PJTa values

Substrates are normally sticky in the reaction direction with the higher maximum velocity, and not always then. In the direction with the lower maximum velocity, substrates cannot normally be sticky because their release rates from the enzyme must exceed the more rapid maximum velocity in the reverse direction. Thus, one needs to use the pH profiles of slow substrates only in the direction with the fastest maximum velocity in the reverse direction, one can use the profiles of the normal substrates for comparison. Further, one should use the V/K profiles for nonsticky substrates to determine the pK a values. Then, one should determine the nature of the catalytic groups by solvent perturbation and temperature variation of the pJta values, and then, by comparison with the V/K profile of the normal sticky substrate determine the stickiness and, from the shape of the profile in the vicinity of the pJCn, how rapidly the protonation state of the group is equilibrated when the substrate is present (Rose et al, 1974 Cleland, 1982). 

Bovine erythrocyte CA shows both oxonase and esterase activity with dimethyl 2,4-dinitrophenyl phosphate and 2,4-dinitrophenyl acetate respectively. Both processes show similar pH dependencies with pJTa values of 7.37 (oxonase) and 7.53 (esterase) and maximum activity in the basic form. Inhibition by acetazol-amide at pH 8.95 yields the same inhibition constant for both reactions, implying involvement of the active-site zinc to the same extent in both processes. Methyl and ethyl pyruvate are susceptible to hydrolysis,... 

The binding of several small molecules to cytochrome c peroxidase has been studied kinetically. In the case of fluoride, the data are consistent with the interaction of F with the protonated form of the enzyme (pJta 5.5) or with the interaction of HF with the ionized form of the protein. The latter is the preferred interpretation, giving a pH-independent rate constant for binding of 5.1 x 10 1 mol s. With cyanide the situation is rather more complex the maximum value of the association rate constant, which occurs between pH 6 and 8, is 1.1 x 10 1 mol S . ... 

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