Study On The Toxicity Of Heavy Metals To The Photosynthesis-related Functions Of Phycocyanin

With the development of industry,the content of heavy metals in water has been increasing,which seriously endangers the ecological environment.As the producer of water,algae are the first to be harmed.Heavy metals can affect the growth and development of algae,their physiological and biochemical functions?including carbon metabolism and photosynthesis?,and their DNA,RNA,protein synthesis and enzyme activities.C-phycocyanin?C-PC?,as a kind of phycobiliprotein,is related to photosynthesis of algae closely,which can absorb and transfer light energy effectively.As an important unit of photosynthesis,C-PC is seriously affected by heavy metal pollution.Therefore,a variety of spectroscopy methods and molecular simulation techniques were used to study the effects of heavy metal ions on the photosynthetic functions of C-PC.This provides a theoretical basis for the study of the toxic mechanism of heavy metal ions on algae,helps to identify the threat of heavy metal pollution in water bodies,and provides a theoretical basis for the formulation of relevant policies and the decision-making of government departments.The effect of heavy metal ions on C-PC absorbing and transmiting light energy was analyzed by UV-vis absorption spectrometry and fluorescence polarization.The results showed that the position of the characteristic absorption peak of C-PC did not change with the heavy metal ions concentration increasing,but the intensity of the characteristic absorption peak decreased with the heavy metal ions concentration increasing.Different heavy metal ions have different effects on the characteristic peak strength of C-PC,which are Zn²⁺,Pb²⁺,Cu²⁺ and Cd²⁺ from the largest to the least,indicating that heavy metal ions can affect the ability of C-PC to absorb light energy.Fluorescence polarization experiments showed that the fluorescence polarization degree of C-PC increased as the heavy metal ions concentration increase.The influences of the four heavy metal ions on the fluorescence polarization degree of C-PC are different,which are Zn²⁺,Pb²⁺,Cu²⁺,and Cd²⁺ from the largest to the smallest,indicating that heavy metal ions can affect the ability of C-PC to transmit light energy.The corre lation analysis of the influences that heavy metal ions have on the abilities of C-PC to absorb and transfer light energy shows that there is a certain correlation between the influence of heavy metal ions on the light energy absorption ability and the inf luence of heavy metal ions on the light energy transfer ability.The effects of heavy metal ions on the secondary and tertiary structures of C-PC were investigated by FTIR and fluorescence probe techniques.The results showed that the interaction of heavy metal ions with C-PC could affect the secondary structure of C-PC,and reduce the content of ?-helix and ?-turn,and increase the content of ?-sheet and random coil.The surface hydrophobicity of C-PC was determined by fluorescence probe assay.The results show that C-PC surface hydrophobicity increased with the concentration of heavy metal ions increasing,so heavy metal ions can affect the C-PC tertiary structure,and cause the increasing of hydrophobic residues on the surface of C-PC,which is correspond ing to the result of effect on C-PC secondary structure.Correlation analysis between the changes of secondary and tertiary structure of C-PC by heavy metal ions and C-PC absorption and light energy were observed.The changes of secondary structure and tertiary structure have a certain correlation with the changes of C-PC absorbing light energy and transferring light energy;and there is also a relationship between the secondary structure change of C-PC and the tertiary structure change.A certain correlation indicates that heavy metal ions may affect the absorption and transmission of light energy by changing the structure of C-phycocyanin,and the greater structural change of C-PC,the function more serious loss.Isothermal Titration Calorimetry and molecular simulation were used to study whether heavy metal ions bind to C-PC and their binding parameters.The results showed that heavy metal ions could bind to C-PC,but the binding parameters were different.The binding force between Cd²⁺,Cu²⁺,Zn²⁺ and C-PC was hydrophobic force,and the binding force between Pb²⁺?hydrated coordination?and C-PC was hydrogen bond and van der Waals force.The binding constant?Ka?of the four heavy metal ions with C-PC were different(Zn²⁺>Pb²⁺>Cu²⁺>Cd²⁺),consistent with the order of influence of heavy metal ions on C-PC structure and function.In addition,the binding sites of the four heavy metal ions with C-PC?Trimer structure?were 7,indicating that the binding sites of the four heavy metal ions with C-PC monomer were 2.The molecular docking simulation results provided two optimal sites for the binding of four heavy metal ions to C-PC monomer,and the analysis of each binding site showed that four heavy metal ions did not bind to the chromophore of C-PC,and the binding sites of Cu²⁺ and Cd²⁺ were close to each other.In summary,heavy metal ions can bind to C-PC,affecting their secondary and tertiary structures,and then affect the C-PC functions?light energy absorption and transmission?.