70 GENERAL CONCEPTS 



unwinding of this helix. Enzyme inactivation is a reversible process if 

 the temperature is not too high and has not been applied more than a 

 short time. Most organisms are killed by exposure to heat because their 

 cellular enzymes are inactivated. The processes of protein denaturation 

 and enzyme inactivation show a striking parallelism and this is one bit 

 of substantiating evidence that enzymes are proteins. The enzymes of 

 man and other warm-blooded animals operate most efficiently at a tem- 

 perature of about 37° C— body temperature— whereas those of plants 

 and cold-blooded animals work optimally at about 25° C. Enzymes are 

 generally not inactivated by freezmg; their reactions continue slowly, or 

 perhaps cease altogether at low temperatures, but their catalytic activity 

 reappears when the temperature is again raised to normal. 



Acidity. All enzymes are sensitive to changes in the acidity and 

 alkalinity— the pH— of their environment, and will be inactivated if sub- 

 jected to strong acids or bases. Most enzymes exert their greatest catalytic 

 effect only when the pH of their environment is within a certain rather 

 narrow range. On either side of this optimum pH, as the pH is raised 

 or lowered, enzyme activity rapidly decreases. The protein-digesting 

 enzyme of the stomach, pepsin, is remarkable in that it has a pH op- 

 timum of 2.0; it will work only in an extremely acid medium. The 

 protein-digesting enzyme secreted by the pancreas, trypsin, in contrast, 

 has a pH optimum of 8.5, well on the alkaline side of neutrality. Most 

 intracellular enzymes have pH optima near neutrality, pH 7.0. This 

 marked influence of pH on the activity of an enzyme is what would be 

 predicted from the fact that enzymes are proteins. The topic is too 

 complex to be discussed in detail, but the number of positive and nega- 

 tive charges associated with a protein molecule, and perhaps the shape 

 of the molecular surface, are determined by the pH. Probably only one 

 particular state of the enzyme molecule, with a particular number of 

 negative and positive charges, is active as a catalyst. From these consid- 

 erations it is clear that the catalytic ability of a protein molecule would 

 be expected to be strongly influenced by the pH of the environment. 



Concentration of Enzyme, Substrate and Cofactors. If the pH and 

 temperature of an enzyme system are kept constant, and if an excess of 

 substrate is present, the rate of the reaction is directly proportional to 

 the amount of enzyme present. This method is used, indeed, to meas- 

 ure the amount of some particular enzyme present in a tissue extract. If 

 the pH, temperature and enzyme concentration of a reaction system are 

 held constant, the initial reaction rate is proportional to the amount of 

 substrate present, up to a limiting value. If the enzyme system requires 

 a coenzyme or specific activator ion, the concentration of this substance 

 may, under certain circumstances, determine the over-all rate of the 

 enzyme system. 



Enzyme Inhibitors. Enzymes can be inhibited by a variety of chemi- 

 cals, some of which inhibit reversibly, others irreversibly. Cytochrome 

 oxidase, one of the "respiratory enzymes," is inhibited by cyanide, which 

 forms a complex with the atom of iron present in the enzyme molecule 

 and prevents it from participating in the catalytic process. Cyanide is 

 poisonous to man and other animals because of its action on the cyto- 



