62 BIOLOGICAL CHEMISTRY 



respectively. In agreement with this rule enzyme action 

 shows a temperature coefficient between two and three at 

 moderate temperatures. 



At higher temperatures a complicating factor occurs. 

 Enzymes are destroyed at high temperatures and it is one of 

 the criteria of an enzyme that its action is abolished by heating 

 to 100 C. The destruction commences at a lower temperature 

 and the rate of destruction increases as the temperature rises. 

 In order to show the activity of an enzyme it is necessary for 

 the enzyme to act upon the substrate for a definite time. 

 For this reason we introduce what is termed the time factor.* 

 Thus the rate of reaction increases as the temperature rises, 

 and if we could measure the rate of reaction instantaneously 

 it should continue rising, but at higher temperatures the rate of 

 destruction becomes so great that the enzyme is destroyed 

 before it can produce any measurable change in the substrate. 

 We therefore find that there is a range of temperature at 

 which the enzyme is most efficient. This temperature is 

 spoken of as the " optimum " temperature. The shorter the 

 time interval of the measurement the higher is the optimum 

 temperature as the increased activity due to rise of temper- 

 atures occurs and owing to the shorter time there is less 

 destruction of the enzyme. f 



INFLUENCE OF ACID AND ALKALI 



Enzymes are extremely sensitive to the presence of acid and 

 alkali. Some act best in acid ; others in alkali and others still 

 near the neutral point. During certain enzyme actions the 

 concentration of hydrogen ions tends to change owing to the 

 formation of substances which can neutralise acid or alkali. 

 For this reason recent experiments have been carried out using 

 stabilising solutions.} In this way the reaction is steadied 

 and the results easier of interpretation. Without the use of 

 the stabiliser the reaction changes all the time, hence the effect 

 of acid and alkali is less clear. 



* F. F. Blackman, Annals of Botany, 1905, vol. 19, p. 281. 



f The rate of destruction may have a very high temperature co- 

 efficient, hence the optimum temperature would not be much affected 

 by a shorter time interval. Compare H. Chick and C. J. Martin. Journ. 

 Physiol., 1910, vol. 40, p. 404. 



J In a previous chapter the real acidity has been shown to be the 

 hydrogen ion concentration, so we will here deal with the effect of 

 the concentration of hydrogen ions on enzyme action. The symbol 

 log [H-] is sometimes used to express the negative exponent of the 

 logarithm to the jbase 10 of the concentration of hydrogen ions. 



