EFFECTS OF TEMPERATURE: ENZYMES 751 



A more general rule may be formulated. We have seen that the inhibition 

 can usually be written in the form: 



(^) (15 7) 



(I) + KIX] 



where [X] represents the effects of various factors on the inhibition. In 



(S) 

 the case of competitive inhibition, Z = 1 H — ^ -, and for the situation 



in which only the ionized form of the inhibitor reacts with the enzyme, 



X = 1 + —^- In general, K/ = K^[X], where, as previously defined, 



K/ is the apparent inhibitor constant which may be obtained from the 

 usual plotting procedures. If the true inhibitor constant K^ is determined, 

 then the calculation of the inhibition must take into account the variation 

 of [X] with temperature. On the other hand, if the apparent constant K^' 

 is determined, thermodynamic data derived from it cannot be directly 

 applied to the binding of the inhibitor to the enzyme. For the complete 

 description of the temperature dependence of an inhibition, it is necessary 

 to investigate the variations in both K^ and X. Inasmuch as X often in- 

 volves two or more dissociation constants when the enzyme or inhibition 

 mechanisms become more complex, the change in the inhibition with 

 temperature may be difficult to estimate. 



It is good practice in determining inhibitions at various temperatures to 

 establish the reversibility of the temperature effects and in this way elim- 

 inate any irreversible changes that may occur. Raising the temperature 

 can occasionally lead to irreversible denaturation changes in the enzyme 

 and this may affect the inhibition kinetics. 



Possible Mechanisms Involved in Temperature Effects on Enzyme Inhibition 



Changes in the equilibrium constants induced by temperature varia- 

 tions and dependent on JH are common to all chemical reactions and bind- 

 ings. Interactions of substances with enzymes often involve additional 

 factors that lead to changes in JH, AS, and JF with temperature and some 

 of these factors will now be discussed briefly. 



(A) Alteration of the water structure. The association between water mol- 

 ecules decreases with rise in the temperature due to the increased thermal 

 movement. It has been shown by X-ray scattering that the water molecules 

 move about 0.02 A further apart on the average for each 10° increase in 

 the temperature. There is also a smoothing of some of the scattering peaks 

 and an elimination of others, indicating a greater randomization of the 

 water structure. The relationships between intermolecular forces and the 

 water structure have been discussed in Chapter 6 and it is evident that, 



