A comparative NMR study of the polypeptide backbone dynamics of hemoglobin in the deoxy and carbonmonoxy forms

Abstract

Model-free-based NMR dynamics studies have been undertaken for polypeptide backbone amide N-H bond vectors for both the deoxy and carbonmonoxy forms of chain-specific, isotopically (15N and 2H) labeled tetrameric hemoglobin (Hb) using 15N-relaxation parameters [longitudinal relaxation rate (R1), transverse relaxation rate (R2), and heteronuclear nuclear Overhauser effect (NOE)] measured at two temperatures (29 and 34 degrees C) and two magnetic field strengths (11.7 and 14.1 T). In both deoxy and carbonmonoxy forms of human normal adult hemoglobin (Hb A), the amide N-H bonds of most amino acid residues are rigid on the fast time scale (nanosecond to picosecond), except for the loop regions and certain helix-helix connections. Although rigid in deoxy-Hb A, beta146His has been found to be free from restriction of its backbone motions in the CO form, presumably due to the rupture of its hydrogen bond/salt bridge network. We now have direct dynamics evidence for this structural transition of Hb in solution. While remarkably flexible in the deoxy state, alpha31Arg and beta123Thr, neighbors in the intradimer (alpha1beta1) interface, exhibit stiffening upon CO binding. These findings imply a role for alpha31Arg and beta123Thr in the intradimer communication but contradict the results from X-ray crystallography. We have also found that there is considerable flexibility in the intradimer (alpha1beta1) interface (i.e., B, G, and H helices and the GH corner) and possible involvement of several amino acid residues (e.g., alpha31Arg, beta3Leu, beta41Phe, beta123Thr, and beta146His) in the allosteric pathway. Several amino acid residues at the intradimer interfaces, such as beta109Val, appear to be involved in possible conformational exchange processes. The dynamic picture derived from the present study provides new insights into the traditional description of the stereochemical mechanism for the cooperative oxygenation of Hb A based on X-ray crystallographic results.

Figure 1

Order parameters (S²) of amide N-H bonds in Hb at 29 °C.

Figure 2

R_ex terms from Model-free analysis for deoxy-Hb A and HbCO A at 34 °C; possible chemical exchange processes are implied at amino-acid residues after 100, including the most dramatic β109Val.

Figure 3

HbCO A at 34 °C with the amino acid residues with significant R_ex values painted red. Amino acid residues in the G and H helices of both α- and β-chains are possibly involved in chemical exchange processes.

Figure 4

Residues α31Arg and β123Thr are shown at the intra-dimer (α₁β₁ or α₂β₂) interface of Hb A. The amide N-H bonds of these two amino acid residues exhibit unusual flexibility in deoxy-Hb A, but both become rigid in HbCO A.