title: Hydrogencarbonate is not a tightly bound constituent of the water-oxidizing complex in photosystem II.
abstract: Since the end of the 1950s hydrogencarbonate ('bicarbonate') is discussed as a possible cofactor of photosynthetic water-splitting, and in a recent X-ray crystallography model of photosystem II (PSII) it was displayed as a ligand of the Mn(4)O(x)Ca cluster. Employing membrane-inlet mass spectrometry (MIMS) and isotope labelling we confirm the release of less than one (~0.3) HCO(3)(-) per PSII upon addition of formate. The same amount of HCO(3)(-) release is observed upon formate addition to Mn-depleted PSII samples. This suggests that formate does not replace HCO(3)(-) from the donor side, but only from the non-heme iron at the acceptor side of PSII. The absence of a firmly bound HCO(3)(-) is corroborated by showing that a reductive destruction of the Mn(4)O(x)Ca cluster inside the MIMS cell by NH(2)OH addition does not lead to any CO(2)/HCO(3)(-) release. We note that even after an essentially complete HCO(3)(-)/CO(2) removal from the sample medium by extensive degassing in the MIMS cell the PSII samples retain > or =75% of their initial flash-induced O(2)-evolving capacity. We therefore conclude that HCO(3)(-) has only 'indirect' effects on water-splitting in PSII, possibly by being part of a proton relay network and/or by participating in assembly and stabilization of the water-oxidizing complex.
authors.affiliation: Max-Planck-Institut für Bioanorganische Chemie, D 45470 Mülheim an der an Ruhr, Germany.
authors.forename: Dmitriy
authors.initials: D
authors.lastname: Shevela
authors.forename: Ji-Hu
authors.initials: JH
authors.lastname: Su
authors.forename: Vyacheslav
authors.initials: V
authors.lastname: Klimov
authors.forename: Johannes
authors.initials: J
authors.lastname: Messinger
mesh_terms: D001639:Bicarbonates / Q000737:chemistry* / Q000378:metabolism; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D002245:Carbon Dioxide / Q000737:chemistry / Q000378:metabolism; D018360:Crystallography, X-Ray; D004795:Enzyme Stability / Q000502:physiology; D005561:Formates / Q000737:chemistry / Q000378:metabolism; D007501:Iron / Q000737:chemistry / Q000378:metabolism; D008024:Ligands; D008345:Manganese / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism; D018515:Plant Leaves / Q000201:enzymology*; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism; D020836:Protein Structure, Quaternary; D011522:Protons; D018724:Spinacia oleracea / Q000201:enzymology*; D014867:Water / Q000737:chemistry* / Q000378:metabolism
chemical_list: D001639:Bicarbonates; D005561:Formates; D008024:Ligands; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D011522:Protons; D014867:Water; C030544:formic acid; D002245:Carbon Dioxide; D008345:Manganese; D007501:Iron; D002118:Calcium