Function of Tyrosine Z in Water Oxidation by Photosystem II:  Electrostatical Promotor Instead of Hydrogen Abstractor

Abstract

Photosynthetic water oxidation by photosystem II is mediated by a Mn4 cluster, a cofactor X still chemically ill-defined, and a tyrosine, YZ (D1-Tyr161). Before the final reaction with water proceeds to yield O2 (transition S4-->S0), two oxidizing equivalents are stored on Mn4 (S0-->S1-->S2), a third on X (S2-->S3), and a forth on YZ(S3-->S4). It has been proposed that YZ functions as a pure electron transmitter between Mn4X and P680, or, more recently, that it acts as an abstractor of hydrogen from bound water. We scrutinized the coupling of electron and proton transfer during the oxidation of YZ in PSII core particles with intact or impaired oxygen-evolving capacity. The rates of electron transfer to P680+, of electrochromism, and of pH transients were determined as a function of the pH, the temperature, and the H/D ratio. In oxygen-evolving material, we found only evidence for electrostatically induced proton release from peripheral amino acid residues but not from YZox itself. The positive charge stayed near YZox, and the rate of electron transfer was nearly independent of the pH. In core particles with an impaired Mn4 cluster, on the other hand, the rate of the electron transfer became strictly dependent on the protonation state of a single base (pK approximately 7). At pH < 7, the rate of electron transfer revealed the same slow rate (t1/2 approximately 35 microseconds) as that of proton release into the bulk. The deposition of a positive charge around YZox was no longer detected. A large H/D isotope effect (approximately 2.5) on these rates was also indicative of a steering of electron abstraction by proton transfer. That YZox was deprotonated into the bulk in inactive but not in oxygen-evolving material argues against the proposed role of YZox as an acceptor of hydrogen from water. Instead, the positive charge in its vicinity may shift the equilibrium from bound water to bound peroxide upon S3-->S4 as a prerequisite for the formation of oxygen upon S4-->S0.

Tyrosine-Z in Oxygen-Evolving Photosystem II: A Hydrogen-Bonded Tyrosinate

“…Such an increase is plausible in view of a more open structure of PSII after removal of the Mn cluster. At pH 5.7 electrochromism was practically absent (Figure 2, blue, dashed line) in agreement with our earlier data (37) and only a small chemical contribution from Y Z ox itself remained around 400 nm (58).…”

“…After this treatment oxygen evolution under continuous illumination (not documented) was below 10% of a control. The pH 9 treated samples were indistinguishable from samples that were inactivated by the classical tris-wash treatment (42,118) as apparent from transients of the reduction of P 680 + at 820 nm (37). Accordingly, it was likely that they were depleted of manganese (37,(42)(43)(44).…”

“…To remove the contribution of Q A / Q A -from our UV spectra we normalized its difference spectrum (taken from Dekker et al (49)) to the observed UV extents at around 325 nm ( Figure 1, UV, dotted lines) and subtracted it from the former. The resulting differences from active centers and from inactive ones at pH 5.7 were scaled by factors of 1.2 to account for the amount of centers where Y Z ox has decayed after 20 µs in the former case and for a proportion of about 20% of total P 680 which stayed oxidized, P 680 + , in equilibrium with Y Z ox (37). To correct for the spectral contribution of P 680 f P 680 + , we furthermore subtracted 20% of its difference spectrum as taken from Gerken et al (50) from the data points at pH 5.7.…”

“…It is probably attributable to the pheophytin of branch A (15,47). We calibrated the contributions of Q A -to the spectra in the blue region on the basis of the electrochromic band shifts in the green ( -was reoxidized with a half-time of about 10 ms (52), and a small fraction of P 680 + was reduced in milliseconds (37). With Mn-depleted material the extents at 1 ms after a flash (repetition rate 0.1 Hz) were recorded with 200 µM DCBQ and 1 mM hexacyanoferrate(III).…”

“…However, the rate of the Y Z oxidation by P 680 + is fast (tens of nanoseconds) (115,116), almost insensitive to H/D substitution (33,(35)(36)(37), and has a low activation barrier (37,38), as characteristic for a pure electron-transfer event. Furthermore, the formation of Y Z ox is accompanied by a large and rapid electrochromic band shift of nearby pigments (15,34,39,40).…”