GENERAL PROPERTIES OF ENZYMES 465 



same quantity of enzyme seems to be present at the end of the reaction as at 

 the beginning. A very small quantity of enzyme can catalyze the trans- 

 formation of relatively large quantities of the substrate. It has been esti- 

 mated, for example, that the enzyme sucrase ("invertase") can effect the 

 hydrolysis of at least 1,000,000 times its own weight of sucrose without exhibit- 

 ing any appreciable diminution in its activity. The cessation of enzymatic 

 reactions is usually due, not to a diminution in the effectiveness of the enzyme, 

 but to other causes. All hydrolytic reactions catalyzed by enzymes are re- 

 versible and accumulation of the end products of the reaction may bring the 

 reaction to an equilibrium point at which the hydrolytic reaction and the op- 

 posite condensation reaction proceed at the same rate. If the end products 

 are removed, however, the reaction often will continue, usually to completion. 



2. Colloidal CoTidition. — Enzymes themselves either exist in a colloidal 

 state or else are so closely associated with other compounds in the colloidal 

 condition as to make it practically impossible to distinguish between the 

 enzyme and the accompanying substances. Willstatter, one of the leading 

 authorities on enzymes, believes them to consist of a small chemically active 

 group bound to a large colloidal carrier. The fact that the most recent 

 chemical analyses indicate that certain enzymes, in as pure a form as it has 

 been possible to prepare them, are either proteins or protein-like compounds 

 probably may be accepted as further evidence of their colloidal condition, 

 since the proteins present in living cells are usually in the colloidal state. 

 Furthermore enzymes dispersed in water exhibit many of the properties of 

 hydrophilic sols, and, like proteins, apparently possess amphoteric properties 

 (Chap. V). 



The catalytic effect of enzymes is often interpreted in terms of an adsorp- 

 tion mechanism. By some this association between enzyme and substrate is 

 considered to be a loose chemical combination, by others a true adsorption 

 phenomenon. Whichever of these pictures is considered to represent the 

 relation between enzyme and substrate it is certain that the adsorption — using 

 this term in its broadest sense to refer to either a loose physical or loose 

 chemical combination of a substrate with an enzyme — is a highly selective 

 process. 



It is supposed that while the enzyme and substrate are bound together 

 in loose physical or chemical combination that the catalytic effect of the 

 enzyme is exerted. The end products, under the conditions usually pre- 

 vailing in hydrolytic reactions, are less strongly bound to the enzyme than 

 the substrate, and hence, as in the case of inorganic catalysts, are released 

 into the reaction medium. 



