Structural basis of heme reactivity in myoglobin and leghemoglobin: thermal difference spectra

Thermal pertubation difference spectra of sperm whale myoglobin (Mb) and soybean leghemoglobin a (Lb a) in the near-ultraviolet reveal similarities in the tryptophan environment of the two proteins. Of the two tryptophans in each protein, one has its indolyl NH group fully exposed to aqueous solvent, while the other behaves as if it were surrounded by motile but nonpolar residues with little access to water. These environments are not significantly altered by removal of the heme group. Assuming conformational homology, the helix-spacing role of Trp-A12 in Mb (Kendrew, J.C. (1962), Brookhaven Symp. Biol. 15, 216-228) may be taken over, in Lb a, by Trp-H8 which, though remote in linear sequence, would occupy a suitable spatial location. Thermal difference spectra in the Soret and visible regions of pure high-spin (fluoroferric) and pure low-spin (cyanoferric) complexes showed a red shift on cooling Mb complexes, reflecting a predominantly nonpolar environment around the heme, but a blue shift on cooling Lb complexes, reflecting a more solvent-exposed environment. Thermal difference spectra using rose bengal as a probe of the heme pockets in the two apoproteins supported these conclusions. Thermal difference spectra for the high-spin complexes of both Mb and Lb are slightly larger in magnitude than in the low-spin complexes. This may reflect a more flexible heme pocket in the high-spin state, as suggested by recent circular dichroic results. A structural basis for the high oxygen affinity of Lb compared with Mb is proposed, based upon the observed differences in polarity and flexibility of the heme pocket and in amino acid substitutions.