Metabolic pathways protein catabolism

CONTENTS Acknowledgments, Margery G. Ord and Lloyd A. Stocken. Introduction. Biochemistry Before 1900. Early Metabolic Studies Energy Needs and the Composition of the Diet. Carbohydrate Utilization Glycolysis and Related Activities. Aspects of Carbohydrate Oxidation, Electron Transfer, and Oxidative Phosphorylation. Amino Acid Catabolism in Animals. The Utilization of Fatty Acids. The Impact of Isotopes 1925-1965. Biochemistry and the Cell. Concepts of protein Structure and Function. Chronological Summary of Main Events Up to ca. 1960. Principal Metabolic Pathways. Index. 

Tissues carry out an enormous number of chemical reactions. Collectively these are referred to as metabolism. Some reactions or reaction sequences (often referred to as metabolic pathways) create complex molecules from simpler molecules, and these processes are then called anabolism. Many reactions, however, serve to break down complex molecules into simple molecules, and these processes are called catabolism. Among the latter is the degradation of foodstuffs, such as proteins, carbohydrates, and fats. Often these are degraded to C02 and HzO, which are excreted by the organism. The purpose of doing this is to acquire useful energy, which is in turn required for the various life processes, including... 

In addition to being incorporated into tissue proteins, amino acids, after losing their nitrogen atoms by deamination and/or transamination, may be catabolized to yield energy or to form glucose. Conversely, the nonessential amino acids may be synthesized from carbohydrate metabolism intermediates and ammonia or from essential amino acids. This section is devoted to the mechanisms of such metabolic processes and their interrelationships with carbohydrate and lipid metabolic pathways. 

The catabolism of lysine merges with that of tryptophan at the level of (3-ketoadipic acid. Both metabolic pathways are identical from this point on and lead to the formation of acetoacetyl-CoA (Figure 20.21). Lysine is thus ketogenic. It does not transaminate in the classic way. Lysine is a precursor of carnitine the initial reaction involves the methylation of e-amino groups of protein-bound lysine with SAM. The N-methylated lysine is then released proteolytically and the reaction sequence to carnitine completed. See Equation (19.6) for the structure of carnitine. 

In addition to the obvious difference in the direction of their metabolic goals, anabolism and catabolism differ in other significant ways. For example, the various degradative pathways of catabolism are convergent. That is, many hundreds of different proteins, polysaccharides and lipids are broken down into relatively few catabolic end products. The hundreds of anabolic pathways, however, are divergent. That is, the cell uses relatively few biosynthetic precursor molecules to synthesize a vast number of different proteins, polysaccharides and lipids. 

Metabolic and catabolic enzymes are specialized proteins that catalyze chemical reactions. In enzymatic catalyzed reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, or products. Almost all processes in a biological cell need enzymes in order to occur at significant rates. Since enzymes are extremely selective for their substrates and accelerate only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell (3973). 

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