Central pathways of energy metabolism

The tricarboxylic acid cycle shown as a part of the central pathways of energy metabolism. (See Figure 8.2, p. 90 for a more detailed view of the metabolic map.)... 

In contrast to transamination reactions that transfer amino groups, oxidative deamination by gutamate dehydrogenase results in the lib eration of the amino group as free ammonia (Figure 19.11). These reactions occur primarily in the liver and kidney. They provide a-ketoacids that can enter the central pathway of energy metabolism, and ammonia, which is a source of nitrogen in urea synthesis. 

Amino acid metabolism shown as a part of the central pathways of energy metabolism. 

Central Pathways of Energy Metabolism (Figure 12.3, Figure 12.4, Figure 12.5, Figure... 

The ways in which energy in the form of ATP is produced and utilized constitute bioenergetics, and will be discussed in greater detail at the end of this chapter. However, before turning to a selection of metabolic pathways, we outline some fundamental notions concerning redox reactions followed by a brief description of the central role of ATP in metabolism as an acceptor and donor of phosphoryl groups, and finally a summary of the types of reactions that we will encounter as we wend our way along a sample of some of the pathways of intermediary metabolism. 

Anabolism and catabolism are complementary to one another, and may be compared with the biologically mediated cycling of carbon (and other materials) in the environment (Fig. 5.2). However, there is an area of metabolism where the two types of metabolism coincide and become indistinguishable. These pathways are known as intermediary metabolism and are the central part of cellular metabolism where the basic requirements for growth are met. At this metabolic crossroads, materials are either rearranged into synthetic precursors or are oxidised to generate energy. 

The citric acid cycle is the central pathway of metabolism. It plays a pivotal role in the production of energy by a cell. 

As the power house of the cell, the mitochondrion is essential for energy metabolism. As the motor of cell death (1), this organelle is central to the initiation and regulation of apoptosis. In addition, mitochondria are critically involved in the modulation of intracellular calcium concentration and the mitochondrial respiratory chain is the major source of damaging reactive oxygen species. Mitochondria also play a crucial role in numerous catabolic and anabolic cellular pathways. 

The objective of this chapter is to place energy metabolism within the context of the biochemistry that takes place in the whole body, in the tissnes and in biochemical pathways or processes. In all of these systems, the generation and ntilisation of ATP are central. Consequently, the final dis-... 

The next part presents the reactions involved in the interconversion of these compounds—the part of biochemistry that is commonly referred to as metabolism (pp. 88-195). The section starts with a discussion of the enzymes and coenzymes, and discusses the mechanisms of metabolic regulation and the so-called energy metabolism. After this, the central metabolic pathways are presented, once again arranged according to the class of metabolite (pp. 150-195). 

A small number of other biosynthetic pathways, which are used by both photosynthetic and nonphotosynthetic organisms, are indicated in Fig. 10-1. For example, pyruvate is converted readily to the amino acid t-alanine and oxaloacetate to L-aspartic acid the latter, in turn, may be utilized in the biosynthesis of pyrimidines. Other amino acids, purines, and additional compounds needed for construction of cells are formed in pathways, most of which branch from some compound shown in Fig. 10-1 or from a point on one of the pathways shown in the figure. In virtually every instance biosynthesis is dependent upon a supply of energy furnished by the cleavage to ATP. In many cases it also requires one of the hydrogen carriers in a reduced form. While Fig. 10-1 outlines in briefest form a minute fraction of the metabolic pathways known, the ones shown are of central importance. 

---