COORDINATION AND SPIN-STATE EQUILIBRIA AS A FUNCTION OF PH, IONIC-STRENGTH, AND PROTEIN-CONCENTRATION IN OXIDIZED DIMERIC SCAPHARCA-INAEQUIVALVIS HEMOGLOBIN

The oxidized homodimeric Scapharca inaequivalvis hemoglobin undergoes changes in coordination and spin state as a function of pH, ionic stre ngth, and protein concentration which have been monitored by optical a bsorption spectroscopy. Three species contribute to the spectra betwee n pH 5.8 and 8.7: (i) a hexacoordinate high spin aquomet derivative, w hose concentration is essentially constant over the whole pH range ana lyzed; (ii) a pentacoordinate high spin component which prevails at al kaline pH values, and (iii) a hexacoordinate low spin hemichrome, whic h is formed at acid pH. The contribution of each of the components to the observed spectra was calculated with the singular value decomposit ion procedure and has been described quantitatively in terms of a link age scheme which accounts for the change in heme coordination and for the observation that the high spin to low spin transition entails diss ociation into monomers. An important feature of the linkage scheme is the cooperative binding of protons to aquomet dimers. Stopped flow exp eriments to study the kinetics indicate that dissociation into monomer s is the rate-limiting process. The unusually strong tendency of oxidi zed HbI to loose the heme-bound water molecule is discussed in terms o f strain in the iron-proximal histidine bond.