142 GENERAL BIOCHEMISTRY 



Temperature 



Changes in temperature characteristically change the rates of all 

 reactions, including those involving enzymes. As the temperature 

 rises, reaction rates increase. The exceptions to this statement are 

 more apparent than real and involve phase changes or the occurrence 

 of competing reactions and the like. 



Reactions of isolated enzyme systems normally accelerate as the 

 temperature rises through the physiological range. However, even- 

 tually a characteristic optimum temperature is reached and further 

 increases cause a decrease in rate. Although harder to establish, this 

 same phenomenon occurs for enzymic reactions in cells. In most cases 

 these maximum rates lie at temperatures where the participating 

 enzymes begin to undergo appreciable denaturation. From this point 

 on, then, the higher the temperature, the more enzyme is denaturated 

 and the lower the overall catalytic rate, since this rate is directly 

 proportional to enzyme concentration. 



Mathematical equations describing the relationship between tem- 

 perature and reaction rate are available but are omitted here for two 

 reasons. First, the mathematics itself is beyond the scope of this book. 

 Second, the equations are largely limited to isolated and purified 

 enzyme systems and to reactions in the absence of complexities like 

 denaturation. 



In general, these expressions depend upon the idea that molecules 

 must be activated beyond some definite energy level before reaction 

 can occur. As shown on page 22 a rise in temperature increases 

 the fraction of molecules sufficiently activated to undergo reaction, and 

 the rate increases. Enzymes themselves are believed to accelerate 

 reactions by somehow lowering the energy level required before the 

 substrate can react. Presumably this reduction in the threshold 

 activation energy is a property of the enzyme-substrate complex, 

 including all necessary cofactors. Possibly chemical bonds of the 

 substrate are strained or otherwise destabilized in the complex, making 

 them susceptible to reaction at lower energy levels and lower tem- 

 peratures. Or perhaps the enzyme molecule serves as an energy source, 

 trapping and transferring sufficient energy to the substrate to activate 

 it for reaction. 



pH 



Changes in the pH of enzyme systems alter the rates of the reactions 

 by exerting any one or any combination of three different influences. 

 The overall effect leads to the occurrence of an optimum pH, a range 



