ENZYMES 47 



change in their effective concentration. It is only the 

 velocity of the reaction which is altered. 



(b) Substrate Concentration. — The term " substrate " 

 is applied to the substance on which the enzyme exercises 

 its cataljrtic properties . Invertase catalyses the conversion 

 of the substrate sucrose into glucose and fructose. 

 Hydrogen peroxide is the substrate which under the 

 action of the enzyme catalase brealis down into water 

 and oxygen. 



With low concentrations of substrate the reaction 

 velocity is in many cases proportional to the substrate 

 concentration for a given concentration of enzyme, but 

 at higher concentrations the rate of reaction rises less 

 rapidly than the concentration. This is very probably 

 due to the saturation of the enzyme surface by adsorption 

 of the substrate to form the hypothetical intermediate 

 compounds which have been postulated in most theories 

 of enzyme action. In some cases, too, the enzyme adsorbs 

 a part of the products of the reaction with a consequent 

 slowing of the rate of reaction. It seems to be a general 

 rule that the oxidising and reducing enzymes are saturated 

 by the substrates at considerably lower concentrations 

 than are the hydro lytic enzymes. 



(c) Heat. — Rise of temperature at first increases the 

 rate of reaction of enzjrmes in the ordinary way common 

 to all chemical reactions ; but at comparatively low 

 temperatures an optimum is reached, and then the activity 

 faUs off with further increase of temperature until at 

 about 70° C. the action of most enzymes is stopped, 

 whilst 100° C. is sufficient to inhibit all known enzyme 

 action. It is seen that the effect of temperature is a result 

 of the competition between the acceleration of chemical 

 reactions by rise of temperature and the gradual destruc- 

 tion of the enzyme at higher temperatures . The optimum 

 temperature varies with the particular enzyme concerned, 

 but the majority of enzymes have temperature optima 

 falling between 35° and 45° C. Freezing has no permanent 



