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 A 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 A 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.