Photosynthetic Water Oxidation at High O₂ Backpressure Monitored by Delayed Chlorophyll Fluorescence

Abstract

The atmospheric dioxygen is produced by photosynthetic organisms. This light-driven process culminates in what appears as one step: a four-electron abstraction from two water molecules bound to the Mn4Ca complex of photosystem II. Recently, an intermediate of the O2-producing reaction sequence was stabilized by elevated oxygen backpressure and detected by UV flash photometry [Clausen, J., and Junge, W. (2004) Nature 430, 480]. We scrutinized its properties by delayed chlorophyll fluorescence measurements. Half-suppression of oxygen evolution was observed at a similar O2 pressure of 2.3 bar, as previously, now with photosystem II membrane particles from spinach, without artificial electron acceptors, and at a high signal-to-noise ratio. The data are tentatively interpreted as the stabilization of a 2-fold oxidized state of the catalytic center (S2*) with bound peroxide and its slow conversion into the normal S2 state by the release of peroxide.

Photosynthetic water oxidation at elevated dioxygen partial pressure monitored by time-resolved X-ray absorption measurements

“…Thus, the pO 2 of Ն10 bar in the XAS measurements discussed above was sufficient to saturate the O 2 effects. A similar pO 2 1/2 value has been observed in previous O 2 experiments on different types of PSII samples (10,11).…”

“…The amplitude of the oxidation-reduction transient is slightly increased at high pO 2 , possibly explainable by additional, nonstandard oxidations of the Mn complex shortly after flash excitation (model a in Table 1). By using at 13 bar and 0.2 bar an identical halftime of Mn reduction of 1.2 Ϯ 0.1 ms, a value that is typical for PSII (10,11), convincing simulations were obtained. Furthermore, an apparent mean halftime of Mn oxidation of Ϸ110 s (Fig.…”

“…They exposed PSII from cyanobacteria to oxygen partial pressures (pO 2 ) ranging from 0.2 to 30 bar and, analyzing flash-induced near-UV absorption changes, discovered a pO 2 effect with a half-saturation pressure of only Ϸ2 bar (10). Subsequently, in measurements of delayed Chl fluorescence, together with Clausen and Junge we confirmed that also the donor side of the plant PSII in its native membrane environment is affected by elevated pO 2 (11). The surprisingly low halfpressure of only 2-3 bar was taken as an indication that the efficiency of the O 2 formation step could be reduced significantly by product inhibition.…”

“…We feel that the X-ray transients of Fig. 4 cannot be reconciled in a reasonably straightforward way with the proposed S 2 * formation in a major fraction of PSII (10,11).…”

“…It has been suggested that at elevated pO 2 , the 3rd flash does not induce a 4-electron reduction of the Mn complex, but a 2-electron reduction resulting in a peroxidic intermediate (S 2 *) (10,11). According to this rationale, a diminished amplitude of the Mn reduction transient on flash 3 may be expected at high pO 2 .…”