EFFECTS OF TEMPERATURE: ENZYMES 765 



unity or less than unity. Changes in the temperature may also alter K^ 

 and jfiC, or K- so that the variation of the inhibition in most cases would 

 be more complex than represented here. A complete description of an 

 inhibition and its dependency on temperature would require the deter- 

 mination of the constants K. K^, and K/, but experimentally this would 

 require an exact elucidation of the mechanism of the inhibition and a great 

 deal of very accurate data obtained at different temperatures and concen- 

 trations of inhibitor. 



The change of K or (E,,)/(E^) with temperature may be roughly estimated 

 if the inactivation process is akin to protein denaturation. The average 

 thermodynamic characteristics for the denaturation of trypsin, chymotryp- 

 sinogen, and soy bean trypsin inhibitor are: JF = — 1.2 kcal/mole, 

 AH = 75 kcal/mole, and AS = 235 cal/mole /degree. From Eq. 15-6 it 

 may be calculated that a 10° rise in temperature in the physiological range 

 would decrease iii by a factor of approximately one-fortieth. However, 

 local structural changes at the active center may not follow over-all pro- 

 tein denaturation thermodynamics, so that any estimates on this basis 

 must be considered to be very tentative. The temperature coefficient, nev- 

 ertheless, would probably be fairly high. 



We must now examine as critically as possible this concept of equilibria 

 between active and inactive forms of enzymes and the bearing this has on 

 inhibition and temperature effects. A marked rise in the inhibition of an 

 enzyme upon increasing the temperature is not in itself sufficient evidence 

 for such an equilibrium, since the inhibition reaction may have a high 

 enthalpy change independent of any equilibrium between active and inac- 

 tive forms. The inhibitor, indeed, may produce a local denaturation of the 

 enzyme and this may occur more readily at higher temperatures, but this 

 does not imply the presence of denatured forms before the inhibitor is 

 added. There is no question about the denaturation of enzymes when the 

 temperature is elevated sufficiently and all enzymes exhibit maxima in 

 their temperature-activity curves. The problem here is: at physiological 

 temperatures between 15° and 38°, where most enzyme and metabolic 

 work is carried out, is such enzyme inactivation of importance? The op- 

 timal temperature for enzyme activity (i.e., the temperature at which the 

 rate is maximal) has been stated to be between 40-50° for most animal en- 

 zymes and between 50-60° for most plant enzymes (Sumner and Somers, 

 1953). Inasmuch as the temperature coefficient is high, this would indicate 

 that the majority of enzymes do not exist appreciably in the inactive form 

 in the physiological temperature range. However, certain enzymes are par- 

 ticularly unstable even at 37° when they are isolated from cells, and in such 

 cases an appreciable fraction may occur in the reversibly denatured or inac- 

 tive form. For most enzymes the situation is probably as shown in Fig. 

 15-6 where the experimental temperature range is sufficiently far below 

 the optimal temperature so that only insignificant amounts of inactive 



