Structural plasticity in the eight-helix fold of a trematode hemoglobin

The three-dimensional structure of recombinant haemoglobin from the trematode Paramphistomum epiclitum, displaying the highest oxygen affinity so far observed for (non)vertebrate haemoglobins, has previously been determined at 1.17 Angstrom resolution (orthorhombic space group P2(1)2(1)2(1)). In the present communication, the three-dimensional structure of wild-type P. epiclitum haemoglobin is reported at 1.85 Angstrom resolution in a monoclinic crystal form (R factor = 16.1%, R-free = 22.0%). Comparison of P. epiclitum (recombinant versus wild-type ferric Hb) structures in the two crystal forms shows structural differences in the haem proximal and distal sites which have not been reported for other known haemoglobin structures previously.