Impact Assessment of heavy metal cations to the characteristics of photosynthetic phycocyanin

This work has assessed the impact of typical heavy metal cations on C-phycocyanin in vitro and in silico. At low concentrations (<2×10^-6 mol/L), the influence of Pb²⁺ is the highest on the light absorption of C-phycocyanin trimer. At higher concentrations, however, a new order of influence on the light absorption has been observed with Cd²⁺ < Cu²⁺ < Pb²⁺ < Zn²⁺. The fluorescence polarization has changed from the order of Cd²⁺ < Pb²⁺≈Cu²⁺ < Zn²⁺ to Cd²⁺ < Cu²⁺ < Pb²⁺ < Zn²⁺, when the metal concentrations reaches 2×10^-6 mol/L. The mechanisms for these findings have been studied using FTIR, hydrophobic probe, isothermal titration calorimetry and molecular docking for the analysis of structure disorder of C-phycocyanin. It has been suggested that the secondary structure of C-phycocyanin affects more to the light absorbance while the fluorescence characteristics relies more on the tertiary structure. The interaction between Pb²⁺ and C-phycocyanin is both enthalpically and entropically favoured, whereas the interactions for Cd²⁺, Cu²⁺ and Zn²⁺ are entropically driven. The ion-molecular docking suggests that the structure disorder of C-phycocyanin relies on the molecular interactions with metal ions. The in silico study also showed that the binding cites of Zn²⁺ are closer to chromophores.