STRUCTURE OF THE SULFIDE-REACTIVE HEMOGLOBIN FROM THE CLAM LUCINA-PECTINATA - CRYSTALLOGRAPHIC ANALYSIS AT 1.5-ANGSTROM RESOLUTION

The crystal structure of the aquo-met form of the sulfide-reactive hem oglobin (component I) from the gill of the symbiont-harboring mollusc, Lucina pectinata, has been solved and refined at 1.5 Angstrom resolut ion, based on synchrotron radiation X-ray diffraction data, and employ ing molecular replacement techniques. The crystallographic R-factor, c alculated for the data in the 15.0 to 1.5 Angstrom resolution range, i s 0.170, with highly regular stereochemical parameters for the protein model, and including 131 water molecules. The monomeric hemoglobin I chain consists of 142 amino acid residues, which have been partly iden tified on the basis of the crystallographic analysis. The molecule is characterized by an unusual distribution of aromatic residues, particu larly in the region surrounding the distal site in the heme pocket. Th e heme distal residue is Gln(64)E7, while other notable amino acid sub stitutions include Trp(21)B2, Phe(29)B10, Leu(46)CD3, Phe(68)E11 and T rp(75)E18. An amino acid insertion (Ser44) is observed between sites C D1 and CD2. In the aquo-met protein, a water molecule is present at th e sixth coordination position of the heme iron, and hydrogen bonded to Gln(64)E7. Simple model building shows that a dioxygen molecule, boun d to ferrous protein, mould contact with its free atom the ring edge o f Phe(29)B10, being thus stabilized at the coordination site by an aro matic-electrostatic interaction. Similarly the unique packing and orga nization of aromatic residues in the surroundings of the heme distal s ite is proposed as the molecular basis of the very high affinity of Lu cina pectinata hemoglobin I for hydrogen sulfide, considered as one of the two physiological ligands of the protein.