Enzymes: How Cells Promote Chemical Activities - 107 



normal substrate 

 NH, 



COOH 

 p-aminobenzoic acid 



sulfanilamide 



N 



^ 



OH 



I 

 O 



H 

 .N 



h,n /C ^n- C -n X 



guanine 



CH 



OH 



H 

 .N 



H 2 N^ NT ^N 

 8-azaguanine 



N 



N 



if- 



OH 



I 

 C- 



^„ 



.CH, 



HO"" ^N' 

 thymine 



II 

 ,CH 



<? 



OH 



I 

 C 





Br 



II 



,CH 



5-bromouracil 



Fig. 5-4. The molecular configuration of an inhibitor substance tends to be similar to that of the normal sub- 

 strate of the particular enzyme. Therefore the inhibitor, or antagonist, is able to compete with the substrate 

 for occupation of the catalytic site. 



Reversibility of Catalyzed Reactions. Theo- 

 retically the enzyme or inorganic catalyst does 

 not determine the direction that a catalyzed 

 reaction will take; that is, the same enzyme 

 is equally effective in accelerating a particu- 

 lar reaction in either direction. In other 

 words, the direction of a reaction is deter- 

 mined not by the catalyst, but by other fac- 



able source of energy. Consequently an en- 

 zyme can accelerate an endothermic reaction 

 only when a suitable source of energy is 

 available. 



Enzymes in Relation to Cell Structure; 

 Coupled Reactions. Catabolic enzymes, which 

 facilitate the exothermic reactions of the cell, 

 tend to retain their activity after they are 



tors. Exothermic reactions, which liberate extracted from the protoplasm. But many 

 energy, tend to occur spontaneously, although anabolic reactions are difficult to duplicate 

 when the proper catalyst is present, such a outside the protoplasm. 



reaction proceeds at infinitely greater speed. 

 To drive an exothermic reaction in the re- 

 verse direction— or, to state the matter more 

 directly, to force the occurrence of an endo- 

 thermic reaction— there must be some avail- 



Such endothermic reactions, which lead to 

 synthesis in the cell, seem to require the 

 cooperation of several enzymes acting as a 

 team; and this degree of teamwork cannot be 

 achieved unless the individual enzymes are 



