30 2. THE KINETICS OF ENZYME REACTIONS 



The most common mechanism by which enzymes accelerate reactions may 

 well be the electron displacement induced in the substrate by which it is 

 made more susceptible to attack by another substance, whether this be a 

 second substrate, a coenzyme, a proton, or the solvent. Most organic reac- 

 tions may be characterized as electrophilic or nucleophilic, depending on 

 whether the attacking group possesses affinity for electrons or a tendency 

 to lose them. The enzyme, by creating or modifying electron-rich and elec- 

 tron-poor regions in the substrate, may thus facilitate such attack. In other 

 cases it may provide a pathway by which electrons or protons can move 

 from one locus to another, these electrons or protons being directly donated 

 or accepted by some enzyme group, the enzyme here truly participating in 

 the reaction. The large number of charged groups on proteins gives them 

 the ability not only to bind substrates but to polarize them. The binding 

 groups may not play a role in the reaction but merely serve to anchor the 

 substrate molecule so that appropriate groups can be polarized. In the large 

 group of hydrolytic enzymes, acidic or basic catalysis may operate under 

 more favorable conditions due to this polarization or the spatial apposition 

 of the susceptible bond to acidic or basic groups on the active site. In the 

 reduction of aldehyde catalyzed by the alcohol dehydrogenase: 



R-CHO + H+ + DPNH -> R-CH2OH + DPN+ 



it has been shown that one hydrogen atom comes from the DPNH and one 

 from an acidic group on the protein, and the importance of this acidic group 

 for the binding indicates that a hydrogen bond may be formed in the ES 

 complex, facilitating the hydrogen transfer (Sizer and Gierer, 1955). It 

 has been postulated that both acidic and basic groups may participate in 

 a so-called " push-pull " type of acid-base catalysis in certain enzymes, 

 this being more efficient than either acidic or basic catalysis alone (Laidler, 

 1955 a). It is becoming more and more evident that in many reactions true 

 chemical compounds, involving covalent bonds, are formed between sub- 

 strate and enzyme at some stage of the reaction sequence and that the 

 molecular transformations involving the activated complexes must include 

 the active center groups as well as the reactants. 



Reaction sequences or energy diagrams, as usually presented, make it 

 appear that the total reaction occurs in a series of stages identified by 

 various intermediate complexes and one comes to conceive of these reac- 

 tions as actually proceeding through these discrete steps. It is possible, how- 

 ever, that on the molecular level such considerations are arbitrary and ar- 

 tificial, since the reaction may proceed quite smoothly without any distin- 

 guishable intermediate compounds or complexes. In fact, two or more steps 

 that are written sequentially may occur simultaneously. The conventions 

 of description should not too much limit our concepts of reality. It is perhaps 

 ideal, whenever possible, to visualize an enzyme reaction in terms of the 



