132 GENERAL BIOCHEMISTRY 



where 2H stands for a particular reducing agent, depending on the 

 enzyme. A diversity of catalyzed processes is a fundamental require- 

 ment of biological systems. Without the enzymes, reaction rates would 

 be too slow to allow the existence of life as we know it. 



Enzyme activity depends upon many factors. For example, a suit- 

 able substrate must be present. This problem is referred to as enzyme 

 specificity or substrate specificity and is discussed in the following sec- 

 tion. With a given enzyme-substrate system, the reaction rate depends 

 upon the concentrations of enzyme, reactants, and products, the pres- 

 ence of cofactors, the presence of inhibitors, the temperature, the pH, 

 and the solvent. These special factors are discussed in a separate divi- 

 sion beginning on page 137. 



Specificity 



The specificity of enzyme action is suggested by the classification sys- 

 tem applied to enzymes in Table 6-1. There the enzymes were 

 grouped according to reaction types or classes of substrates. In either 

 case, the assumption is implicit that an enzyme placed in any category 

 does not directly affect the reactions catalyzed by enzymes from any 

 other category. For a long time this assumption was believed im- 

 mutable, but a few enzymes are now known to promote changes in 

 substrates of difi:erent types when studied in vitro. As an example, 

 chymotrypsin, formed in the pancreas and important in digestion, 

 catalyzes the hydrolysis of peptide and amide bonds in proteins and 

 peptides. In addition, chymotrypsin acts as a transpeptidase, transfer- 

 ring amino acids from one peptide to another. Moreover, this same 

 enzyme catalyzes the hydrolysis of certain synthetic ethyl esters. Thus 

 chymotrypsin could be placed among the proteases, transpeptidases, or 

 esterases. Ordinarily it is placed in the first class since it definitely 

 functions in proteolysis in animals. The biological importance of the 

 last two types of activity is still in doubt. 



Although some other enzymes also can perform dual functions, 

 the ntimber known is still quite small, helping to simplify problems of 

 classification and specificity. Eventually additional activities may be 

 discovered for many enzymes, but there is no simple systematic means 

 of searching for all the functions of enzymes. Therefore, until more 

 or less accidental discoveries bring many additional activities to light, 

 it is easiest to recognize the problem and solve it by learning the 

 known individual exceptions. 



There are widely differing degrees of specificity. Pepsin splits most 

 proteins except the fibrous types into peptides. It attacks some poly- 

 peptides and a few known small peptides. Thus this enzyme is specific 



