Enzymes—Catalytic Power, Specificity, and Regulation
Enzymes are characterized by three distinctive features: catalytic power, specificity, and regulation.
Enzymes display enormous catalytic power, accelerating reaction rates as much as 1016 over uncatalyzed levels, which is far greater than any synthetic catalysts can achieve, and enzymes accomplish these astounding feats in dilute aqueous solutions under mild conditions of temperature and pH. For example, the enzyme jack bean urease catalyzes the hydrolysis of urea:
At 20°C, the rate constant for the enzyme-catalyzed reaction is 3 x 104/sec; the rate constant for the uncatalyzed hydrolysis of urea is 3 x 10-10/sec. Thus, 1014 is the ratio of the catalyzed rate to the uncatalyzed rate of reaction. Such a ratio is defined as the relative catalytic power of an enzyme, so the catalytic power of urease is 1014.
A given enzyme is very selective, both in the substances with which it interacts and in the reaction that it catalyzes. The substances upon which an enzyme acts are traditionally called substrates. In an enzyme-catalyzed reaction, none of the substrate is diverted into nonproductive side-reactions, so no wasteful by-products are produced. It follows then that the products formed by a given enzyme are also very specific. This situation can be contrasted with your own experiences in the organic chemistry laboratory, where yields of 50% or even 30% are viewed as substantial accomplishments (Figure 14.3). The selective qualities of an enzyme are collectively recognized as its specificity. Intimate interaction between an enzyme and its substrates occurs through molecular recognition based on structural complementarity; such mutual recognition is the basis of specificity. The specific site on the enzyme where substrate binds and catalysis occurs is called the active site.
Figure 14.3 •A 90% yield over 10 steps, for example, in a metabolic pathway, gives an overall yield of 35%. Therefore, yields in biological reactions must be substantially greater; otherwise, unwanted by-products would accumulate to unacceptable levels.
Regulation of enzyme activity is achieved in a variety of ways, ranging from controls over the amount of enzyme protein produced by the cell to more rapid, reversible interactions of the enzyme with metabolic inhibitors and activators. Chapter 15 is devoted to discussions of enzyme regulation. Because most enzymes are proteins, we can anticipate that the functional attributes of enzymes are due to the remarkable versatility found in protein structures.
Date: 2016-01-03; view: 1300