Relaxation studies of enzymes: rapid isomerization in deoxyribonuclease I

Temperature-jump relaxation studies in deoxy-ribonuclease I were carried out at 10 degrees C and [I] = 0.1 M. The single observed relaxation time, which varied from 10(-4) to 10(-5) s, was characterized as a function of enzyme concentration, pH, and indicator concentration. The concentration and pH dependences of the relaxation time are in quantitative agreement with a mechanism involving an isomerization of the enzyme coupled to a rapid proton ionization process. The best fit forward and reverse isomerization rate constants are 6.5 X 10(3) and 7.2 X 10(4) s-1, respectively; the apparent pK is 5.7. The addition of urea brought about reductions in both the amplitude of the relaxation effect and the enzyme activity.