400 CHEMICAL DYNAMICS 



must employ only a single enzyme. Anyone who possesses an 

 extensive knowledge of the literature on protein hydrolysis by 

 enzymes will readily admit that these conditions have very rarely 

 been realized. 



3. The Kinetics of Protein Hydrolysis by Enzymes. — In 

 several instances it has been found that very small quantities 

 indeed of proteolytic enzyme will suffice to convert large quan- 

 tities of protein; thus one part of rennet will curdle from 400,000 

 to 800,000 parts of milk (42) and a pepsin powder has been pre- 

 pared which in seven hours dissolved 500,000 times its weight 

 of fibrin. From these facts it has been urged by many observers 

 that the fermentative splitting of proteins is an instance of 

 "typical" catalysis, in the limited sense of the term described 

 in section 1. The justification for this view is totally inadequate, 

 for it has not been shown that at the end of extensive hydrolysis 

 the enzyme can be recovered from the digest, wholly unaltered 

 in efficiency, without the expenditure or absorption of any 

 energy. In fact, in many cases it is known that the activity of 

 the enzyme is impaired during the hydrolysis. Since this im- 

 pairment of activity is even more pronounced when proteins 

 are not present, it is^ usually conceded that autohydrolysis of 

 the enzyme itself suffices to account for the whole of this effect; 

 this is clearly not necessarily the case; while autohydrolysis of 

 the enzyme renders it very difficult to ascertain whether the 

 enzyme is actually exhausted by the hj^drolysis or not, yet it does 

 not exclude the possibility that such exhaustion occurs. Now 

 it must be constantly borne in mind that the proteins are very 

 nearly thermoneutral substances; their hydrolysis (Cf. section 1) 

 is accompanied by the disengagement of heat, it is true, but this 

 disengagement of heat is very minute. Hence the energy change 

 which is involved in a shift of the equilibrium between protein 

 and the products of its hydrolysis must be very minute and a 

 shift in the equilibrium of a correspondingly minute quantity 

 of a less thermoneutral substance might very well suffice to 

 provide or absorb the requisite energy to bring about a very 

 decided shift in the equilibrium between protein and its prod- 

 ucts. We cannot assume, therefore, from the smallness of the 

 amount of enzyme which will transform large quantities of pro- 

 tein, that the enzyme does not exert any influence upon the 

 equilibrium which the protein finally attains with its products. 



