ON CHEMICAL REACTION RATES; ENZYMES 201 



The role of a catalyst is to provide an alternate path which is energetically 

 easier. Thus the catalyst, because of the energetic advantages it offers, acts 

 as a guide-post to direct the reaction through preferred channels or path- 

 ways (see Figure 8-4 (b)). This subject is now explored further. 



Catalyzed Reactions; Enzymes 



There are many chemical reactions and physical processes whose rate or 

 pathway is controlled by one or more catalysts. Far surpassing all the rest 

 in importance as biological catalysts are the enzymes. These are large pro- 

 tein molecules, which are often bound with metallic ions and are always 

 heavily hydrated. They have the special property that at some site(s) on the 

 surface both the kinds of atoms and their arrangement are such that more or 

 less specific adsorption of a "substrate" molecule can occur. The substrate 

 molecule is the one which is to undergo hydrolysis, hydrogenation, trans- 

 ammination, or some other reaction. 



In addition to the kind of atoms and their arrangement, a third essential 

 requirement of the enzyme seems to be the presence, in the vicinity, of a 

 large electric charge, usually in the form of a metallic ion such as Mg ++ , or a 

 charged chemical group, such as -PCV 2 . The role of the charged group is 

 to distort the electronic structure of the substrate molecule as it adsorbs on 

 the enzyme, thus to make it energetically easier for the desired reaction to 

 occur. The most easily measured manifestation of a catalyzed process is a 

 lowered activation energy, E*. Some values are collected in Table 8-4. Note 

 especially the numbers for the decomposition of H 2 2 . 



"The suffix "ase" denotes enzyme. 



