ENZYMES 



(Readings: Weisz, pp. 135-141 and 271-273. S.P.T., pp. 93-96. Villee, 

 pp. 57-65, 306.) 



Living cells have the remarkable capacity to 

 perform rapidly and under mild conditions 

 chemical reactions which under the same cir- 

 cumstances would proceed extremely slowly 

 outside the organism. A homely example: sugar 

 exposed to oxygen burns to carbon dioxide and 

 water, generating considerable heat in the proc- 

 ess. If you touch a match to the sugar, thus pro- 

 viding energy of activation, this reaction goes 

 very rapidly, as you know. Without the match, 

 i.e., at room temperature, the same reaction goes 

 in exactly the same way, yielding just as much 

 carbon dioxide, water, and heat, but so slowly 

 as to be negligible. In a frog at room tempera- 

 ture, however, or in yourself at a slightly higher 

 temperature, the same reaction occurs rapidly, 

 yielding exactly the same products, and exactly 

 the same amount of energy, though the latter, 

 before being degraded finally to heat, is used 

 for all the multiple activities of the organism. 



The enzymes of living cells greatly accelerate 

 such chemical reactions, and by governing their 

 relative rates, regulate the overall directions of 

 metabolic change. Enzymes are catalysts: they 

 greatly speed a chemical reaction, without them- 

 selves being used up in the process. It is not 

 that they don't take part in the reaction. They 

 do, by combining for a moment with the react- 



ant, the substrate; but at the end of the reaction 

 the enzyme is returned and can be used again: 



enzyme -f substrate ^ 



enzyme-substrate complex ;^ 



enzyme -|- products. 



This is what we mean by a catalyst; and for 

 this reason a little enzyme goes a long way. 



Since the enzyme is returned unchanged at 

 the end of the reaction, it can contribute nothing 

 to the final result. If the reaction is reversible, 

 the presence of the enzyme hastens, but does 

 not change, the final equilibrium. That is, in 

 any reversible system, the enzyme speeds up 

 equally the forward and the back reaction. This 

 behavior also is typical of all catalysts. Thus 

 the pancreatic enzymes you have already used 

 catalyze equally well the hydrolysis and the 

 synthesis of peptide linkages; yet because the 

 equilibrium of this pair of opposed reactions 

 lies far over toward hydrolysis, and because the 

 reaction usually occurs in the presence of over- 

 whelming concentrations of water, an almost 

 irreversible hydrolysis is the end result. 



All known enzymes are proteins, and many 

 of their properties depend upon this fact. Their 

 activity depends, as do many other protein 



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