Studies On The Photoassembly Of Water-oxidizing Complex Of Photosystem Ⅱ

Photosynthetic water oxidation is one of the most important biochemical processes on Earth. Water cleavage into O2 and four protons takes place via a sequence of four redox steps at a manganese containing catalytic site within a protein complex, called Photosystem II(PSII). The catalytic center of the water oxidation contains a 4-manganese cluster, a Ca²⁺ ion, 1-2 Cl^-ions and a redox-active tyrosine Yz(D1-Y161) as essential cofactors. During the assembly of PSII to its functional state, the catalytic center is the last unit introduced. This last step is called photoassembly. Photoassembly is the process of binding and photooxidation of the inorganic confactors(manganese, calcium, and chloride) within the apoprotein compontents of photosystem II. The unraveling of the structure and function of the water-oxidizing complex is one of the great challenges in photosynthesis research. It is therefore highly motivating to work on problems of photoaessembly and contribute to our understanding in this fascinating field. In this thesis, the process of photoassembly of the water-oxidizing complex of photosystem II membranes has been examined in different bioinorganic chemistry conditions. The results obtained here were summarized as follows: 1. Manganese complexes coordinated to nitrogen atoms of imidazole ligand and manganese complexes without coordination to nitrogen atoms of imidazole ligand were used to reconstitute the WOC and some interesting relationship between the coordination environment of manganese atom in the complexes with their efficiency in restoring electron transport and oxygen evolution was found. Our results demonstrate that the capability of Mn complexes to reconstitute the WOC is mainly determined by the coordination of Mn atom. If Mn within the complex is coordinated to nitrogen atoms of imidazole ligand within the complex it can restore significant rates of electron transport and oxygen evolution. Otherwise, the efficiency of complexes in restoring electron transport and the O₂-evolution activity is relatively low or is unable to be found. It is suggested from our results that the nitrogen atoms of imidazole ligand is a ligand of manganese cluster. 2. Efficiency of two binuclear and one trinuclear manganese complexes in reconstituting electron transport and O₂ evolution activity in Mn-depleted Photosystem II preparations is analyzed. Trinuclear Mn-complexes participates in a more effective reactivation of oxygen evolution than binuclear Mn-complexes. However, trinuclear Mn complex is less effective as an electron donor than other two binuclear Mn-complexes. It is inferred from our results that recovery of electron transport and O2 evolution activity with polynuclear Mn-complexes is affected with different factors. Moreover, Trinuclear Mn-complex is extremely sensitive to the addition of CaCl2. It is suggested that there is an interaction between Ca2+ and carboxyl in the trinuclear Mn-complex during photoactivation and this interaction support the ligation of Mn atom and formation of active WOC. The efficiency of three Mn-complexes in the reconstitution of WOC is in an order: trinuclear Mn3(III)>binuclear Mn(III)Mn(III)>binuclear Mn(III)Mn(IV). 3. Effects of lanthanide on the properties of the photoassembly of photosystem II is found. Here we show that La3+, Tb3+ inhibited photoassembly markedly in isolated photosystem II. The exact La3+, Tb3+ concentrations necessary for inhibition depended on the concentration of calcium. It is proposed that La3+, Tb3+ bind competitively to the essntial Ca2+ site in potosystem II during potoactivation. Kenetic analysis suggests that La3+, Tb3 function as a mixed-type competitor for Ca2+. Moreover, the concentration where 50% inhibition of photoactivation occurred is 1/10 of that need to inhibit 50% of the oxygen evolving activity of functional PSII membranes in the presence of 10 mmol/L Ca2+. 4. The inhibitory effect of Co2+, Ni2+ on the structure and function was investigated in native and NaCl treated(depleted in extrinsic polypeptides) photosystem II submembrane preparations. Inhibition of oxygen evolution activity by two heavy ions was found in these two samples. The inhibition was much stronger in protein depleted preparations in comparison to the native form. It was found that Ca2+ significantly reduced the inhibition It was showed that both the metal ions significantly dissociated the 23 kDa, 17 kDa polypeptides. Moreover, our results suggest that Co2+, Ni2+ have no obvious effects on the photoassembly of OEC of PSII.