168 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



In the case of peptidases, doubt has long been cast on the validity of this 

 finding : it appeared that the speed of hydrolysis increased less rapidly 

 than the enzyme concentration. Northrop has provided an explanation of 

 this phenomenon, in the case of pepsin and trypsin, by showing that the 

 effect is due to the presence of impurities exercising an inhibitory action 

 on the enzyme, 



B. Influence of pH 



If the pH is varied, one normally observes that the initial velocity passes 

 through a maximum which is called the "optimum pH". This is a con- 

 sequence of the protein nature of the enzyme. The phenomenon is ex- 

 plained by postulating that the active part of the enzyme consists of a 

 — C00~ group and a — HN3+ group associated through their charges. 

 Changes in pH consequently change the concentration of the active form 

 of the enzyme in the solution. 



H+ 

 OH- 



NH2— COO-^ NH3+— COO- ^ NH3+— COOH. 



Inactive enzyme 



H+ 

 OH- 



Active enzyme 



Inactive enzyme 



Activity 



10 n pH 



Fig. 33 (Michaelis and Davidson) — Activities of j8-fructosidase (invertase) 

 and trypsin at difTerent pH values. 



