways. We will now examine the regulatory enzymes that control The rate of carbohydrate breakdown along The Glycolytic Pathway.

Let us first consider how the initial entry of glucose residues into the glycolytic pathway is regulated. The involvement of glucose residues in Glycolysis is driven by two key reactions, both of which are controlled by regulatory enzymes. The first such reaction is the hexokinase-catalyzed phosphorylation of free glucose at the C-6 position using ATP. In certain Tissues, such as Skeletal Muscle, hexokinase Functions as an allosteric enzyme and is inhibited by the reaction product, glucose-6-phosphate, as shown in Fig. 15-13. Whenever the intracellular concentration of glucose-6-phosphate rises sharply—that is, when it is produced faster than it is consumed—inhibition occurs: hexokinase is shut down by glucose-6-phosphate, and no further phosphorylation of glucose takes place until the excess glucose-6-phosphate is utilized. In the Liver, however, another enzyme predominates: glucokinase, which is not inhibited by glucose-6-phosphate (Section 15.6, a). Therefore, in the liver, which is capable of storing large amounts of Glycogen, excess Blood glucose can be phosphorylated to yield glucose-6-phosphate, which is subsequently converted via glucose-1-phosphate into glycogen, a storage polysaccharide. When blood glucose levels rise, the hormone Insulin, secreted into the blood by the Pancreas, stimulates glucokinase synthesis. In diabetes and during fasting, glucokinase activity is decreased.

Class="center">

Fig. 15-13. Mechanism regulating the incorporation of glucose residues into glycolysis and their subsequent breakdown along this pathway. Regulatory inhibition is indicated by dashed red arrows pointing to the blocked step (a red bar across the arrow indicating the direction of the reaction); regulatory stimulation is indicated by bold red arrows parallel to the reaction arrows. G1P, glucose-1-phosphate; G6P, glucose-6-phosphate; F6P, fructose-6-phosphate; FDP, fructose-1,6-diphosphate; TP, triose phosphate; 3PG, 3-phosphoglycerate; 2PG, 2-phosphoglycerate; PEP, phosphoenolpyruvate.

In the second reaction supplying glucose residues for glycolysis, glycogen serves as the substrate. This reaction is catalyzed by Glycogen phosphorylase, which is also a regulatory enzyme. In both The Liver and Muscles, glycogen phosphorylase occupies a strategic position between the fuel reservoir (glycogen) and the glycolytic system, whose purpose is to utilize that fuel. In skeletal muscle, this enzyme exists in two forms: the catalytically active phosphorylated form (phosphorylase a) and the significantly less active dephosphorylated form (phosphorylase b). Phosphorylase a has been obtained in crystalline form (molecular mass 190,000). Its molecules consist of two identical subunits, each containing a Serine residue essential for catalytic activity in its phosphorylated form (Fig. 15-14). The rate of conversion of glycogen structural units into glucose-1-phosphate in muscle is regulated by The ratio of active phosphorylase a to less active phosphorylase b.

Fig. 15-14. Regulation of Glycogen phosphorylase activity. The enzyme molecule consists of two subunits, each containing a serine residue critical for catalytic activity. The hydroxyl groups of these serine residues in both subunits are phosphorylated by phosphorylase kinase, yielding phosphorylase a. This reaction is stimulated by Ca²⁺ ions. The dephosphorylation of phosphorylase a is inhibited by Ca²⁺ ions and AMP. Phosphorylase b can also be activated by the non-covalent binding of AMP to allosteric sites on the enzyme molecule. The conformational changes undergone by the enzyme are illustrated schematically here.

The interconversion of these two forms of glycogen phosphorylase is mediated by specific enzymes that catalyze covalent modification (Section 9.22) of the phosphorylase. Phosphorylase a is converted into the less active phosphorylase b by an enzyme known as phosphorylase a phosphatase; by catalyzing the hydrolytic Cleavage of bonds, this enzyme removes the phosphate groups necessary for catalytic activity from the phosphorylase a molecule (Fig. 15-14). Phosphorylase b is converted back into active phosphorylase a by phosphorylase b kinase, which catalyzes a reaction in which ATP phosphorylates serine residues at the Active Site of the phosphorylase b molecule, resulting in The formation of phosphorylase a. Thus, through the action of these two enzymes—phosphorylase a phosphatase and phosphorylase b kinase—the ratio of active phosphorylase a to the relatively inactive phosphorylase b in The Cell can be dynamically adjusted.

A second mechanism regulates glycogen phosphorylase activity in muscle. Phosphorylase b, the relatively inactive form, can be activated through the non-covalent binding of an allosteric modulator, AMP. The concentration of AMP in muscle rises as ATP is hydrolyzed during contractile activity (Fig. 15-14, see also Section 14.17). The activation of phosphorylase b by AMP is antagonized by ATP, which acts as a negative modulator. Consequently, The activity of phosphorylase b is determined by the AMP-to-ATP ratio. Unlike phosphorylase b, phosphorylase a is not activated by AMP; therefore, phosphorylase a is sometimes referred to as the AMP-independent form, and phosphorylase b as the AMP-dependent form. Thus, skeletal muscle glycogen phosphorylase is subject to two regulatory mechanisms: (1) covalent modification via phosphorylation or dephosphorylation of serine residues at the active site of the enzyme, and (2) Allosteric Regulation of phosphorylase b through the non-covalent binding of AMP or ATP. In resting muscle, almost all of the phosphorylase is in the inactive, or b-form, because the intracellular concentration of ATP vastly exceeds that of AMP.