Metabolism & its regulation, a central theme in biochemistry, is one of the most remarkable features of living organisms. Metabolism is a highly coordinated cellular activity in which many multienzyme systems (metabolic pathways) cooperate (1) to obtain chemical energy by degrading energy rich nutrients from the environment; (2) convert nutrient molecules into the cell's own characteristic molecules, including precursors of macromolecules; (3) polymerize monomeric precursors into macromolecules proteins, nucleic acids, and polysaccharides and; (4) synthesize and degrade biomolecules required for specialized cellular functions, such as membrane lipids, intracellular messengers, etc.
Glucose is not only an excellent fuel, it is also a remarkably versatile precursor, capable of supplying a huge array of metabolic intermediates for biosynthetic reactions. A comprehensive study of the metabolic fates of glucose would encompass hundreds or thousands of transformations. By storing glucose as a high molecular weight polymer such as starch or glycogen a cell can stockpile large quantities of hexose units while maintaining a relatively low cytosolic osmolarity. When energy demands increase glucose can be released from these intracellular storage pools and used to produce ATP either aerobically or anaerobically. In humans, glucose has four major fates: it may be used in the synthesis of complex polysaccharide destined for the extracellular space; stored in cells (as a polysaccharides; glycogen); oxidized to a three-carbon compound (pyruvate) via glycolysis to provide ATP and metabolic intermediates; or oxidized via the pentose phosphate (Phospho-gluconate pathway) to yield ribose 5-phosphate for nucleic acid synthesis and NADPH for reductive biosynthetic processes. also discussed. In fact, glucose (glucose 6-phosphate) has other possible fates in hepatocytes too; it may, for example, be used to synthesize other sugars, such as glucosamine galactose, galactosamine, fucose, and neuraminic acid, and there is another minor biosynthetic pathway, the via uronic acid pathway, for the synthesis of proteoglycans and pentoses.
Metabolism of carbohydrates encompass the study of the individual reactions of glycolysis, gluconeogenesis and the pentose phosphate pathway and the functional significance of each pathway. Organisms when they do not have access to glucose from other sources must make it. Cells of certain tissues can make glucose from simpler three- and four-carbon precursors by the process of gluconeogenesis, effectively reversing glycolysis in a pathway that uses many of the glycolytic enzymes. Various metabolic fates of the pyruvate produced by glycolysis are described, and there are the pathways that feed various sugars from mono-, di-, and polysaccharides in to the glycolytic pathway. The pyruvate is converted to acetyl groups, then the entry of those groups takes place into the citric acid cycle, also called the tricarboxylic acid (TCA) cycle or the Krebs cycle, with the cycle reactions and the enzymes that catalyze them. The citric acid cycle is a hub in metabolism, with degradative pathways leading in and anabolic pathways leading out, and it is closely regulated in coordination with other pathways. The intermediates of the citric acid cycle are also siphoned off as biosynthetic precursors, and these intermediates are replenished in the cycle.
The discussion of glucose metabolism continues where the processes of carbohydrate synthesis and Degradation to illustrate the many mechanisms by which organisms regulate metabolic pathways. The general principles of metabolic regulation are illustrated for regulation of the metabolism of carbohydrates. 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. This need to regulate every aspect of cellular metabolism becomes clear as one examines the complexity of metabolic reactions. Firstly, the general roles of regulation in achieving metabolic homeostasis are looked at to introduce metabolic control analysis, a system for analyzing complex metabolic interactions quantitatively. Then the specific regulatory properties of the individual enzymes of glucose metabolism are explored, for example, the catalytic and regulatory properties of the enzymes of glycogen synthesis and breakdown, one of the best studied cases of metabolic regulation. The metabolism of carbohydrates is regulated by a variety of hormones and other molecules. There is the integrative role of hormones, such as interplay of insulin, glucagon, and epinephrine, in coordinating fuel metabolism in muscle, Liver and adipose tissue. The metabolic disturbances in diabetes further illustrate the importance of hormonal regulation of metabolism. These are involved in long-term hormonal regulation of body mass and, finally, have role in obesity, development of the metabolic syndrome and diabetes. Note that in selecting carbohydrate metabolism to illustrate the principles of metabolic regulation, the metabolism of fats and carbohydrates are artificially separated. In fact, these two activities are very tightly integrated and regulated.

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