Enzymes: How Cells Promote Chemical Activities - 109 



a measurement possible. This turnover nurn- to their ionic environment is related to 



ber is unusually high, to be sure, but many changes in the secondary and tertiary struc- 



enzymes can easily handle more than a mil- ture of the protein component. Too great a 



lion molecules of substrate per minute, under degree of folding in the peptide chain (p. 



optimal conditions. Thus it is safe to say that 87) may distort or block off the catalytic site, 



enzymes are generally much more active than 

 inorganic catalysts. The best calculations 

 indicate, indeed, that often there may be only 

 about 100 molecules of a particular enzyme 

 in a given cell — which puts a heavy burden 

 of activity upon each enzyme molecule. 



Ptyalin, the amylase present in human 

 saliva, is capable of activating the hydrolytic 

 breakdown of more than a million times its 

 weight of starch. Theoretically no potency 

 should be lost, even with infinite activity. But 

 enzymes, like other proteins, do not possess 

 a very stable structure. Consequently some 

 deterioration occurs even though the catalyst 

 is not consumed in the reaction it catalyzes. 

 Thus the maintenance of enzymic activity in 

 the living cell depends upon a gradual re- 

 placement of its enzymes through the agency 

 of constructive metabolism. 



Sensitivity of Enzymes to Hydrogen and 

 Other Ions. The activity of many enzymes is 

 sensitive to the influence of various ions 

 present in the reaction medium. This is par- 

 ticularly true as regards the hydrogen and 

 hydroxyl ions. Each specific enzyme exhibits 

 its most vigorous activity only when the acid- 

 ity or alkalinity of the medium is adjusted to 

 the proper level. Several examples of such 

 sensitivity may be taken from the digestive 

 enzymes of man. Pepsin, the powerful pro- 

 tease of the gastric juice, acts very sluggishly 

 except when the medium is strongly acid; 

 trypsin and the other enzymes of the pan- 

 creatic juice (Table 5-1) display full activity 

 only when the medium is slightly alkaline. 

 Ptyalin, the salivary amylase, requires a 

 nearly neutral solution, and is quite inactive 

 when chloride ion is absent from the me- 

 dium. Most metabolic enzymes, which exert 

 their influence in the protoplasm proper, at- 



rendering it ineffective or only partially 

 effective, and the same is true for any exces- 

 sive unfolding. In other words, the stereo- 

 chemical configuration of the active site can- 

 not be changed unduly without destroying its 

 catalytic potency. 



Enzymes in Relation to Temperature. Heat 

 accelerates the movements of molecules and 

 ions, increasing the frequency of their con- 

 tacts; consequently chemical reactions pro- 

 ceed more rapidly when the temperature of 

 the medium is raised. As a general rule, each 

 increase of 10° C doubles, or even trebles, the 

 rate of most reactions. 



The foregoing temperature rule applies 

 equally for both catalyzed and uncatalyzed 

 reactions; but enzyme systems constitute a 

 very special case. Up to the point where the 

 heat begins to unfold and denature the en- 

 zyme protein, warming the medium increases 

 the reaction rate. But there is a critical tem- 

 perature beyond which each enzyme cannot 

 be carried. Above this point the enzyme be- 

 comes denatured (p. 88) and loses its catalytic 

 power. Then the reaction rate drops quickly 

 to the uncatalyzed level, which may be very 

 slow, despite the elevated temperature. 



The case of ptyalin will exemplify the 

 thermal behavior of enzymes generally. If a 

 small amount of saliva is mixed with a starch 

 solution, the starch is hydrolyzed to maltose 

 (Fig. 5-5). This reaction proceeds rather 

 slowly at 0° C, but more and more rapidly as 

 the temperature of the test tube is raised. 

 Maximum rapidity is reached at about 40° C. 

 If the temperature is then raised still fur- 

 ther, chemical activity comes to a virtual 

 standstill. Furthermore, if the damaging tem- 

 perature is maintained for many minutes, no 

 trace of amylase activity will persist after the 



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tain greatest potency when the medium is system has been cooled again. In other words, 

 approximately neutral. an irreversible denaturation of the enzyme 



Probably the unique sensitivity of enzymes protein has occurred. 



