Metmyoglobin/Azide: The Effect of Heme-Linked Ionizations on the Rate of Complex Formation

Abstract

The kinetics of formation and dissociation of the horse metmyoglobin/azide complex has been investigated between pH 3.5 and 11.5. The ionic strength dependence of the reaction has been determined at integral pH values between 5 and 10. Hydrazoic acid, HN3, binds to metmyoglobin with a rate constant of (3.8 ± 1.0) × 105M−1s−1. Protonation of a group with an apparent pKaof 4.0 ± 0.3 increases the rate of HN3binding 6.5-fold to (2.5 ± 0.8) × 106M−1s−1. The ionizable group is attributed to the distal histidine, His-64. The azide anion, N−3, binds to metmyoglobin with a rate constant of (4.7 ± 0.3) × 103M−1s−1, about two orders of magnitude slower than HN3. Conversion of aquometmyoglobin to hydroxymetmyoglobin slows azide binding significantly. Binding of HN3to hydroxymetmyoglobin cannot be detected, while N−3binds to hydroxymetmyoglobin with a rate of 5.7 ± 3.2 M−1s−1, almost three orders of magnitude slower than N−3binding to aquometmyoglobin. Protonation of the distal histidine facilitates HN3dissociation from the complex. HN3dissociates from the metmyoglobin/azide complex with a rate constant of 18 ± 6 s−1, while the azide anion dissociates with a rate constant of 0.16 ± 0.02 s−1, about 100 times slower. The apparent pKafor His-64 is essentially the same in metmyoglobin and the metmyoglobin/azide complex, 4.0 ± 0.3 and 4.4 ± 0.2, respectively. The ionic strength dependence of the observed association rate constant is influenced by both primary and secondary kinetic salt effects. The primary kinetic salt effect is anomalous, with the rate of N−3binding decreasing with increasing ionic strength above the isoelectric point of metmyoglobin where the protein has a net negative charge. The ionic strength dependence of the dissociation rate constant can be described solely in terms of the ionic strength dependence of the acid dissociation constant for His-64 in the metmyoglobin/azide complex, a secondary kinetic salt effect.