Proton-linked Subunit Kinetic Heterogeneity for Carbon Monoxide Binding to Hemoglobin from Chelidonichthys kumu

Massimo Coletta,Paolo Ascenzi,Rossana D'Avino,Guido Di Prisco

doi:10.1074/jbc.271.47.29859

Abstract

The pH dependence of CO binding kinetics to Chelidonichthys kumu hemoglobin (Hb) and human adult Hb has been investigated between pH 2.0 and 9.0 at 20 degrees C. For both Hbs, CO binding kinetics is characterized by two proton-linked transitions, with different pKa values for alpha- and beta-chains in C. kumu Hb, leading to a relevant functional kinetic heterogeneity at most pH values. On the other hand, in human adult Hb the CO binding does not display a functional heterogeneity. Lowering the pH from 9 to 6 brings about a decrease of the CO binding rate constants, to a different extent for human adult Hb and the two chains of C. kumu Hb. Further lowering the pH from 6 to 2 induces an enhancement of CO binding rate constants, probably related to the protonation of proximal HisF8 Nepsilon atom and the cleavage (or severe weakening) of the HisF8-Fe bond. The presence of physiological concentrations of ATP (approximately 3 mM) affects the pH dependence of CO binding kinetics to C. kumu. Moreover, the effect of temperature (between 8 degrees C and 38 degrees C) on CO binding kinetics has been investigated in the absence of ATP at different pH values. These results allow to interpret the functional kinetic heterogeneity of C. kumu Hb on the basis of different regulatory aspects in the alpha- and beta-subunits, as suggested by structural considerations.

Highlights

In mammalian hemoglobins (Hb), the subunit structural heterogeneity of ␣- and ␤-chains usually does not bring about any relevant functional heterogeneity, this being especially true for CO binding both in the T and R quaternary states [1, 2]
No kinetic functional heterogeneity has been detected for human adult Hb, as indicated by the observation that alteration of the proximal HisF8-Fe bond may be described as an apparent single protonation process
A marked functional heterogeneity has been observed for CO binding to some fish Hbs [11,12,13], and it becomes especially evident whenever experimental conditions tend to stabilize the T quaternary state in the liganded form

Summary

Introduction

In mammalian hemoglobins (Hb), the subunit structural heterogeneity of ␣- and ␤-chains usually does not bring about any relevant functional heterogeneity, this being especially true for CO binding both in the T and R quaternary states [1, 2].

Results

Conclusion