. Principles of modern biology. Biology. 104 - The Cell. Fig. 5-1. Crystals of purified pepsin. (Courtesy of J. A. Northrop, Rockefeller Institute for Medical Re- search.) enzyme has an essential protein component, without which the enzyme loses all catalytic power. Indeed, some enzymes (for example, pepsin and trypsin, Table 5-1) are solely pro- tein. Many others, however, consist of a pro- tein part, called the apoenzyme, and a non- protein part, called a cofactor. Quite a few such cofactors have been identified. Each has proved to be a relatively simple phosphory- lated substance, chemicall
. Principles of modern biology. Biology. 104 - The Cell. Fig. 5-1. Crystals of purified pepsin. (Courtesy of J. A. Northrop, Rockefeller Institute for Medical Re- search.) enzyme has an essential protein component, without which the enzyme loses all catalytic power. Indeed, some enzymes (for example, pepsin and trypsin, Table 5-1) are solely pro- tein. Many others, however, consist of a pro- tein part, called the apoenzyme, and a non- protein part, called a cofactor. Quite a few such cofactors have been identified. Each has proved to be a relatively simple phosphory- lated substance, chemically united to some one or another of the vitamin compounds, such as niacin, thiamine, riboflavin, etc. (Chap. 18). In fact, recent developments indi- cate that the principal role of most vitamins is to act as cofactors in various metabolic en- zyme systems—as will be discussed more fully in Chapters 8 and 18. Coenzymes vs. Prosthetic Groups. The pro- tein and nonprotein parts, in some enzymes (for example, succinic dehydrogenase, p. 108), are very firmly united, and a separation of the parts leads to an irreversible loss of cata- lytic activity. In such cases, the nonprotein cofactor is spoken of as a prosthetic group, and the total enzyme complex can be re- garded as a conjugated protein. However, in other cases (for example, lactic dehydro- genase, p. 108), the affiliation is loose, and full activity can be restored after the parts have been separated and then brought together again. Such readily separable cofactors, in- deed, are called coenzymes. Moreover, several cases are known (for example, coenzyme A, p. 153) wherein the same coenzyme may team up with more than one apoenzyme, thus cata- lyzing several different metabolic reactions. This indicates that the specificity of enzyme action (p. 108) is determined mainly by the structural configuration of the protein part of each different enzyme complex. HOW ENZYMES ACT The precise mechanisms of enzyme catalysis are still not
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