FACTORS AFFECTING ENZYMATIC REACTIONS 475 



enzymatic reactions can, however, be readily determined. It is only neces- 

 sary to dilute a given extract to one-half, one-fourth, one-eighth, etc. of 

 its initial concentration and to study the rates of reaction resulting when a 

 series of such diluted extracts is employed. In general the activity of ex- 

 tracts of most enzymes is approximately proportional, at least within limits, 

 to the concentration of the enzyme (Fig. 102). 



5. Concentration of the Substrate. — The velocity of an enzymatic reaction 

 usually increases with increase in the concentration of the substrate up to a 

 certain maximum, after which the 

 relative amount acted upon per unit g 

 of time may decrease with increase o 

 in the substrate concentration. The 

 retarding effect of relatively high 

 concentrations of the substrate upon 

 enzj-me activity may be due in part 

 to the more rapid accumulation of 

 the end products of the reaction (see 

 next section). Furthermore as the 

 concentration of the dissolved or dis- 



2 3 4 5 



SUCRASE-C.CPER 100 C.C. 



Fig. I02. Relation between concentra- 

 tion of sucrase and its hydroljtic activity, 

 persed substrate in a reaction mix- Data of Nelson and Hitchcock (1921). 

 ture increases, the concentration of 



water decreases. The smaller proportion of water present at higher substrate 

 concentrations prevents the action of the enzyme at its maximum efficiency. 



6. Concentration of the End Products. — Enzymatic reactions, like all 

 other chemical reactions, are subject to the laws of chemical equilibrium and 

 mass action. Hence as the end products of the reaction accumulate the 

 apparent rate of the reaction decreases. Similarly if the concentration of 

 the end products in the reaction mixture is initially high the rate of the 

 reaction will be slower than if the end products are lacking. In some enzy- 

 matic reactions the enzyme itself combines with one of the end products. 

 This results in a reduction in the active concentration of the enzyme present, 

 and since the rate of the reaction is proportional to the active concentration 

 of the enzyme, its velocity is slowed down. 



7. Accelerators. — Certain specific chemical substances are known to exert 

 a stimulating effect upon enzyme activity. Such substances are generally 

 called accelerators or activators. Some accelerators seem to be general, i.e. 

 increase the activity of all or most enzymes, while others are specific for cer- 

 tain enzymes. Among the former are low concentrations (2-5 per cent) of 

 the salts of the alkali and alkaline earth metals. Accelerators may exert their 

 influence either by some direct effect upon the enzyme, or by eliminating the 



