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fourth class, the isomerases, changes the isomeric form. Opsin, discussed 

 in Chapter 19, belongs in this class because it acts as a photoisomerase. 

 The fifth class of enzymes remove C0 2 ; they are called carboxylases. 



Finally, as the sixth class, one may list the respiratory enzymes. The 

 kinetics of some of their reactions are discussed in the following chapter. 

 Respiratory enzymes include those which enter into oxidative reactions. 

 Either 2 or H 2 2 may be the eventual source of oxygen, depending on 

 the system of enzymes. These respiratory enzymes have been studied 

 extensively by spectroscopic methods because many of them undergo 

 changes in their absorption (and fluorescence) spectra during reaction. 

 Many respiratory enzymes also have altered magnetic susceptibility 

 during reactions. 



Under each of the six major classifications just given, there are sub- 

 classes and within each of these there are large numbers of enzymes. 

 A major activity of biochemistry since about 1930 has been the dis- 

 covery of more and still more enzymes. The number of known enzymes 

 seems astronomical and is still growing. Almost every reaction which 

 occurs in a living organism, or just outside it, is catalyzed by a specific 

 enzyme. Complex reactions such as the oxidation of glucose, referred 

 to earlier, involve many enzymes. Specifically, in the oxidation of 

 glucose, more than 50 different enzymes participate, to form a complex 

 pathway. 



It is interesting that the same types of respiratory enzymes occur in 

 almost every living cell from yeast to mammals. (A partial exception to 

 this are the anaerobic bacteria.) On a molecular basis, the description 

 of life must include a description of enzymes and enzyme kinetics. On a 

 kinetic basis the similarities between the respiratory enzymes of various 

 types of cells are much more impressive than their differences. 



It has been facetiously remarked that the living cell is a bag full of 

 enzymes. This is a gross oversimplification. Some of the enzymes are 

 associated with particulate matter; others, such as digestive enzymes, 

 act outside the cell altogether. The cell also contains many compounds 

 not normally considered as enzymes. One class of compounds whose 

 members act as catalysts, but are not considered enzymes, is the nucleic 

 acids. These are discussed in Chapter 15. Nucleic acids are not 

 considered enzymes because they are not proteins. Enzymes are usually 

 distinguished from transport proteins such as hemoglobin, although here 

 the basis of distinction is very weak. 



At the start of this chapter, an enzyme was defined as a biological 

 catalyst of a primarily protein nature. The word "primarily" was 

 inserted because many enzymes consist not only of a protein molecule 

 but also of a smaller molecule. In this case, the large protein portion of 

 the enzyme is referred to as the apoenzyme. The smaller part is called 



