202 



SPEEDS OF SOME PROCESSES IN BIOLOGICAL SYSTEMS 



A typical process can be illustrated as in Figure 8-5, and described as fol- 

 lows, a hydrolysis serving as the example: First the molecule to be hy- 

 drolyzed (the substrate molecule, S) bumps into the hydrated enzyme mole- 

 cule, E; and if the collision occurs at the active site and is energetic enough, 

 a slight bond will be made between the two, forming the enzyme-substrate 

 complex, ES. The complex can then do one of two things: it can fall apart 

 again (in which case we lose interest); or it can be "activated" — i.e., given 

 excess energy by favorable collisions with its neighbors — and associate with 

 a water molecule close by, to form the activated complex, ESK This in turn 

 can either fall apart the way it was formed, or it can proceed on to split up in 

 a new way — as reaction products — with the substrate molecule hydrolyzed 

 and the enzyme ready to go again. 



The process is sketched at the top of Figure 8-5, the energy of the reac- 

 tion path at the bottom, and the formal equation in the middle. For the 

 purpose of formulation of the rate equations, the reaction can be written: 



E + S^ ES (formation of the M jchaelis complex, ES) 1 



*-i 



ES ~^> products (activation and reaction to products) 2 



where k t , k_ x , and k 2 are the specific rate constants for the respective steps. 



prod. 2 

 *• E + products 



Reoctonts Complex Activated complex 



State — 



Products 



Figure 8-5. Schematic Representation of Catalyzed Hydrolysis Reaction, Showing 

 Formation and Activation of the Intermediate Complex, ES. 



