High resolution proton nuclear magnetic resonance studies of interaction between deoxyhaemoglobin and small molecules. Dithionite and diphosphoglycerate

Abstract

90 MHz proton n.m.r. spectra have been measured on solutions of deoxyhaemoglobin (Hb) in D₂O in the presence and absence of dithionite (DT) and D-2,3-diphosphoglycerate (DPG) ions.

Application of deconvolution and interpolative baseline correction software has enabled the pH dependence of the histidine C₂ proton chemical shifts to be observed without the necessity of using high fields or elaborate pulse sequences. The n.m.r. data are interpreted in terms of a strong fast exchange interaction with DPG and a much weaker one, at the same site, for DT.

Histidines β2 and β143 at the DPG binding site have been assigned to peaks in the proton n.m.r. spectrum. From the chemical shift behaviour of a selected resonance at different DPG concentrations, it has been shown that DPG and Hb bind with a 1:1 stoichiometry. The measured affinity constant is (5.1 ± 0.4)× 10⁴ mol^–1 and is independent of the ionised species of DPG present in the pH range 7.1–7.3.