Properties of human hemoglobins with increased polarity in the α- or β-heme pocket -: Carbonmonoxy derivatives

The spectroscopic, conformational, and functional properties of mutant carbonmonoxy hemoglobins in which either the beta-globin Val(67)(Ell) or the alpha-globin Val(62)(Ell) is replaced by threonine have been investigated. The thermal evolution of the Soret absorption band and the stretching frequency of the bound CO were used to probe the stereodynamic propel-ties of the heme pocket. The functional properties mere investigated by kinetic measurements. The spectroscopic and functional data were related to the conformational properties through molecular analysis. The effects of this nonpolar-to-polar isosteric mutation are: (i) increase of heme pocket anharmonic motions, (ii) stabilization of the A(o) conformer in the IR, spectrum, (iii) increased CO dissociation rates. The spectroscopic data indicate that for the carbonmonoxy derivatives, the Val --> Thr mutation has a larger conformational effect on the beta-subunits than on the alpha-subunits. This is at variance with the deoxy derivatives where the conformational modification was larger in the heme pocket of the alpha-subunit (Cupane, A., Leone, Ri., Militello, V,, Friedman, R. K., Koley, A. P,, Vasquez, G. P,, Brinigar, W, S., Kara; vitis, M., and Fronticelli, C. (1997) J. Biol. Chern. 272, 26271-26278), These effects are attributed to a different electrostatic interaction between O-gamma of Thr(E11) and the bound CO molecule. Molecular analysis indicates a more favorable interaction of the bound CO with Thr OY in the beta-subunit heme pocket.