A comparative study of the reduction of EPR signal II slow by iodide and the iodo-labeling of the D2-protein in photosystem II

Two tyrosines that changed the world: Interfacing the oxidizing power of photochemistry to water splitting in photosystem II

Photosystem II (PSII), the thylakoid membrane enzyme which uses sunlight to oxidize water to molecular oxygen, holds many organic and inorganic redox cofactors participating in the electron transfer reactions. Among them, two tyrosine residues, Tyr-Z and Tyr-D are found on the oxidizing side of PSII. Both tyrosines demonstrate similar spectroscopic features while their kinetic characteristics are quite different. Tyr-Z, which is bound to the D1 core protein, acts as an intermediate in electron transfer between the primary donor, P(680) and the CaMn₄ cluster. In contrast, Tyr-D, which is bound to the D2 core protein, does not participate in linear electron transfer in PSII and stays fully oxidized during PSII function. The phenolic oxygens on both tyrosines form well-defined hydrogen bonds to nearby histidine residues, His(Z) and His(D) respectively. These hydrogen bonds allow swift and almost activation less movement of the proton between respective tyrosine and histidine. This proton movement is critical and the phenolic proton from the tyrosine is thought to toggle between the tyrosine and the histidine in the hydrogen bond. It is found towards the tyrosine when this is reduced and towards the histidine when the tyrosine is oxidized. The proton movement occurs at both room temperature and ultra low temperature and is sensitive to the pH. Essentially it has been found that when the pH is below the pK(a) for respective histidine the function of the tyrosine is slowed down or, at ultra low temperature, halted. This has important consequences for the function also of the CaMn₄ complex and the protonation reactions as the critical Tyr-His hydrogen bond also steer a multitude of reactions at the CaMn₄ cluster. This review deals with the discovery and functional assignments of the two tyrosines. The pH dependent phenomena involved in oxidation and reduction of respective tyrosine is covered in detail. This article is part of a Special Issue entitled: Photosystem II.

Photoinactivation of photosystem II by in situ-photoproduced hydroxyurea radicals

Oxygenic photosystem (PS) II complex from spinach photooxidized hydroxyurea (HU) to produce its aminoxy radical, which was identified by its electron spin resonance spectrum. HU was apparently photooxidized by the water-oxidizing enzyme (WOE) since the photooxidation reaction was blocked by carbonyl cyanide m-chlorophenylhydrazone (CCCP). HU radicals photoproduced by the WOE inhibited the electron transfer between the redox-active tyrosine residue (YZ), which is involved in electron transfer from the WOE to the reaction center chlorophyll of PS II, and the secondary quinone electron acceptor. Treatment of PS II complex with Tris resulted in the appearance of a CCCP-insensitive photooxidation site for HU. Photoproduced HU radicals in oxygenic and Tris-treated PS II complex decayed with first-order kinetics, an indication that the radicals reacted primarily with surrounding molecules rather than decayed through spontaneous dismutation or recombination. HU inhibited the diphenylcarbazide-supported photoreduction of 2,6-dichlorophenolindophenol (DCIP) in Tris-treated PS II complex preincubated only under illumination, but this inhibition was suppressed when ascorbate was added to scavenge HU radicals. If examined in darkness, HU radicals could not, however, inhibit subsequent photoreduction of DCIP. Therefore, the photoproduced HU radicals interact with a photogenerated site(s) in the PS II complex. The photoproduction of YZ., a radical of YZ, was suppressed to about 40% in Tris-treated PS II complex by the in situ-photogenerated HU radicals, and the yield of a cation radical of chlorophyll, close to the PS II reaction center, was increased, while the production of a radical of another redox-active tyrosine residue in PS II (YD.) was hardly affected.(ABSTRACT TRUNCATED AT 250 WORDS)