STRUCTURE OF METMANGANOGLOBIN

The structure of metmanganoglobin, in which Mn(III) replaces Fe(III) as the heme metal, was compared with that of native Hb [horse] by X-ray difference Fourier techniques. Their quaternary structures were identical and their tertiary structures were similar, as expected both from the close stereochemical correspondence between Mn porphyrins and the corresponding Fe porphyrins, and from the similarities in functional properties previously reported. There were a number of small but significant differences. The .alpha.-heme was essentially unperturbed by the metal substitution, in contrast to the .beta.-heme. The 6th ligand of the .beta.-heme, a water molecule in native Hb, was apparently lost, and that the resultant 5-co-ordinate .beta.-heme in metmanganoglobin was distinctly ruffled in accord with quasi-S4 symmetry. The loss of non-covalent ligand-globin interactions together with the heme ruffling produced numerous small perturbations in the .beta.-globin, which were transmitted across the .alpha.1-.beta.1 interface to the .alpha.-globin. The .alpha.1-.beta.2 interface was only slightly perturbed. Metmanganoglobin displayed some, but not all, of the structural features to be expected in a partially liganded Hb: those arising directly from ligand loss from the .beta.-hemes, and contraction of the ligand pocket in the .beta.-chain, but not others, such as those which accompany a marked alteration in the distance of the proximal histidine from the mean plane of the porphyrin. The basic similarity in the structural and functional properties of manganoglobin and native Hb demonstrated that Hb function was not uniquely dependent on the co-ordination properties of the metal.