FACTOKS DETERMINING RATE OF ENZYME REACTIONS 31 



motions of electrons, protons, and atoms from the initial molecular con- 

 figuration of the substrate to the final one of the products. Due to our lack 

 of knowledge of the intimate meclianisms and kinetics of most enzyme 

 reactions, this visuaHzation is frequently difficult at the present time but it 

 should be more and more realizable as progress in accurate investigation 

 is made. 



It is clear from the rough picture that has been developed of the enzyme 

 complexes and the possible mechanisms of reaction that any phase of the 

 total sequence may be affected by modification of the enzyme, either at the 

 active center or at some distance from it. The effects of activators, inhibi- 

 tors, pH, temperature, ionic strength, and similar factors should be inter- 

 preted in such terms. An inhibitor may interfere with the binding of the 

 substrate, this being the most common assumption, but it could also modify 

 the formation of any of the subsequent complexes, hindering the necessary 

 movements of electrons, protons, or groups either directly by effects exerted 

 by the inhibitor molecule (electrical, chemical, or steric) or indirectly through 

 the protein by modifying the active center groups involved in the reaction, 

 perhaps by increasing or decreasing the electron or proton density in the 

 critical regions. An inhibitor need not interfere with the reaction between 

 two substrates to retard the process; in transphosphorylation or transacety- 

 lation reactions, for example, the inhibitor may hinder the phosphorylation 

 or acetylation of the enzyme by modification of the acceptor group on the 

 active center. It is the desire to present a comprehensive concept of inhibitor 

 action that has prompted us to discuss in some detail the normal phases of 

 the enzyme reaction and the participation of the active center. 



FACTORS DETERMINING THE RATE OF ENZYME REACTIONS 



Inhibitors of the simple enzymes reduce the rates at which the products 

 are formed. It will facilitate later discussion if the factors that determine 

 these rates are stated clearly and briefly. In Michaelis-Menten kinetics and 

 in many extensions of this theory the rate is assumed to be expressed by 

 V = Z^a (ES). The over-all rate is thus dependent on two basic factors: the 

 concentration of the complex and the intrinsic rate at which the complex 

 breaks down into the products. If one compares the rates with different 

 substrates and a single enzyme, variation in rates may be due to different 

 tightnesses of binding (K^) or different rates of breakdown of the correspond- 

 ing complexes (A-.,). It is not necessary, as has sometimes been assumed, that 

 any parallelism be exhibited between the affinities of the enzyme for a series 

 of substrates and the rates observed, inasmuch as J1.2 can vary independently. 

 Some substances may be bound very tightly but react only slowly; in fact, 

 most competitive inhibitors are substances binding reasonably tightly to the 

 enzyme but for which k^ is low or zero. If k^ remains constant, then changes 



