Low-resolution molecular structures of isolated functional units from arthropodan and molluscan hemocyanin

Synchrotron x-ray scattering measurements were performed on dilute solution s of the purified hemocyanin subunit (Bsin1) from scorpion (Buthus sindicus ) and the N-terminal functional unit (Rta) from a marine snail (Rapana thom asiana). The model-independent approach based on spherical harmonics was ap plied to calculate the molecular envelopes directly from the scattering pro files. Their molecular shapes in solution could be restored at 2-nm resolut ion. We show that these units represent stable, globular building blocks of the two hemocyanin families and emphasize their conformational differences on a subunit level. Because no crystallographic or electron microscopy dat a are available for isolated functional units, this study provides for the first time structural information for isolated, monomeric functional subuni ts from both hemocyanin families. This has been made possible through the u se of low protein concentrations (less than or equal to 1 mg/ml). The obser ved structural differences may offer advantages in building very different overall molecular architectures of hemocyanin by the two phyla.