

CATALYSIS AND ENZYMES 311 



/3-glucosides, which are almost innumerable- in number. Some investigators seem to l>e 

 prepared to postulate a separate enzyme for each glucoside. This question requires more 

 detailed discussion in a later page. 



As to the action of heat, the sensibility of enzymes varies considerably, according 

 to the conditions present. As a rule, they are coagulated or precipitated by heat, 

 but in some cases the enzymes seem to be merely carried down by adsorption or 

 changed in their physical state, reversibly. In practice, however, this property is 

 frequently useful in deciding the nature of a particular agent. If the action is 

 stopped by moderate heat, say up to that of boiling water, it is almost certainly 

 due to an enzyme or to the action of living protoplasm, using this latter name for 

 the present as a cloak for ignorance. To distinguish an enzyme action from it, use 

 is made of antiseptics, which have a more powerful action on what we call " vital 

 activity" than on that of enzymes. But, again, the distinction is one of degree 

 only, some enzymes are very sensitive to antiseptics, others not; the difference 

 probably depends on complexity of structure. Invertase is comparatively 

 insensitive, zymase is very sensitive. Meyer hof (1913 and 1914) finds the in- 

 hibiting effect of indifferent narcotics on enzymes to be completely reversible, and 

 interprets it as being due to the driving off of adsorbed substrate by the more 

 strongly adsorbed narcotic. We shall see later that preliminary adsorption is a 

 phase in the action of enzymes. 



We may take it, then, that enzymes are a special class of catalysts ; this being 

 so, their function is to alter the rate of reactions. The factors involved in the 

 velocity with which a reaction takes place have been incidentally dealt with to 

 some extent, but it may be well to spend a little more time on the question, as it 

 affects catalytic action especially. 



VELOCITY OF REACTIONS 



When one large molecule is undergoing the process of being divided up into 

 smaller ones, spontaneously, or under the action of a catalyst, there is no difficulty 

 in seeing that the number of molecules split -in a given time is proportional to 

 those present. 



This is the law of mass action in its simplest form. The history of this law, 

 which is the foundation both of chemical statics and dynamics, will be referred 

 to below, under the head of equilibrium. 



dC 



If C is the concentration at the time t, then - ' is the velocity of change 



at 



during so short a time that the rate does not alter, and, according to the law 

 of mass action, 



dc *r 

 Tt =kc > 



where k is a constant, varying with each individual case and known as the velocity 

 constant. 



In order to use this equation for any practical purpose, it must, of course, 

 be integrated, so that the change during a measurable time can be investigated. 

 The reader will note that we have another case of the " compound interest law," 

 and the simplest form of the integral is 



1 C, 



k =r~T lo &c> 



l/f, t<l \JQ 



where Cj and C., are the concentrations of the molecule undergoing change, ^ 

 and t.-, are the times after the commencement of the reaction at which the 

 concentrations are found to be C x and C 2 . This is known as a unimolecular 

 reaction. 



It is obvious that, in practice, any quality of the substance concerned which is a mathe- 

 matical function of its concentration, such as electrical conductivity or optical rotation, can 

 be used to represent C, provided that it is known what function of the concentration this 

 quality is. 



Let us take a step further and suppose that the reaction is one in which two 



