Effect of Hæm-linked and Other lonizations on Hæmoprotein Reactions

Abstract

THE ionization of a proton from a ‘hæm-linked’ acidic group, generally believed to be on the amino-acid residue to which the hæm is attached, is known to alter the affinity of the iron atom for a ligand, as in the combination of the ferrous hæmoprotein, hæmoglobin, with oxygen (Bohr effect)¹. If a similar effect operates in the formation of ferric hæmo-protein complexes, then it may be very difficult to decide, from the variation with pH of equilibrium or of velocity constants, or from determinations of the pH changes which accompany complex formation in unbuffered solutions, whether complexes formed with ligands possessing acid-base properties contain the undissociated ligand, EX, or its anion, X ⁻ bonded to the iron atom². However, a knowledge of the acid strengths of hæm-linked groups, obtained from reactions where the effect of pH comes entirely from their ionizations, such as the oxygenation and electrode reactions, does enable a definite choice to be made. In this way we have recently shown² that the fluoride and cyanide complexes of the ferric hæmoprotein, metmyoglobin, involve bonding of fluoride and cyanide ions.