The 2.4-angstrom crystal structure of Scapharca dimeric hemoglobin. Cooperativity based on directly communicating hemes at a novel subunit interface

The crystal structure of the cooperative dimeric hemoglobin from the arcid clam, Scapharca inaequivalvis , has been determined in the carbonmonoxy state. The phase problem was solved for reflections with Bragg spacings greater than 3 angstrom using anomalous scattering from the porphyrin iron atoms measured at a single wavelength in combination with molecular averaging. The model built into this electron density map has been refined at 2.4 angstrom resolution by means of stereochemically restrained least squares minimization to a conventional R-value of 0.156. The root mean square deviation from ideal bond lengths and angles are 0.013 angstrom and 1.7 degree , respectively. In addition to the 2336 hemoglobin atoms, 214 water molecules have been incorporated into the model.