FRANK H. JOHNSON 271 



librium reaction, together with the catalytic reaction of a limiting 

 enzyme, are sufficient to account for a major part of the temperature- 

 activity curve. For convenience of discussion, we may designate the 

 equilibrium constant for the reversible denaturation as Ki, and the 

 specific rate constant of the catalytic reaction as ki. 



Figure 4 illustrates the observed intensity (by visual photometry) 

 of steady-state luminescence in a suspension of A. fischeri cells during 

 brief exposures to various temperatures, above as well as below the 

 optimum. The soHd line is a curve calculated in accordance with 

 the equation and constants given in the figure, assuming only the two 

 reactions, with constants Ki and ki, referred to above. Although the 

 theory is obviously oversimplified, the curve fits the data within the 

 limits of experimental error, except at the highest temperatures where 

 destructive reactions with high-temperature coefficients complicate 

 the simplified picture. With some other species of bacteria, it has not 

 proved possible to describe corresponding data with the same accu- 

 racy, showing again that the theory is oversimplified in not including 

 additional reactions which appreciably influence the quantitative var- 

 iation of the overall process with temperature. 



Among other processes, the simplified theory describes with con- 

 siderable accuracy the rate of reproduction of Escherichia coli as a 

 function of temperature, from somewhat above to well below the 

 normal optimum of 37° to 39° under the conditions involved (John- 

 son and Lewin, 1946). Bacteriostasis occurs at about 45° C, but 

 growth is immediately resumed on cooling to 37° C. Analysis of the 

 data indicates that more than one equilibrium reaction is involved 

 in the reversible bacteriostasis at high temperatures, although most 

 of the temperature-activity curve can be accounted for on the same 

 basis as that of bacterial luminescence. 



Volume Changes of Activation (AV*) in 

 Enzyme-Catalyzed Reactions 



Reactions involving only small molecules are not likely to be accom- 

 panied by volume changes of activation exceeding a few cubic centi- 

 meters per mole (cf. Stearn and Eyring, 1941). With large molecules, 

 such as proteins and enzymes, there is the possibility of large volume 

 changes of activation, of the order of 50 to 100 cc per mole, depend- 



