Spectroscopic Properties of C-Phycocyanin and of Its α and β Subunits

Abstract

Abstract Synechococcus sp. (Anacystis nidulans) C-phycocyanin and its α and β subunits have been examined with respect to spectroscopic properties and aggregation behavior. In 0.05 m phosphate buffer at pH 7.0, and a protein concentration of 2.1 x 10-5 m, the α subunit exhibited a λmax of 620 nm (em = 0.98 x 105 m-1 cm-1), and a maximum molecular ellipticity, [θ]620, of 30.2 x 104 deg cm2 per dmole, while the β subunit exhibited a λmax of 608 nm (em = 1.43 x 105 m-1 cm-1), and a maximum [θ]600 of 56.1 x 104 deg cm2 per dmole. Fluorescence polarization spectra of native phycocyanin indicate the presence of two different chromophore types in this protein, those that absorb at shorter wave length (s type) sensitizing those that absorb and emit at longer wave length (f type) (see Teale, F. W. J., and Dale, R. E. (1970) Biochem. J. 116, 161–169). The absorption characteristics of the β subunit indicate that it contains the s type of chromophore, whereas those of the α subunit are consistent with the presence of the f type of chromophore. As reported earlier by others, aggregation of phycocyanin results in a red shift of the absorption maximum, and a large increase in the extinction coefficient. Phycocyanin, under conditions where it is essentially monomeric, i.e. 7.2 x 10 -7 m protein in 0.05 m phosphate buffer at pH 7.0, is characterized by a λmax of ∼615 nm (em ∼ 2.3 x 105 m-1 cm-1), while hexameric phycocyanin, in 0.2 m acetate buffer at pH 5.5, exhibits a λmax of 621 nm (em = 3.33 x 105 m-1 cm-1). Changes in the circular dichroism spectrum observed upon phycocyanin aggregation suggest strong intermolecular interaction between the chromophores. Phycocyanin, formed by recombination of α and β subunits, was found to be indistinguishable from native phycocyanin with respect to circular dichroism and absorption spectra.