Chemical evolution of the water oxidizing complex: Speciation and cofactor binding in the assembly of the tetra-manganese cluster in photosystem II.

摘要

The oxidation of water into di-oxygen is an energy demanding chemical reaction that is carried out by the enzyme photosystem II, by coupling the free energy of the chlorophyll photochemistry with an inorganic catalyst (Mn 4CaOx) in its active site. In the introductory chapter we review and propose possible mechanisms of water oxidation based on structural data from X-ray diffraction studies and other spectroscopic techniques on PSII (Phys. Chem. Chem. Phys., 2004). In the second chapter, we hypothesize (the 'carbonate' hypothesis) the chemical reasons that might have led to the evolution of the tetra-manganese cluster from anoxygenic reaction centers that lack the ability to split water. It is proposed that CO 2/HCO3-/CO32- solution equilibrium in the Archean era is crucial to the aqueous speciation of Mn 2+, as Mn-bicarbonate complexes ultimately led to the assembly of this inorganic catalyst (Proc. Natl. Acad. Sci. 2001).; The complexation of Mn2+ in carbonate solution was investigated using electrochemistry and EPR spectroscopy. The oxidation potentials of the formed complexes were found to decrease significantly from E0 = 1.18 V for Mn2+aq to 0.67 V and 0.52 V for the 1:1 and 1:2 complexes, respectively ( J. Phys. Chem. B accepted).; Bicarbonate plays an important role in the light driven assembly reaction (known as photoactivation) of the tetra-manganese cluster in PSII. In presence of bicarbonate there is 20% and 400% stimulation in the rate of photoassembly of the cluster at low and high cofactor (Mn2+ and Ca 2+) concentrations, respectively. Borate, a chemical analogue of bicarbonate, shows 400% rate stimulation at low cofactor concentrations but shows no effect at high values. Thus borate acts only as a base to deprotonate the protein residues near the Mn-binding site in apoPSII. However, using CW and pulsed EPR spectroscopy we have shown that bicarbonate coordinates to Mn2+ bound to the high-affinity site in apoPSII to form a ternary complex [HCO3--Mn2+-apoPSII]. This work clearly establishes that bicarbonate is a required cofactor in the photoassembly process.; Finally, to test our 'carbonate' hypothesis the first set of experiments confirming light-dependent oxidation of Mn2+-bicarbonate complexes by anoxygenic reaction centers from Rhodovulum iodosum have been reported.