318 Enzyme Kinetics of Hydrolytic Reactions /I7 : 2 



2. Enzymes 



Although many biophysicists have studied enzyme kinetics, most enzyme 

 studies have not been the work of biophysicists. In this section, a 

 minimal outline will be presented of the many types of enzymes known, 

 primarily from a result of biochemical and physiological studies. It is 

 hoped that this section will be of use to those readers with comparatively 

 little biochemical background. 



A complete classification of types of enzymes, in terms of the reactions 

 they catalyze, can be found in many biochemistry texts. The first 

 reference at the end of the next chapter has appealed particularly to the 

 author and his students; the following four paragraphs contain a brief 

 summary of the functional classification of enzymes found in that text. 

 Six major classes are described. Most of the classes are named by 

 attaching the suffix -ase to a word describing the reaction catalyzed. 

 This same naming procedure is used for many individual types of 

 enzymes. 



The first such class consists of the hydrolases, which split molecules, 

 adding H to one part and OH to the other. The kinetics of the hydro- 

 lases are discussed in this chapter. One example of hydrolases are the 

 phosphatases which catalyze the addition of water to an organic phos- 

 phate to form the corresponding alcohol and phosphoric acid; sym- 

 bolically, this may be represented as 



R— O— P0 3 H 2 + H 2 ^ ROH + H 3 P0 4 



In a like manner, proteolytic enzymes catalyze the splitting of peptide 

 bonds adding H and OH to the two split parts. Other hydrolases are : 

 glycosidases which catalyze the splitting of complex sugars to simpler 

 sugars with the addition of water ; and lipases and esterases, both of which 

 catalyze the equilibrium between an ester and its hydrolyzed com- 

 ponents. Acetyl cholinesterase, introduced in Chapter 4, is an example 

 of this last group. 



A second class of enzymes are the transferases, which catalyze the 

 transfer of a piece of one molecule to another. For instance, trans- 

 phosphorylases catalyze reactions in which phosphate groups are changed 

 from one molecule to another. (If either the acceptor or donor of the 

 phosphate is water [or phosphoric acid], the enzyme is a phosphatase 

 rather than a transphosphorylase.) Other transferases may be defined 

 in a similar manner; they include trans glycosidases, transpeptidases, trans- 

 aminases, transmethylation systems, and transacy loses. The reaction cata- 

 lyzed by each of these is contained in the name. 



A third major group of enzymes are the addition enzymes, which 

 catalyze the addition of two molecules, to form a single molecule. A 



