Structural Basis of Heme Reactivity in Myoglobin and Leghemoglobin: Thermal Difference Spectra

Abstract

Thermal perturbation difference spectra of sperm whale myoglobin (Mb) and soybean leghemoglobin a (Lb a) in the near-ultraviolet reveal similarities in the tryptophan environments 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 sequ..