766 



15. EFFECTS OF VAEIOUS FACTORS ON INHIBITION 



enzyme are present. Thus it would appear that for an E^/E^ equilibrium 

 to be assumed to explain the effects of temperature on enzyme inhibition, 

 experimental evidence that such an equilibrium exists in the temperature 

 range studied for the particular enzyme under experimental conditions 

 must be demonstrated. 



There is some indirect evidence that E,,/E^ equilibria are not important 

 for the majority of enzymes. We have seen above that noncompetitive 



RATE 



Fig. 15-6. Generalized variation of enzyme 

 rate with the temperature within the common 

 experimental range of temperatiu'es shown. 



inhibition would depend to some extent on the substrate concentration if 

 an inactive form of the enzyme occurs, and yet there are many instances 

 where noncompetitive inhibition has been shown to be independent of the 

 substrate concentration in the temperature range studied. Also it may be 

 noted that according to Eq. 15-17 for an uninhibited reaction, the apparent 

 substrate constant would involve K, i.e. KJ = KJil + (1/^). If K varies 

 markedly with the temperature, one would expect the experimentally de- 

 termined Michaelis constants also to vary strongly with the temperature, 

 which usually does not seem to be the case. The evidence from high pres- 

 sure studies will be considered later. 



A method of calculating K, the equilibrium constant for the interconver- 

 sion of E„ and E^, has been suggested by Hultin (1955) and applied to the 

 hydrolysis of tributyrin by pancreatic lipase at subzero temperatures. 

 Inactivation of the enzyme becomes progressively greater as the tempera- 

 ture is reduced below — 5^ as indicated in the accompanying tabulation 

 where Hultin's K has been altered to fit the convention adopted above. 

 The enthalpy change of the inactivation was calculated to be — 27 kcal/ 



