ENZYMES AND THEIR ACTION 195 



pages 56 to 58), is an added indication that there is some kind 

 of a union between the enzyme and the substrate as a first step 

 in the catalytic process. 



As to the nature of this supposed combination of substrate and 

 enzyme, two theories are held. The first is that this union is in the 

 form of an actual molecular combination, or chemical compound, 

 and the other is that it is a purely physical, or colloidal, complex. 

 The latter view has by far the greater weight of theoretical and 

 experimental evidence in its support. The relation of electrolytes 

 to the catalytic effect of enzymes, the appearance of the reacting 

 masses under the ultra-microscope, and the effect of heat Upon the 

 reacting mixtures, all point to the conclusion that the phenomenon 

 is colloidal rather than molecular in character. This view also 

 makes the remarkable catalytic effects which take place in living 

 protoplasm, w r hich undoubtedly exists in the colloidal condition, 

 much more easily understood. This phase of the matter will 

 be much more apparent after the chapter dealing with the physical 

 chemistry of the protoplasm has been studied. 



A further indication that the mechanism of enzyme activity is 

 colloidal in character lies in the fact that, so far as is known, all 

 reactions which are catalyzed by specific enzymes are reversible 

 and the same enzyme will accelerate the velocity of the reaction 

 in either direction, the direction in which the reaction goes being 

 determined by the conditions surrounding the reacting material 

 at the time. It was formerly supposed that enzymes catalyze 

 only decomposition reactions and that the synthetic reactions of 

 living tissues are produced by means of some other force or agency. 

 This view supported the idea of a chemical union of the enzyme 

 with the substrate which, when it breaks down, breaks the mole- 

 cule of the substrate material into some simpler form, or forms. 

 But it is now known that the reaction which is influenced by the 

 enzyme will be catalyzed in either direction by the specific enzyme 

 which " fits " the particular substrate material at every point of 

 its molecular configuration, as the glove fits the hand. The con- 

 trast between this fitting of the enzyme to the entire configuration 

 of the molecule, and the union at a single point or group which is 

 characteristic of chemical linkages, is apparent. As examples of 

 the synthetic action of the same enzyme which, under other con- 

 ditions, accelerates the decomposition of the same material, there 

 may be cited the demonstrated synthesis of isomaltose from glu- 



