Oxygen-evolving complex evolution

The S states represent the accumulation of positive charges on the oxygen-evolving complex ((TEC), and O2 is evolved only when 4 charges are accumulated. Starting with dark-adapted chloroplasts (or intact photosynthetic cells), O2 evolution is maximal at the third flash, then proceeds with a periodicity of 4, because the state Si is the most abundant at equilibrium in the dark. After a number of cycles of the system, the periodicity tends to disappear due to misses and double hits , which finally randomize the PS II units into the 4 S states [8,9]. 

Fig. 2. Pattern showing removal and restoration of cofactors in the oxygen-evolving complex in a PS-II particle from spinach. The PS-II complex consists of the D1/D2 proteins, as shown In Fig. 1. Legend for the extrinsic proteins is shown at upper left Manganese atoms are represented by four small dots. See text for other details. Scheme adapted from Yamamoto (1989) Molecular organization of oxygen-evolution system in chioropiasts. Bot Mag (Tokyo) 102 572.

Bricker TM, Ghanotakis DF. Introduction to oxygen evolution and the oxygen-evolving complex. In Ort DR, Yocum CF, eds. Advances in Photosynthesis The Light Reactions. Vol. 4. Dordrecht Kluwet Academic Publishers, 1996 113-136. 

The restoration of the oxygen-evolving complex (DEC) Includes both photoinduced and dark steps. The rate of 02-evolutIon increases as a function of the ordinal number of I-mln light expositions separated by a 2-min dark period and Is higher than that observed upon one 5 mln con-tlnlous Illumination /2-4/. 

The correlation of the extent to which Tris inactivates photos)mthetic O2 evolution with the loss of 2/3 of the Mn bound to thylakoid membranes provided the first evidence that the oxygen-evolving complex (OEC) contained four bound Mn (1). Due to the linear dependence of activity and bound Mn, Tris was h) othesized to cause the concerted release of all four Mn from each reaction center. Later experiments which quantitated functional Mn after the removal of adventitious Mn by washing thylakoids with 50 mM CaCl2 confirmed the ratio of 4 Mn/OEC (2). However, in these experiments the amount of Mn released during inactivation by Tris or NH2OH was variable with the conditions of the extraction. This partial extraction of Mn was interpreted to result from a loss of one to three Mn/OEC in all of the reaction centers rather than the loss of 4 Mn/OEC in 1/4 to 3/4 of the reaction centers, respectively. Tris and NH2OH are now known to remove the extrinsic membrane proteins associated with the OEC (3,4). In the absence of these proteins, the mM concentrations of Ca " " used to remove adventitious Mn from thylakoids also activate the OEC and inhibit the loss of Mn from the enzyme (5-7). 

Another selective inhibitor of the oxygen evolving complex is NH2OH. Treating PS II particles with NH OH (Fig 3) effectively inhibited the oxygen evolution and the water to DCIP reaction. The treatment also resulted in an almost complete (97 %) loss of Mn. The release of Mn correlated well to the inhibition of oxygen evolution and water to DCIP reaction. In contrast, the pseudo catalase activity was only inhibited to some 60 % and was not at all correlated to the release of Mn. Thus, the pseudo catalase activity is not dependent, on either bound or free, Mn. 

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