Very high resolution structure of a trematode hemoglobin displaying a TyrB10-TyrE7 heme distal residue pair and high oxygen affinity

Monomeric hemoglobin from the trematode Paramphistomum epiclitum displays v ery high oxygen affinity (P-50 < 0.001 mm Hg) and an unusual heme distal si te containing tyrosyl residues at the B10 and E7 positions. The crystal str ucture of aquo-met P. epiclitum hemoglobin, solved at 1.17 <Angstrom> resol ution via multiwavelength anomalous dispersion techniques (R-factor = 0.121 ), shows that the heme distal site pocket residue TyrB10 is engaged in hydr ogen bonding to the iron-bound ligand. By contrast, residue TyrE7 is unexpe ctedly locked next to the CD globin region, in a conformation unsuitable fo r heme-bound ligand stabilisation. Such structural organization of the E7 d istal residue differs strikingly from that observed in the nematode Ascaris suum hemoglobin (bearing TyrB10 and GlnE7 residues), which also displays v ery high oxygen affinity. The oxygenation and carbonylation parameters of w ild-type P. epiclitum Hb as well as of single- and double-site mutants, wit h residue substitutions at positions B10, E7 and E11, have been determined and are discussed here in the light of the protein atomic resolution crysta l structure. (C) 2001 Academic Press.