Structure and Function of Human Semihemoglobins α and β

Abstract

Semihemoglobins α and β are derivatives of human hemoglobin which possess the usual chain composition (α2β2 or αβ) but which carry the heme prosthetic group on only one kind of chain (α or β), the complementary chain (β° or α°) being heme-free. In this paper, new results on two compounds of this type, namely, semiHbαD (αβ°) and semiHbβ (α°β) have been presented; existing data on other earlier preparations have also been reviewed. Methods for preparation of semiHbαD and semiHbβ have been described. Some of their properties including optical absorption, circular dichroic spectra, oxygen equilibrium and kinetics of carbon monoxide binding have been measured; these properties are then compared with those possessed by intact hemoglobin (α2β2), isolated chains (α,β) and the heme-free apoprotein (α°β°). 1 The contribution of the heme moiety to the optical absorption and circular dichroic spectra of the semihemoglobins are found to be intermediate between those of hemoglobin and those of the corresponding isolated chain. This result indicates that the effect of interaction between the unlike chains of these molecules are transmitted as far as the heme prosthetic group. The structure of the heme-free chain is also modified as the result of this interaction as revealed by titration of sulfhydryl groups, although circular dichroic spectra in the far ultraviolet are not conclusive. 2 The oxygen affinity and the Bohr effect of the semihemoglobins are again intermediate between those of hemoglobin and isolated chain, the semiHbβ being closer to hemoglobin than semiHbαD. The same is true concerning the velocity of carbon monoxide binding. 3 The kinetics of fixation of carbon monoxide to semihemoglobins is strongly biphasic. It is suggested that this property could reflect the existence of semihemoglobins as two types of dimers (αβ) but differing among themselves on account of different contact regions brought into play. (α1β1 and α1β2 in the nomenclature of Perutz.) 4 The significance of the fact that semiHbα is obtained in multiple forms is considered. 5 The ensemble of the results are discussed in the context of chain-chain interactions.