Reducing Side Of Photosystem Research Articles

Inhibitory Effects of Synthetic Lanthanum-Crown Ether at the Reducing Side of Photosystem II

Abstract Inhibitory effects of lanthanum-crown [La-(Pic)3 (15-crown-6) 3H2O].was investigated on the O2 evolution activity of photsystem II particles. Lanthanum (La)-crown inhibited the electron flow at the reducing side of PS II complex. Short duration (1-2 min) treatment of PS II membranes with trypsin partly developed resistance to La-crown inhibition. However, longer proteolytic treatment (>2 min) appeared to expose newer site(s) for La-crown inhibi­tion. The inhibitory constant (Ki) for La-crown was nearly 0.17 | μᴍ . This inhibitory capacity is about 4 to 5 times less than the potent PS II inhibitor diuron which also binds at the acceptor side of PS II. The number of binding sites for La-crown was found to be 1 per 20 chlorophyll molecules. The Hill plot analysis showed the presence of three distinct straight lines suggesting that the compound acts at least at three sites. Furthermore, from the slope value (Hill coefficient) it is suggested that two of these sites provide minimum of two binding domains for the inhibitor.

Identification of Surface-Exposed Domains on the Reducing Side of Photosystem I

Photosystem I (PSI) is a multisubunit enzyme that catalyzes the light-driven oxidation of plastocyanin or cytochrome c6 and the concomitant photoreduction of ferredoxin or flavodoxin. To identify the surface-exposed domains in PSI of the cyanobacterium Synechocystis sp. PCC 6803, we mapped the regions in PsaE, PsaD, and PsaF that are accessible to proteases and N-hydroxysuccinimidobiotin (NHS-biotin). Upon exposure of PSI complexes to a low concentration of endoproteinase glutamic acid (Glu)-C, PsaE was cleaved to 7.1- and 6.6-kD N-terminal fragments without significant cleavage of other subunits. Glu63 and Glu67, located near the C terminus of PsaE, were the most likely cleavage sites. At higher protease concentrations, the PsaE fragments were further cleaved and an N-terminal 9.8-kD PsaD fragment accumulated, demonstrating the accessibility of Glu residue(s) in the C-terminal domain of PsaD to the protease. Besides these major, primary cleavage products, several secondary cleavage sites on PsaD, PsaE, and PsaF were also identified. PsaF resisted proteolysis when PsaD and PsaE were intact. Glu88 and Glu124 of PsaF became susceptible to endoproteinase Glu-C upon extensive cleavage of PsaD and PsaE. Modification of PSI proteins with NHS-biotin and subsequent cleavage by endoproteinase Glu-C or thermolysin showed that the intact PsaE and PsaD, but not their major degradation products lacking C-terminal domains, were heavily biotinylated. Therefore, lysine-74 at the C terminus of PsaE was accessible for biotinylation. Similarly, lysine-107, or lysine-118, or both in PsaD could be modified by NHS-biotin.

Inhibition on the reducing side of photosystem II by carbonylcyanide m-chlorophenylhydrazone and lithium 3,5-diiodosalicylate.

Several investigators have shown that carbonylcyanide m‐chlorophenylhydrazone (CCCP) inhibits photosystem II of chloroplasts by acting on a site close to the oxygen‐evolving center. Evidence is presented in this paper which suggests that CCCP interferes also with the electron transfer between the primary acceptor Q and the secondary acceptor pool, A. A similar action in this electron acceptor complex is described for the photosynthesis inhibitor salicylaldoxime, and other phenolic compounds like salicylate and, in particular, lithium 3,5‐diiodosalicylate. The latter two agents are known for their ability to increase the solubility of hydrophobic peptides and proteins in aqueous solutions. Hence, our findings may be of help in attempts to understand the structure‐activity relationship of the large class of structurally diverse herbicides which inhibit photosynthetic electron transport between Q and A.