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Chapter22SUMMARY - Summary 22.1 What Is Gluconeogenesis and...

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Unformatted text preview: [ Summary 22.1 What Is Gluconeogenesis, and How Does It Operate? Gluconeogenesis is the generation (genesis) of new (nee) glucose. In addi- tion to pyruvate and lactate, other noncarbohydrate precursors can be used as substrates for gluconeogenesis in animals, including most of the amino acids, as well as glycerol and all the TCA cycle intermediates. On the other hand, fatty acids are not substrates for gluconeogenesis in ani- mals. Lysine and leucine are the only amino acids that are not substrates for gluconeogenesis. These amino acids produce only acetyl-CoA upon degradation. Acetyl-CoA is a substrate for gluconeogenesis when the gly- oxylate cycle is operating. The major sites of gluconeogenesis are the liver and kidneys, which account for about 90% and 10% of the body’s gluconeogenic activity, respectively. 22.2 How Is Gluconeogenesis Regulated? Glycolysis and glu- coneogenesis are under reciprocal control, so glycolysis is inhibited when gluconeogenesis is active, and vice versa. When the energy status of the cell is low, glucose is rapidly degraded to produce needed energy. When the energy status is high, pyruvate and other metabolites are uti- lized for synthesis (and storage) of glucose. The three sites of regulation in the gluconeogenic pathway are glucose-G-phosphatase, fructose-1,6— bisphosphatase, and the pyruvate carboxylase—PEP carboxykinase pair, respectively. These are the three most appropriate sites of regulation in gluconeogenesis. Glucose-6-phosphatase is under substrate-level con- trol by glucose-6-phosphate. AcetyLCoA allosterically activates pyruvate carboxylase. Fructose-1,6-bisph'osphatase is inhibited by AMP and acti- vated by citrate. Fructose-2,6—bisphosphate is a powerful inhibitor of fructose—1,6-bisphosphatase. 22.3 How Are Glycogen and Starch Catabolized in Animals? Almost 100% of digestible food is absorbed and metabolized. Digestive breakdown of starch and glycogen is an unregulated process. On the other hand, tissue glycogen represents an important reservoir of poten- tial energy, and the reactions involved in its degradation and synthesis are carefully controlled and regulated. Glycogen reserves in liver and muscle tissue are stored in the cytosol as granules exhibiting a molecu- lar weight range from 6 X 106 to 1600 X 106. These granular aggregates Q contain the enzymes required to synthesize and catabolize the glycogen, as well as all the enzymes of glycolysis. The principal enzyme of glycogen catabolism is glycogen phosphorylase, a highly regulated enzyme. The glycogen phosphorylase reaction involves phosphorolysis at a nonre- ducing end of a glycogen polymer. 22.4 Howls Glycogen Synthesized? Luis Leloir, a biochemist in Argentina, showed in the 1950s that glycogen synthesis depended upon sugar nucleotides. The glycogen polymer is built around a tiny protein core. The first glucose residue is covalentlyjoined to the protein glyco- genin via an acetal linkage to a tyrosine—OH group on the protein. Sugar units are added to the glycogen polymer by the action of glyco- gen synthase. The reaction involves transfer of a glucosyl unit from UDP—glucose to the C-4 hydroxyl group at a nonreducing end of a glycogen strand. The mechanism proceeds by cleavage of the C—0 bond between the glucose moiety and the B—phosphate of UDP—glucose, leaving an oxonium ion intermediate, which is rapidly attacked by the C—4 hydroxyl oxygen of a terminal glucose unit on glycogen. 22.5 How Is Glycogen Metabolism Controlled? Activation of glycogen phosphorylase is tightly linked to inhibition of glycogen syn< thase, and vice versa. Regulation involves both allosteric control and covalent modification, with the latter being under hormonal control. Glycogen synthase is regulated by covalent modification. Storage and uti- lization of tissue glycogen are regulated by hormones, including insulin, glucagon, epinephrine, and the glucocorticoids. Insulin stimulates glyco- gen synthesis and inhibits glycogen breakdown in liver and muscle, whereas glucagons and epinephrine stimulate glycogen breakdown. 22.6 Can Glucose Provide Electrons for Biosynthesis? The pentose phosphate pathway is a collection of eight reactions that provide NADPH for biosynthetic processes and ribosec5-phosphate for nucleic acid synthesis. Several metabolites of the pentose phosphate pathway can also be shuttled into glycolysis. Utilization of glucose-fi-P in the pentose phosphate pathway depends on the cell’s need for ATP, NADPH, and ribose—5—P. ...
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