Planorbis corneus haemoglobin: Circular dichroism and susceptibility to proteases

The purified haemoglobin of Planorbis corneus was subjected to protease digestion and the resulting products characterised by gel filtration and detergent-gel electrophoresis. Small functional subunits of molecular weights approximately 20 000 were obtained corresponding to a single haem group, but multiples of this unit were also always obtained even at high proteolytic enzyme : haemoglobin ratios. This suggested that the subunits of the native molecule (one-tenth containing perhaps ten O₂-binding sites) were made up of single-binding site domains linked by regions of polypeptide chains having different susceptibilities to proteases. The far ultraviolet CD of the native haemoglobin indicated the presence of a high helix content (75–80%) in the protein. The near ultraviolet and visible CD spectra of oxy- deoxy-, and CO-haemoglobin were reported. Planorbis haemoglobin CD was more like that of vertebrate haemoglobins than that of annelid haemoglobins. Nevertheless the Soret CD of Planorbis oxyhaemoglobin had only about half the rotational strength of that of human haemoglobin A, and was halved again upon removal of the ligand. Also in contrast to Lumbricus and human haemoglobins there was only a small decrease in rotational strength in the 260 nm band when Planorbis oxyhaemoglobin was deoxygenated.