BICARBONATE MAY BE REQUIRED FOR LIGATION OF MANGANESE IN THE OXYGEN-EVOLVING COMPLEX OF PHOTOSYSTEM-II

It was previously shown in the photosystem II membrane preparation DT- 20 that photoxidation of the oxygen-evolving manganese cluster was blo cked by 0.1 mM formate, unless 0.2 mM bicarbonate was present as well [Wincencjusz, H., Allakhverdiev, S. I., Klimov, V. V., and Van Gorkom, H. J. (1996) Biochim. Biophys. Acta 1273, 1-3]. Here it is shown by m easurements of EPR signal II that oxidation of the secondary electron donor, Y-Z, is not inhibited. However, the reduction of Y-Z(+) is grea tly slowed and occurs largely by back reaction with reduced accepters. Bicarbonate is shown to prevent the loss of fast electron donation to Y-Z(+). The release of about one or two free Mn2+ per photosystem II during formate treatment, and the fact that these effects are mimicked by Mn-depletion, suggests that formate may act by replacing a bicarbo nate which is essential for Mn binding. Irreversible light-induced reb inding in an EPR-silent form of Mn2+ that was added to Mn-depleted DT- 20 was indeed found to depend on the presence of bicarbonate, as did t he reconstitution in such material of both the fast electron donation to Y-Z(+) and the UV absorbance changes characteristic of a functional oxygen-evolving complex. It is concluded that bicarbonate may be an e ssential ligand of the functional Mn cluster.