Role of electrostatic interaction in the stability of the hexamer of constitutive phycocyanin from Fremyella diplosiphon

The role of electrostatic interactions between the charges carried by the titratable groups in the different aggregates of constitutive phycocyanin from Fremyella diplosiphon has been studied by using a simple theoretical approach based on the modified Tanford-Kirkwood model. The electrostatic potential has also been calculated by means of a numerical solution of the linearized Poisson-Boltzmann equations using the finite-differences technique. The pH dependence of the electrostatic contribution to free energy suggests an electrostatic stabilization of the alpha- and beta-subunits as well as of the (alpha beta)-monomer over a broad pH interval. The charge distributions in the individual alpha- and beta-subunits produce electrostatic complementarity and promote the assembly of the subunits to the (alpha beta)-monomer, as well as of the monomers to the larger trimeric and hexameric aggregates. Trimer-trimer electrostatic interactions exhibit strong pH dependence, predicting an association/dissociation equilibrium with a midpoint at pH 6. The electrostatic trimer-trimer interactions correspond to the steric fit, suggesting that electrostatic interactions may initially help to orient the trimers during aggregation. The distribution of the electrostatic potential of the monomers and of the higher aggregates suggests that it plays an important role also in phycocyanin-linker protein binding.