Functional unit of the Rapana thomasiana (Grosse) (marine snail, gastropod) hemocyanin

The amino terminal functional unit (domain a) of the Rapana hemocyanin "heavy" structural subunit, designated as Rta, was obtained after limited trypsinolysis of the whole polypeptide chain. Mass spectrometric analysis showed a molecular mass of 49,698 daltons for the electrophoretically homogeneous fragment. Twenty-five amino acid residues were sequenced directly from the N-terminus of Rta, which allowed the location of the domain in the polypeptide chain of the subunit. Physicochemical parameters were determined by absorption and fluorescence spectroscopy and circular dichroism. Comparison with the respective parameters of the whole Rapana hemocyanin showed that the polypeptide backbone folding, binuclear active site and capability of oxygen binding of the isolated functional unit are identical to those of the native hemocyanin. Comparison of N-terminal sequences of functional units from different molluskan hemocyanins and located at different positions revealed some evolutionary relationships.