A) Activation by imidazole (pKa = 7.0) and B) 2-aminoimidazole (pKa = 8.5) of R367A AblKD at pH 6.8 and 8.2 were fit to equation 1. Reactions were performed at 30 ° in 100 mM HEPES, 10 mM MgCl2, 1 mM ATP, 440 µM Abltide, 0.1 U/µL pyruvate kinase, 0.05 U/µL lactate dehydrogenase, 1 mM phosphoenolpyruvate, 150 µM NADH, and 0.5 mM Na3VO4 at the indicated pH.