Enzymes 



6 



NATURE AND FUNCTION 



Enzymes are proteins with catalytic activities. When the definition 

 of proteins from page 11 1 is inserted in the sentence above, enzymes are 

 defined as macromolecules of biological origin consisting largely of 

 amino acids condensed via peptide bonds and capable of altering the 

 rates of reactions. Thus the chemical and physical properties of 

 enzymes are those of proteins in general, excepting of course, the 

 catalytic function. Hence it is not necessary to discuss further the 

 basic structure of enzymes. Rather the present chapter will be devoted 

 to enzymes as catalysts. 



It happens that no other types of naturally occurring macromolecu- 

 lar catalysts are now known. Perhaps some will ultimately be dis- 

 covered, but the enzymes are clearly fundamental units in the control 

 of biological reactions. In fact, some workers feel that the term 

 enzyme should be applied to any and all conceivable natural macro- 

 molecular catalysts without regard to their chemical constitution. This 

 conception hardly seems warranted of chemists or biochemists who 

 traditionally classify molecular types on chemical bases. If some large 

 non-protein catalysts are demonstrated, they would be more properly 

 grouped with chemically related compounds than with proteins. 

 Nowadays whenever a giant molecule is found that accelerates a reac- 

 tion, it is customary to call it an enzyme, all too often without the 

 demonstration of protein properties. Such an assumption may well be 

 valid but can hardly be justified on the sole possible basis of a gen- 

 eral rule derived only from past experience. 



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