Principles Of Metabolic Regulation Glucose And Glycogen

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15.1 The Metabolism of Glycogen in Animals 562

15.2 Regulation of Metabolic Pathways 571

15.3 Coordinated Regulation of Glycolysis and Gluconeogenesis 575

15.4 Coordinated Regulation of Glycogen Synthesis and Breakdown 583

15.5 Analysis of Metabolic Control 591

Formation of liver glycogen from lactic acid is thus seen to establish an important connection between the metabolism of the muscle and that of the liver. Muscle glycogen becomes available as blood sugar through the intervention of the liver, and blood sugar in turn is converted into muscle glycogen. There exists therefore a complete cycle of the glucose molecule in the body . . . Epinephrine was found to accelerate this cycle in the direction of muscle glycogen to liver glycogen ... Insulin, on the other hand, was found to accelerate the cycle in the direction of blood glucose to muscle glycogen.

-C. F. Cori and G. T. Cori, article in Journal of Biological Chemistry, 1929

Metabolic regulation, a central theme in biochemistry, is one of the most remarkable features of a living cell. Of the thousands of enzyme-catalyzed reactions that can take place in a cell, there is probably not one that escapes some form of regulation. Although it is convenient (and perhaps essential) in writing a textbook to divide metabolic processes into "pathways" that play discrete roles in the cell's economy, no such separation exists inside the cell. Rather, each of the pathways we discuss in this book is inextricably intertwined with all the other cellular pathways in a multidimensional network of reactions (Fig. 15-1). For example, in Chapter 14 we discussed three possible fates for glucose 6-phosphate in a hepatocyte: passage into glycolysis for the production of ATP, passage into the pentose phosphate pathway for the production of NADPH and pentose phosphates, or hydrolysis to glucose and phosphate to replenish blood glucose. In fact, glucose 6-phos-phate has a number of other possible fates; it may, for example, be used to synthesize other sugars, such as glucosamine, galactose, galactosamine, fucose, and neu-raminic acid, for use in protein glycosylation, or it may be partially degraded to provide acetyl-CoA for fatty acid and sterol synthesis. In the extreme case, the bacterium Escherichia coli can use glucose to produce the carbon skeleton of every one of its molecules. When a cell "decides" to use glucose 6-phosphate for one purpose, that decision affects all the other pathways for which glucose 6-phosphate is a precursor or intermediate; any change in the allocation of glucose 6-phosphate to one pathway affects, directly or indirectly, the metabolite flow through all the others.

Such changes in allocation are common in the life of a cell. Louis Pasteur was the first to describe the large (greater than tenfold) increase in glucose consumption by a yeast culture when it was shifted from aerobic to anaerobic conditions. This phenomenon, called the

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