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there not be another cause, if the pepsin were to find in the gel 
protein which it could attack? 
It has long been known that enzymes are capable of binding 
other substances, not only substances which can be decomposed 
under the influence of the enzyme, but also substances of quite a 
different nature, on which the enzyme does not exert any influence 
at all. Pepsin e.g. combines not only with proteins but also with 
carbon. 
In a concise review on the nature and the action of enzymes *) 
I have endeavoured to show that this combination is effected in 
various ways. First of all there is adsorption. If, as in the case of 
pepsin, the enzyme and the substance bound by it are both colloidal 
substances and consequently a difference in surface-tension is of little 
importance for the adsorption, it is especially the difference in the 
electric charge of the molecules that comes into play. Owing 
to this the particles of one substance aggregate as closely as possible 
on the periphery of those of the other substance. In an acid solution 
pepsin is charged negatively, protein positively. 
The compound thus formed is to a large extent independent of 
the nature of the two substances. Just as finely divided carbon can 
bind pepsin as well as all sorts of other enzymes, trypsin also 
combines not only with protein but also with starch and compounds 
have been obtained of amylase not only with starch, but also with 
casein. Adsorption promotes the action of the enzymes by increasing 
the concentration of the substrate in the immediate neighbourhood 
of the enzyme or the concentration of the enzyme in the immediate 
environment of the particles of the substrate. This action is, however, 
only of a promotive character. For a chemical change the enzyme 
must combine with it in a manner that depends on the molecular 
constitution of the substrate as well as of the enzyme. As B. Fiscumr 
has put it: the enzyme must fit to the substrate, or what BEYERINCK 
terms the “zymotele’, like a key to a lock. Only when this kind 
of combination is effected, can the decomposition of the substrate, 
generally with addition of water, take place. In this process the 
enzyme is detached from the substrate in order to combine again 
with other still intact particles of it. Consequently a small amount 
of the enzyme can continually decompose new particles of the 
substrate, unless the enzyme itself is destroved by noxious influences, 
as e.g. is the case with trypsin by alkaline reaction of the solution, 
which however aids the action of the enzyme. 
1) Some Remarks on Enzymes. Recueil des Trav. Bot. néerl. XVI, 207. 
