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

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 Å resolution (orthorhombic space group P2₁2₁2₁). In the present communication, the three-dimensional structure of wild-type P. epiclitum haemoglobin is reported at 1.85 Å 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.