270 iENZYMES 



ISature of enzyme action 



It has been stated that enzymes are biological catalysts. This means 

 that they are agents which affect the rates of metabolic reactions. How- 

 ever, although they greatly affect the speed of reactions, they do not 

 influence the extent of the chemical change concerned, that is, they do 

 not influence the final position of chemical equilibrium. The latter is 

 determined by the nature (particularly the energy content) of the react- 

 ing substances and the products formed (see Chap. 16). The rates of 

 metabolic reactions, however, are all-important for living organisms, since 

 they must be able to utilize foods fast enough to keep up with their 

 metabolic needs. For example, the same amount of glucose can be ob- 

 tained from starch as from cellulose on chemical hydrolysis; yet, while 

 the former will support growth in man, the latter will not. The explana- 

 tion is that enzymes are present in the human digestive tract which accel- 

 erate the hydrolysis of starch, whereas there are none that attack cellulose. 

 The uncatalyzed breakdown of cellulose to glucose is much too slow 

 to be of use to the body. Cattle and other ruminants, however, have in 

 their paunch vast numbers of bacteria which contain enzymes that can 

 break down cellulose to organic acids and thus provide the animal with 

 utilizable food. It follows from the above that any agent or condition 

 which affects the catalytic ability of one or more of the enzymes involved 

 in the metabolism of a vital food will have a profound effect on the 

 development of the whole organism. 



Many of the chemical reactions which occur easily in living organisms 

 are difficult to reproduce in the laboratory in good yields and require 

 drastic conditions of pressure, temperature, or pH to proceed at adequate 

 speed; yet these reactions take place under much milder conditions in 

 living cells. Enzymes accomplish this end, since by virtue of their 

 specificity they guide reactions to the desired products, and because they 

 can lower the energy of activation of the reaction {i.e., the energy neces- 

 sary to get it started). 



Mechanism of action 



According to the most widely accepted theory an enzyme functions 

 through union with its substrate to form a labile intermediate compound 

 or "enzyme-substrate complex," which in turn decomposes with forma- 

 tion of the end products of the reaction and regeneration of the enzyme. 

 This mechanism can be schematically represented as follows: 



enzyme -|- substrate ^ enzyme-substrate — > product (s) + enzyme 



complex 



