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The metabolism of cells includes:
(1) All the individual chemical reactions,
(2) The sequences of these reactions,
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(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).
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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.
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.
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.
