Oxygen-evolving complex inhibition

The simplest picture of donation to P-680 is one in which a single component, Z, acts as a carrier for electrons between P-680 and the oxygen-evolving complex. Time-resolved absorption spectra of minus Z in Tris-inhibited [171,172] and in... 

It is worth to mention that chloride is a cofactor required for the optimal function of the oxygen-evolving complex. Its presence may increase the activity of the photosystem II sub-membrane fractions and in such way to modify the extent of inhibition. In addition, it has been shown that the inhibitory effect of mercury is reversed by chloride. 

The kinetic mechanism for the interaction of Cl and Ca" " with the oxygen evolving complex obtained with NaCl is representative of an ordered reaction mechanism overlaid with an inhibition process (data not shown). In the... 

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. 

Sandusky PO, Yocum CF. The chloride requirement for photosynthetic oxygen evolution. Analysis of the effects of chlorie and other anions on amine inhibition of the oxygpn-evolving complex. Biochim Biophys Acta 1984 776 603-611. 

Cyanide binds to Fe in heme-containing proteins. This inhibits the terminal cytochrome complex IV of the electron transport chain. The blocklock of complex IV by cyanide depletes ATP culminating in cell death. Oxygen is unable to reoxidize the reduced cytochrome a3. Thus, cellular respiration is inhibited, as well as ATP production, in essence depriving the cells, tissue, and, ultimately, the whole body of oxygen. Hypoxia evolves into metabolic acidosis and decreased oxygen saturation. The extent of lactic acidosis indicates the severity of the cyanide... 

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