Mechanism of irreversible inhibition of O2 evolution in photosystem II by Tris(hydroxymethyl)aminomethane.
The dark reaction of tris(hydroxymethyl)aminomethane (Tris) with the O2-evolving center of photosystem II (PSII) in the S1 state causes irreversible inhibition of O2 evolution. Similar inhibition is observed for several other amines: NH3, CH3NH2, (CH3)2NH, ethanolamine, and 2-amino-2-ethyl-1,3-propanediol. In PSII membranes, both depleted of the 17- and 23-kDa polypeptides and undepleted, the rate of reaction of Tris depends inversely upon the Cl- concentration. However, the rate of reaction of Tris is about 2-fold greater with PSII membranes depleted of the 17- and 23-kDa polypeptides than with undepleted PSII membranes. We have used low-temperature electron paramagnetic resonance (EPR) spectroscopy to study the effect of Tris on the oxidation state of the Mn complex in the O2-evolving center, to monitor the electron-donation reactions in Tris-treated samples, and to observe any loss of the Mn complex (forming Mn2+ ions) after Tris treatment. We find that Tris treatment causes loss of electron-donation ability from the Mn complex at the same rate as inhibition of O2 evolution and that Mn2+ ions are released. We conclude that Tris reduces the Mn complex to labile Mn2+ ions, without generating any kinetically stable, partially reduced intermediates, and that the reaction occurs at the Cl(-)-sensitive site previously characterized in studies of the reversible inhibition of O2 evolution by amines.
Rickert
K W
KW
Department of Chemistry, Yale University, New Haven, Connecticut 06511.
Sears
J
J
Beck
W F
WF
Brudvig
G W
GW
D004578:Electron Spin Resonance Spectroscopy; D007700:Kinetics; D008345:Manganese / Q000032:analysis; D010100:Oxygen / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000037:antagonists & inhibitors*; D045332:Photosystem II Protein Complex; D010944:Plants / Q000378:metabolism*; D014325:Tromethamine / Q000494:pharmacology*
D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D014325:Tromethamine; D008345:Manganese; D010100:Oxygen