754 



15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



nates, dodecyl sulfate, and other surfactant substances (Klotz, 1953). These 

 results also indicate that the binding may either increase or decrease with 

 a rise in temperature, inasmuch as AH may be positive or negative. 



The K, for the inhibition of beef heart succinic dehydrogenase by malon- 

 ate was given by Singer et al. (1957) as 0.041 mM at 38o and 0.025 mM 

 at 20-23°. At a constant malonate concentration the inhibition would thus 

 decrease with rising temperature. Assuming that Eq. 15-6 is valid, one may 

 calculate that at 38° the binding of malonate to the enzyme is characterized 

 by the following thermodynamic quantities: JF = — 6.26 kcal/mole, 

 AH = — 5.48 kcal/mole, and AS = 2.6 cal/mole/degree. The enthalpy 

 values for the substrates, succinate and fumarate, are similar in magni- 

 tude, supporting the concept that malonate is bound like the substrates 

 to the enzyme active site. The temperature effects on the inhibition of 

 fumarase by malonate and succinate are more complex, in that the for- 

 mation of the EI complex is exothermic at low temperatures and endotherm- 

 ic at higher temperatures (Massey, 1953 b). The plots of log K^ against 

 1/T show linear portions connected by sharp breaks (Fig. 15-2), AH being 



2.0 



1.5 



-LOG K 

 1.0 



UCCINATE 



310 



320 



330 



340 



350 



360 



l/T 



370 



Fig. 15-2. Effects of temperature on the inhibitor constants for the 



inhibitions of fumarase by malonate and succinate at pH 6.35. AH 



given in kcal/mole. (From Massey 195.3b.) 



around — 20 kcal/mole above 27° and 5 kcal/mole below this temperature. 

 Since K^ would be the same for two different temperatures for each inhibitor, 

 and thus the AF the same at these temperatures, the markedly different 

 values of AH would indicate that the entropies of binding at high and low 

 temperatures are very different. One might readily interpret this striking 

 change with temperature as resulting from a modification of the structure 

 of the active center and supporting the temperature-dependent flexibility 

 of the enzyme. The inhibition of fumarase by thiocyanate is weakly de- 

 pendent on temperature, the yyK, changing from 2.3 to 2.2 as the temper- 

 ature rises from 25° to 35° (Massey and Alberty, 1954). 



