Photo inactivation reactions

Photosystem II reaction center is susceptible to light induced damage and steady state photosynthesis is maintained by continual repair of the photo-inactivated photosystems. An increase in the rate of inactivation (by over excitation of PS II) or a decrease in the rate of repair of PSII centers can result in accumulation of damaged PS II and consequent photoinhibition of photosynthesis. Since the assembly and stability of PS II complex involves close interaction with membrane lipids, photoinhibition and recovery of PS II centers are likely to be affected by the lipid composition of the thylakoid membrane. 

For Aspergillus niger extracellular endo-D-galacturonanase, the role of histidine in the enzyme reaction was investigated by the method of photo-oxidative inactivation, catalyzed by Methylene Blue.140 The inactivation of the enzyme was paralleled by the decomposition of histidine. The similarity of pH profiles, as well as the values of the rate constants of enzyme inactivation (4.0 X 10-2 min-1) and of decomposition of histidine (3.9 X 10-2 min-1), indicate that one of the five histidine residues present in the molecule of the enzyme141 is essential for its activity. 

Evidence has been accumulated that shows that transaldolase is a half the sites enzyme, in which binding of substrate to one site almost completely inactivates the other active site. As mentioned above, only 1 mole of substrate is bound to the active site, even though the enzyme is a dimer. In the presence of excess F-6-P a burst of one equivalent of G-3-P is produced followed by a slower conversion of F-6-P to G-3-P plus DHA. It appears therefore, that the blocking of one active site by adduct formation with dihydroxyacetone results in a modified second active site which is able to slowly cleave F-6-P [76], This second site behavior is identical to that found in transaldolase in which one active site has been modified by reducing the dihydroxyacetone adduct with borohydride. Photo-oxidation of one of the two histidine residues in isoenzyme III results in the loss of activity of the enzyme, as is consistent with half-sites reaction [77]. The dye l-anilino-8-naphthalene sulfonate on the other hand is bound with 2 moles of dye per mole of enzyme, indicating two binding sites for F-6-P [78]. 

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). 

Also the reaction center activity, measured by the yield and extent of photo-oxidation of the primary electron donor P-798, was completely retained. The absorbance difference spectra for the complexes from both species and the absorbance difference spectrum for //. chlorum membranes are shown in Fig. 4. The reduction of P-798 was slow as compared to membranes but could be speeded up upon addition of PMS. The absence of a fast decay component showed that the electron acceptor chain was at least partially intact. Cytochrome c-553 photo-oxidation was not observed, indicating that the cytochrome was either lost or inactivated during the isolation. 

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