SOLUTION H-1-NMR STRUCTURE OF THE HEME CAVITY IN THE LOW-AFFINITY STATE FOR THE ALLOSTERIC MONOMERIC CYANO-MET HEMOGLOBINS FROM CHIRONOMUS-THUMMI-THUMMI - COMPARISON TO THE CRYSTAL-STRUCTURE

Solution H-1-NMR studies of the heme cavity were performed for the cya nomet complexes of monomeric hemoglobins III and IV from the insect Ch ironomus thummi thummi, each of which exhibit marked Bohr effects. The low pH 5, paramagnetic (S = 1/2) derivatives were selected for study because the large dipolar shifts provide improved resolution over diam agnetic forms and allow distinction between the two isomeric heme orie ntations [Peyton, D. H., La Mar, G. N. & Gersonde, K. (1988) Biochim. Biophys. Acta 954, 82-94]. The crystal structure for the low-pH form o f the hemoglobin III derivative, moreover, has been reported and showe d that the functionally implicated distal His58 side chain adopts alte rnative orientations, either in or out of the pocket [Steigemann, W. & Weber, E. (1979) J. Mel. Biol. 127, 309-338]. All heme pocket residue s for the low-pH forms of the two hemoglobins were located, at least i n part, and positioned in the heme cavity on the basis of nuclear Over hauser effects to the heme and each other, dipolar shifts, and paramag netic-induced relaxation. The resulting structure yielded the orientat ion of the major axis of the paramagnetic susceptibility tenser. The h eme pocket structure of the cyanomet hemoglobins III and IV were found to be indistinguishable, with both exhibiting a distal His58 oriented solely into the heme cavity and in contact with the ligand, and with two residues, Phe100 and Phe38, exhibiting small but significant displ acements in solution relative to hemoglobin III in the crystal.