Temperature dependence of the primary kinetic hydrogen isotope effect as a mechanistic criterion

Catalase Reaction by Myoglobin Mutants and Native Catalase

“…In these reactions large KIEs in a range of 9 -29 are commonly observed due to a tunneling effect (27). We have also observed the tunneling effect (A H /A D ϭ 0.07) for the catalatic reaction of H64A Mb-I with KIE of 23 at 5°C, suggesting the involvement of hydrogen abstraction through a tunneling process.…”

“…On the other hand, in the reactions of H 2 O 2 and Mb-I of L29H/ H64L and H64X (X ϭ Asp, Ala, and Ser), the KIEs are extremely larger (10 -29) than those of F43H/H64L Mb and MLC (Table II). Large KIEs are due to the tunneling effect, which is confirmed by the Arrhenius parameter ratio (Table III) (27).…”

“…The A H /A D values (0.07) derived from the Arrhenius plots for H64A Mb, thus, indicate that large KIEs are ascribed to the linear tunneling effect (Fig. 5) (27). In contrast to H64A Mb, F43H/H64L Mb and MLC exhibited small KIEs of 2.1 and 4.0, respectively (we could not determine their thermodynamic parameters due to less stability and rapid reaction above 5°C, respectively).…”

“…Such large primary kinetic isotope effects suggest the occurrence of tunneling. Accurate measurements of the primary kinetic isotope effects as a function of temperature are very important for the examination of the tunneling effect (27). The incidence of tunneling results in the curvature of the Arrhenius plot (ln k ϭ ln A Ϫ E a /RT) in a wide range of temperature, which is larger for hydrogen as compared with deuterium.…”

“…The incidence of tunneling results in the curvature of the Arrhenius plot (ln k ϭ ln A Ϫ E a /RT) in a wide range of temperature, which is larger for hydrogen as compared with deuterium. In the temperature range of measurements (5-25°C), a larger A D value than A H , derived from an apparent linear Arrhenius plot, can be used as criteria to recognize the tunneling (A H /A D Ͻ 0.6) (Table III) (27). The A H /A D values (0.07) derived from the Arrhenius plots for H64A Mb, thus, indicate that large KIEs are ascribed to the linear tunneling effect (Fig.…”