FACTORS AFFECTING ENZYMATIC REACTIONS 473 



initial velocity of the reaction may be maintained almost indefinitely at lower 

 temperatures, but the higher the temperature, the more markedly it decreases 

 with time. 



The optimum temperature for most enzymatic reactions, if measured in 

 terms of hours, lies between 40° and 50° C, although for a few it is as 

 high as 60° C. If only a very short interval of time is allowed to elapse 

 before a determination of the amount of substrate hydrolyzed is made, a 

 higher optimum will usually be found, than if longer time intervals are used 

 as a basis of measurement. The optimum is influenced, however, not only by 

 the time factor, but also by other conditions prevailing in the medium such 

 as the pH, relative concentration of enzyme and substrate, etc. 



At temperatures above the optimum point for any enzymatic reaction, 

 inactivation of the enzyme occurs so fast that the rate of the reaction is rapidly 

 decreased. It is also possible that the retarding influence of relatively high 

 temperatures may sometimes be due to a greater acceleration of the reverse 

 reaction (whereby the substrate is resynthesized from the end products) 

 than the hydrolytic reaction. 



At temperatures below those at which an enzyme is rapidly inactivated 

 a 10° C. rise in temperature usually increases the rate of the reaction cata- 

 lyzed by that enzyme from two to three times. In other words the tempera- 

 ture coefficient (Qio) of an enzymatic reaction is generally between two 

 and three. 



2. Hydrogen Io?i Concentration. — Sorensen, who first proposed the pH 

 system of notation as a method of indicating hydrogen ion concentration, was 

 one of the first workers to realize clearly that the speed of reactions catalj^zed 

 by enzymes is greatly influenced by the pH of the medium in which the re- 

 action occurs. This is one of the most important known effects of hj'drogen 

 ion concentrations in the realm of biological phenomena. This fact fits in 

 very well with the generally held presumption that enzymes are amphoteric 

 substances. The hydrogen ion concentration of the medium exerts a con- 

 trolling effect on the sign and the magnitude of the charges carried by the 

 particles of ampholytes. Apparently the activity of an enzyme is at its maxi- 

 mum when it carries a certain charge and inactive when the charge deviates 

 too far from the value in either direction. Hence the pH value at which 

 a charge of the proper magnitude and sign is present on the enzyme is 

 the one at which the enzyme will operate at its maximum efficiency. The 

 pH of the medium may also influence the charge on the substrate as well as 

 the charge on the enzyme. The influence of hydrogen ion concentration on 

 enzymatic activity may be due partly to this effect. 



In general there appears to be an optimum hydrogen ion concentration 



