320 Enzyme Kinetics of Hydrolytic Reactions / 1 "7 : 3 



the cof actor, coenzyme, or prosthetic group. No sharp dividing line separates 

 these three types of smaller groups. By and large, the word "cofactor" 

 is used for a loosely bound, small inorganic ion. Coenzyme usually 

 implies a larger group which separates easily from the enzyme, such as 

 diphosphopyridine nucleotide (DPN) illustrated in Chapter 18. Co- 

 enzymes probably separate and recombine with the various apoenzymes 

 during normal physiological conditions. There may be many different 

 types of apoenzymes for any given coenzyme. Although apoenzymes, 

 like most other proteins, are irreversibly destroyed by prolonged boiling, 

 the coenzymes in general are not so destroyed. 



Prosthetic groups are small, nonprotein parts of enzymes which are 

 attached so firmly that they cannot be easily removed without irreversibly 

 altering the enzyme. Among the more frequently studied prosthetic 

 groups are the hemes. A molecule consisting of a heme group and a 

 protein is referred to as a hemoprotein. Examples include the carrier 

 protein, hemoglobin, and the respiratory enzymes myoglobin, catalase, 

 peroxidase, and about 20 different cytochromes. The chemical struc- 

 ture of the heme group is presented in Section 1 of the next chapter. 



Hemoproteins are convenient to study because their optical absorption 

 spectra change during their reactions. Their magnetic susceptibility 

 also changes, which helps to elucidate their chemical structure. The 

 occurrence of these heme compounds in relatively large amounts in most 

 types of cells has also contributed to the ease of studying them. These 

 studies have formed the basis of many of the accepted ideas of how 

 specific enzymes catalyze reactions. The material in Sections 1 and 2 

 of the next chapter is concerned with the reactions of two heme enzymes, 

 catalase and peroxidase. 



3. Michaelis-Menten Kinetics of Hydrolases 



The hydrolases were listed as the first group of enzymes because from 

 many points of view they are the simplest. Most extracellular enzymes, 

 such as those of the digestive tract, are hydrolases. By and large, it is 

 relatively easy to obtain large quantities of these enzymes in an active 

 form. Furthermore, most of them attack specific bonds rather than 

 forming part of a complex chain or pathway. 



Symbolically, one may represent these reactions by 



S + H 2 -* Products 



where S represents the substance hydrolyzed. It is called the substrate. 

 For instance, the substrate might be sucrose, in which case the enzyme 



