An ultracentrifuge study of C-phycocyanin aggregation

The molecular weight of C-phycocyanin has been determined as a function of protein concentration in a sodium acetate buffer solution of pH 4.8 at 21.2 degrees C by the Yphantis method. The higher aggregate, the dodecamer (19 S), is first removed by dissociation simply by dialysis against pH 3.9 acetate buffer solution. The molecular weight data are best interpreted by the simultaneous presence of monomers, trimers, and hexamers. Assuming a monomer K13 in equilibrium trimer K36 in equilibrium hexamer equilibrium system, the equilibrium constants have been calculated, and the values are K13 = 1.4 x 10(12) (L/mol)2 and K36 = 6.1 x 10(5) L/mol. The corresponding free energies for each step have been calculated to be -16.3 kcal/mol (delta G degrees 13) and -7.76 kcal/mol (delta G degrees 36). With these values of K13 and K36, the weight-average molecular weight as a function of concentration is calculated. Satisfactory agreement is obtained between the calculated curve and the experimental data. Sedimentation velocity studies performed with a band-forming centerpiece, on the same protein solutions used for the sedimentation equilibrium studies, confirm the presence of three species: monomers, trimers, and hexamers.