Conformation of the Phosphoprotein, Phosvitin

Abstract

Studies of viscosity, optical rotatory dispersion (ORD), and circular dichroism (CD) are reported on the phosphoprotein, phosvitin. From viscosity measurements it was shown that phosvitin in some of its properties resembles a polyelectrolyte. At low ionic strength the protein molecules, because of the strong electrostatic repulsion between the charged phosphate groups of the phosphoserine residues, attain an extended shape, whereas in the ionic strength interval of 0.02 to 0.05 the molecules contract to a more compact shape. From the ORD measurements in the wave length range of 600 to 300 mµ as function of pH and ionic strength, it was shown that a conformational transition occurs in the pH range of 5.5 to 7.0. The ORD spectra below 300 mµ show that at neutral and alkaline pH the principal trough of phosvitin is at 205 mµ, whereas in the pH range of 3.0 to 4.0 two minima at 207 mµ and 232 mµ are present. The CD spectra are characterized by a strong negative band at 198 mµ. In addition, a positive band at 220 mµ and a negative one at 233 mµ were recorded. Based on these results and in analogy with ORD and CD measurements on polypeptides with known conformation it is suggested that at alkaline pH phosvitin has an unordered conformation, whereas at pH 3.0 to 3.6 helical regions and β structures are present within an otherwise unordered structure.