ENZYMES — KILBY 



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and in such cases it is nearly always found that only one form will 

 work with the enzyme. The two forms of lactic acid are shown in 

 figure 1, and the enzyme lactic dehydrogenase which removes two 

 hydrogen atoms to convert lactic acid into pyruvic acid will func- 

 tion with the naturally occurring form, but not with the other, its 

 mirror image. The active center might be thought of as a hand and 

 the lactic acid as a glove, when only one glove of the pair will fit the 

 hand. 



When an enzyme reaction takes place, one may picture the sub- 

 strate molecule colliding with the active center, forming a complex, 

 reaction occurring and the products then leaving the center which 

 is then free for another cycle to take place. The enzyme molecule 

 (if it has a single active center) can thus deal only with a single 



H 



Figure 1.^ — Models of the two forms of lactic acid, CH3CH(0H)C00H, which are mirror 

 images. These models give the best representation that is possible of the actual shape 

 of molecules. (From Organic Chemistry, by L. F. and M. Fieser.) 



molecule of substrate at a time, but the cycle may be repeated very 

 rapidly; a single molecule of catalase, for instance, can break up 

 at least 5 million molecules of hydrogen peroxide in a minute. Wlien 

 measurements are made of the speed of an enzyme reaction in the 

 presence of increasing amounts of substrate, a curve of the type 

 shown in figure 2 is usually obtained. This has the form of a rec- 

 tangular hyperbola. If there is ample substrate, the enzyme is work- 

 ing full out all the time and increasing the concentration of substrate 

 has little effect as at A. At low concentrations of substrate, as at 

 B, the active centers are unoccupied for much of the time, and so 

 increasing the amount of substrate has a direct and proportional 

 effect on the speed. 



As an analogy, one might consider a crowd of football fans wait- 

 ing to enter a football ground. The individual fans are the mole- 



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