846 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



(Laidler and Beardell, 1955). A rise in the pressure leads to an increase in 

 k^ and an increase in K,^^. This means that the binding of ATP to the enzyme 

 involves an increase in the volmne, this presumably resulting from the neu- 

 tralization of ionic charges and the release of water of hydration. This 

 effect alone would decrease the enzyme rate but the increase in k^ compen- 

 sates for this and, especially at high substrate concentrations, the rate is 

 increased at high pressures. The increase in A'2 with pressure and the cor- 

 responding decrease in volume on forming the activated complex were 

 attributed to changes in the configuration of the enzyme; however, it is 

 also possible that there is an increase in polarity during the activation and 

 a consequent electrostriction of the water. The high pressure data conform 

 quite well to the entropy changes determined by other means. 



Effects of Pressure on Enzyme Inhibition 



There has been, unfortunately, only one study on the response of en- 

 zyme inhibition to high pressure, the inhibition of yeast fructofuranosid- 

 ase by urethane. The extensive investigations on the inhibition of bio- 

 luminescence may relate to the luciferase system but this is by no means 

 certain so that this work will be discussed in the following section. Al- 

 though pressure studies will probably never be common, due to the technical 

 problems, it is likely that some enzyme inhibitions will be profitably at- 

 tacked in this way in the future, so that a brief discussion of some of the 

 factors that may be involved is not out of place. 



When the inhibition is the result of the simple complexing of the inhi- 

 bitor with the enzj^me, the principal effect of high pressure will be on the 

 binding affinity as manifested by changes in A',. Many inhibitors are bound 

 to enzymes by electrostatic forces between oppositely charged ionic groups 

 so that the formation of the EI complex in these cases will result in the 

 release of some water of hydration, increasing the volume. The effect of 

 high pressure will then be to reduce the binding and the inhibition. The 

 change in K^ may be estimated from Eq. 15-118 or, conversely, pressure 

 data may be used to determine the volume changes upon inhibition. Of 

 course, other factors, such as changes in the pA,/s of acidic groups or al- 

 terations in the enzyme structure, may be important. Also certain inhi- 

 bitions involve the interaction of nonpolar surfaces and a reduction in the 

 volume due to the displacement of the water. Just as in the effects of the 

 dielectric constant and the ionic strength, the final result will depend in the 

 case of a complex inhibitor on the balance of the forces participating in the 

 binding. 



Competitive inhibition will be affected by pressure in a more complex 

 fashion because A",„ will ])e changed along with K^. Inasmuch as competitive 

 inhibition usually involves an inhibitor structurally related to the sub- 

 strate, the forces binding the two substances to the enzvme will often be 



