On the Bigness of Enzymes 233 



in which we denote the enzyme by E ± and the substrate by S. Not 

 only does the substrate displace water molecules bound to the enzyme, 

 but it also changes the organization of water molecules in the vicinity 

 of the enzyme. The free-energy change of the reaction must be rela- 

 tively small (of the order of RT) since appreciably larger values would 

 lead to a self-poisoning, either by the substrate or by water. It would 

 seem that in those reactions in which the substrate is a charged mole- 

 cule the value of —AH should be large since there would be large 

 interactions of charged groups of the substrate and the enzyme. On 

 the other hand those reactions involving uncharged substrates should 

 have a large value of AS since the substrate should partly destroy the 

 ordered arrangement of water molecules around the polar enzyme. 



The enzymatic reaction as usually studied is the resultant of at least 

 three reactions: 



S + E • (H 2 0) n ^ S • E • (H 2 0)„. y + </H 2 (2) 



S-E-CHaOV, ^ P-E-(H 2 0) n _j,_ r + rH 2 (3) 



P-E-(H 2 0) w + {y + r)H 2 ^ E-(H 2 0) n + P (4) 



where E, S, and P have their usual meanings and n, m, and r are not 

 necessarily integral. The sum of the three reactions is the reaction 

 usually written 



S -» P (5) 



and AH and AS for this last reaction is the sum of these quantities for 

 reactions 2, 3, and 4. It is not surprising that so much time and effort 

 spent in determining the AH and AS for reaction 5 has thrown so little 

 light on the individual enzymatic reactions involved. Studies of en- 

 zymatic reactions in deuterium and 18 -labeled water have furnished 

 methods for the study of these individual steps. 3 A comprehensive 

 study of the elementary reactions involved in the enzymatic catalysis 

 might be illuminating. 



The aspect of enzyme action, other than geometrical, involves the 

 energetics of interaction of enzyme and substrate. That these two 

 aspects are not mutually exclusive is illustrated by the previous para- 

 graphs; geometry merges into energetics without a sharply defined 

 boundary. The question of the energetics of enzyme-catalyzed reac- 

 tions is one about which we know the least. Formally we know that 

 an enzyme speeds up the rate of a reaction by reducing the value of 

 the activation energy. This, however, tells us little new since, in 

 general, fast reactions have low activation energies, and vice versa. 

 The method by which an enzyme reduces the energy of activation of 



