The metabolism of cells includes:

(1) All the individual chemical reactions,

(2) The sequences of these reac­tions,

(3) The interrelationships that exist among the reaction sequences, and

(4) The various mechanisms that regulate the reactions.

Individual reactions may be energy pro­ducing (the exergonic reactions) or energy consuming (endergonic reactions). Commonly, the primary reactants or substrates are converted into final products by means of a sequence of reactions, each reaction enzymatically catalyzed and requiring a product of the prior reaction as the substrate.

The overall reaction sequences may be classified as catabolic (i.e., degradative) if the ultimate products of the reaction sequence are considered to be subunits or parts of the initial substrate. Alternatively, if the products are a result of the combining of two or more different substrates, the sequence is considered to be anabolic (i.e., synthetic).

A sequence of reactions is usually referred to as a metabolic pathway. Some metabolic pathways are common to all living organ­isms or cells and several of these are considered in this article. Some pathways are especially active, receiv­ing as substrates the products of a variety of other, less active pathways. Alternatively, these central pathways may feed substrates into a number of other, less active pathways.

Figure 10-1 diagrammatically shows some of the relationships between the catabolic and anabolic path­ways that are followed by the major groups of cellular compounds. Intermediates in the breakdown of carbo­hydrates can be diverted to lipid synthesis or to the formation of nitrogenous compounds such as nucleo­tides and amino acids.

Lipids in microbial and plant (and to a limited extent animal) cells can be converted into carbohydrates and nitrogen compounds. Like­wise, nitrogen compounds, once deaminated, can be converted into lipids or carbohydrates. All these com­pounds may be further degraded, their catabolism act­ing as sources of reducing power (e.g., NADH and NADPH) to be used in cellular anabolic reactions.

The Major Path-way of Metabolism

It is possible to identify specific sites within a cell where particular metabolic pathways are operative. For example, the enzymes necessary for the tri-carb­oxylic acid (Krebs) cycle reactions are located in the mitochondria; the primary reactions of steroid synthe­sis are associated with the smooth endoplasmic reticu­lum; fatty acids are oxidized by reaction sequences in mitochondria and are synthesized in the cytosol; the reaction sequences that successively break down sug­ars to form pyruvic acid or lactic acid also take place in the cytosol; proteins are synthesized by the cell’s ribosomes.

The intermediates, as well as the end products, of a pathway may be drawn off and used in other path­ways. For example, during the breakdown of carbohy­drates, a large number of intermediate compounds are formed before the ultimate products, CO2 and H2O are formed.

Some of these intermediates may be diverted from the catabolic process and used in the formation of fatty acids. Other intermediates may be used in the formation of amino acids. A number of mechanisms are used by cells to regulate the activity of metabolic pathways.

Some of the major pathways com­mon to cells are discussed. For convenience the major pathways of carbohydrate metabolism will be re­viewed first, followed by those of lipid and nitrogen metabolism. Although they are described separately, it should be remembered that in the cell many path­ways are operative at the same time, and one pathway may influence both the rate and the direction of reac­tions of another pathway.

The terms used to identify metabolic intermediates vary somewhat in the chemical and biological litera­ture. For example, pyruvic acid is sometimes identi­fied as -pyruvate, lactic acid as lactate, and so on. In situ, many acids dissociate and the anion (e.g., pyru­vate) is often the more common form. The chemical formula may be presented as the acid or the anion.

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Also variable are the names assigned to phosphoryla- ted compounds, the position of the phosphate groups in the molecules is identified at either the beginning or end of the compound’s name. Hence, 3-phosphoglyc- eraldehyde and glyceraldehyde-3-phosphate refer to the same chemical substance.

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