Effect of acetic acid on phycocyanin-phycoerythrocyanin mixture extracted from Anabaena variabilis

Exposure of a phycocyanin-phycoerythrocyanin mixture extracted from Anabaena variabilis to sodium acetate, pH 3.8, ionic strength of 0.1, results in dissociation of the phycoerythrocyanin's beta subunit from its alpha subunit. The alpha subunit obtained by this method has a strong absorption transition at 508 nm. This transition is a consequence of the subunit's specific conformation, rather than of a new chromophore. The behavior of the phycocyanin-phycoerythrocyanin mixture in acetate buffers of variable compositions suggests that interactions which involve carboxylic amino acid residues play an important role, along with hydrophobic associations, in the association of phycoerythrocyanin subunits into monomers (alpha beta) and between this protein and phycocyanin. This work also indicates that the linkage between alpha and beta subunits of phycoerythrocyanin is labile and may be weaker than the association of these subunits with phycocyanin under acidic conditions.