SAXS investigation on the temperature dependence of the conformation of Carcinus aestuarii 5S hemocyanin subunit

The small-angle X-ray scattering technique has been used to study the spatial distribution of a subunit isolated from Carcinus hemocyanin, in solution at pH 7.5 in the 20 degrees C-40 degrees C temperature range. From the obtained scattering profiles, two species with different gyration radius have been detected by Guinier approximation: one species with R-g1 approximate to 25 Angstrom is assigned to the 75 kDa 5S subunit whereas a second species with R-g2 approximate to 48 Angstrom, and accounting for approximate to 3% of the total protein, is attributed to the 450 kDa 16S hexamer. Whereas R-g2 decreases slightly (approximate to 10%) and reversibly on increasing the temperature, R-g2 decreases more markedly (approximate to 30%), but irreversibly, The scattering data have been analysed also on the basis of the impenetrable spheres model and by means of the distance distribution function: the temperature dependence of the geometrical dimensions of the particles is confirmed. Ln addition, for the 5S subunit also the cross-section gyration radius decreases appreciably (15%) and reversibly with temperature. These results are interpreted on the basis of temperature induced structural rearrangements among the three domains of 5S subunit leading to an increased compactness of the molecule and a more elongated form. In contrast, the effect on the hexamer is assigned to its irreversible dissociation to monomers, This interpretation agrees with the analysis of the distance distribution functions, calculated from the Fourier's transforms of the scattering curves at the different temperatures.