Calcium reconstitutes high rates of oxygen evolution in polypeptide depleted Photosystem II preparations

Abstract

Exposure of highly resolved Photosystem II preparations to 2 M NaCl produces an 8OYo inhibition of oxygen-evolution activity concomitant with extensive loss of two water-soluble polypeptides (23 and 17 kDa). Addition of Ca'+ to salt-washed PS II membranes causes an acceleration in the decay of Zt, the primary donor to P-680+, and we show here that this acceleration is due to reconstitution of oxygenevolution activity by Ca'+. Other cations (Mg'+, Mn2+, Sr'+) are much less effective in restoring oxygen evolution. On the basis of these observations we propose that Ca2+, perhaps in concert with the 23 kDa polypeptide, is an essential cofactor for electron transfer from the 'S'-states to Z on the oxidizing side of PS II.Photosystem II Oxygen evolution Polypeptide Calcium

The PSII calcium site revisited

“…First, in the absence of Ca 2+ , PsbP increases the threshold temperature for the S 1 to S 2 transition from 200 K to 250 K (Ono and Inoue 1990a;Ono et al 1992). Second, after addition of Ca 2+ , the presence of the extrinsic polypeptides accelerates restoration of the native conformation of the binding site (as evidenced by recovery of EGTA-insensitive O 2 evolution activity), from hours to minutes (Ghanotakis et al 1984a;Miyao and Murata 1986;Ono and Inoue 1988;Ä delroth et al 1995).…”

“…The highest extent of activity reconstitution is produced by Ca 2+ addition to these samples. It is generally agreed that among all other metals tested, only Sr 2+ is capable of reconstituting O 2 evolution activity, but at lower rates (Ghanotakis et al 1984a;Boussac and Rutherford 1988b). Lockett et al (1990) reported that VO 2+ could also replace Ca 2+ in restoration of O 2 evolution activity and formation of the S 2 multiline signal.…”

“…Early experimentation to unravel the structure and function of the components of PSII showed that extraction of PsbP and PsbQ from spinach PSII preparations results in a strong inhibition of steady state O 2 evolution activity. Intensive efforts to understand why reconstitution of the polypeptides alone would not restore activity led to the discovery (Ghanotakis et al 1984a;Miyao and Murata 1984) that Ca 2+ would restore activity. This, in turn, has spawned a number of efforts in many laboratories to understand the role of Ca 2+ in the structure and function of the OEC.…”

“…The first method involves exposure of the intact enzyme to high ionic strength (1-2 M NaCl) (Ghanotakis et al 1984a;Miyao and Murata 1984), which generates a PSII sample that lacks PsbP and Q, and that exhibits low activity in steady state assays. The highest extent of activity reconstitution is produced by Ca 2+ addition to these samples.…”

“…So far as we can determine, there have been no further experiments on this phenomenon. The original procedure for Ca 2+ extraction has been amended to include illumination with continuous room light, which accelerates the rate of activity loss (Miyao and Murata 1986), and inclusion of a chelator (1 mM EDTA or EGTA) is recommended to suppress the high concentrations of adventitious Ca 2+ found in PSII preparations (Ghanotakis et al 1984a). The effect of illumination on Ca 2+ release from the OEC was in fact first noted by Dekker et al (1984), who exposed salt-washed PSII preparations to single-turnover flashes, and found that the 200 ls decay phase of the Q A -P680 + recombination reaction increased in amplitude as a function of flash number, out to about 150 flashes; the original amplitude was restored by Ca 2+ addition.…”