Protective effect of bicarbonate against extraction of the extrinsic proteins of the water-oxidizing complex from Photosystem II membrane fragments

Abstract

A protective effect of bicarbonate (BC) against extraction of the extrinsic proteins, predominantly the Mn-stabilizing protein (PsbO protein), during treatment of Photosystem II (PS II) membrane fragment from pea with 2 M urea, and at low pH (using incubation in 0.2 M glycine-HCl buffer, pH 3.5 or 0.5 M citrate buffer, pH 4.0-4.5) was detected. It was shown that the extraction of the proteins with Mw 24 kDa (PsbP protein) and 18 kDa (PsbQ protein) by the use of highly concentrated solutions of NaCl does not depend on the presence of BC in the medium. An optimal concentration of BC at which it produces the maximum protecting effect was shown to be between 1 mM and 10 mM. The addition of formate did not influence the protein extraction but it reduced the stabilizing effect of BC. Independence of the stabilizing effect on the presence of the functionally active Mn within the water-oxidizing complex indicates that the protecting effect of BC is not related to its interaction with Mn ions. The fact that there is a preferable sensitivity of the PsbO protein to the absence of BC in the medium during all the treatments makes it possible to suggest that either BC interacts directly with the PsbO protein or it binds to some other sites within PS II and this binding facilitates the preservation of the native structure of this protein.

Flash-Induced FTIR Difference Spectroscopy Shows No Evidence for the Structural Coupling of Bicarbonate to the Oxygen-Evolving Mn Cluster in Photosystem II

“…In this case, the structure of bicarbonate does not necessarily change before and after proton release. Probably, bicarbonate plays major roles in the photoassembly process of the Mn cluster as a ligand to its precursor ,,,– , and stabilization of the Mn cluster through an indirect interaction by binding at PsbO or some site in PSII proteins ,, .…”

“…Since 1995, various data have been accumulated showing that bicarbonate is indeed functional in oxygen evolution (9,10,12,(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). In addition to the earlier proposal of a catalytic intermediate (15,16,36), it has been suggested that bicarbonate functions as a ligand to the Mn cluster or an integral cofactor of OEC (20,24,26,33), a cofactor crucial in the photoassembly process of the Mn cluster as a transient ligand to Mn ions (20,21,25,(27)(28)(29)(30)(31)35), a proton transfer mediator bound to a residue in the vicinity of the Mn cluster (32), and an element to indirectly stabilize OEC by binding to PSII proteins or extrinsic proteins (20,21,34). In fact, the X-ray structure by Ferreira et al (4) at 3.5 Å resolution tentatively assigned a part of the nonprotein density to bicarbonate as a ligand bridging Mn and Ca ions.…”

“…Since 1995, various data have been accumulated showing that bicarbonate is indeed functional in oxygen evolution ,,,– . In addition to the earlier proposal of a catalytic intermediate ,, , it has been suggested that bicarbonate functions as a ligand to the Mn cluster or an integral cofactor of OEC ,,, , a cofactor crucial in the photoassembly process of the Mn cluster as a transient ligand to Mn ions ,,,– , , a proton transfer mediator bound to a residue in the vicinity of the Mn cluster , and an element to indirectly stabilize OEC by binding to PSII proteins or extrinsic proteins ,, . In fact, the X-ray structure by Ferreira et al at 3.5 Å resolution tentatively assigned a part of the nonprotein density to bicarbonate as a ligand bridging Mn and Ca ions.…”