Surface charge asymmetry and a specific calcium ion effect in chloroplast Photosystem II

Abstract

We have used the decay kinetics of Signal IIf in Tris-washed chloroplasts as a direct probe to reactions on the oxidizing side of Photosystem II. A study of the salt concentration dependence of the rate of reduction of Z ⨥ by the ascorbate monoanion has been interpreted by using the Gouy-Chapman diffuse double layer model and allows the calculation of an inner membrane surface charge density of −3.4 ± 0.3 μC · cm −2 at pH = 8.0 in the vicinity of Photosystem II. We have also measured the outer membrane surface charge density at this pH in Tris- and sucrose-washed chloroplasts by monitoring the rate of potassium ferricyanide oxidation of Q −, and arrive at values of −2.2 ± 0.3 μC · cm −2 and −2.1 μC · cm −2, respectively. From these experiments we conclude that in dark-adapted chloroplasts at pH 8.0 there exists a transmembrane electric field in the vicinity of Photosystem II which arises from this surface charge asymmetry. In the presence of 10 mM monovalent salts, the transmembrane potential difference is of the order of 20 mV, corresponding to a field of 4 · 10 4 V · cm −1 (negative inside) for a 50 Å membrane. It is both smaller in magnitude and in the opposite direction compared to the photoinduced transmembrane field which gives rise to the 515 nm absorption change. We have also found non-double layer Ca 2+ effects on the decay kinetics of Signal IIf with both charged (ascorbate monoanion) and neutral (diphenylcarbazide) donors. These results suggest a change in the environment of Z from lipophilic to hydrophilic upon specific binding of Ca 2+.