Conformational changes in coupling factor 1 may control the rate of electron flow in spinach chloroplasts.

The relationship between the rate of electron flow, internal H⁺ concentration and the magnitude of the H⁺ concentration gradient (ΔpH) in chloroplasts illuminated at various light intensities has been examined. At an external pH of 7.0, the internal H⁺ concentration is a linear function of the rate of electron flow except at saturating light intensity. In contrast, at pH 8.1, this relationship between electron flow and internal H⁺ concentration holds only at values of ΔpH below about 2.8 – 2.9 units. At higher ΔpH values, the rate of electron flow increases much more dramatically than the internal H⁺ concentration. ATP (0.1 mM) prevents this increase. It is suggested that at pH 8.1 but not at pH 7.0, the conformation of coupling factor 1 is altered at high ΔpH values. Its altered conformation may result in an increased efflux of H⁺ from the chloroplasts. This notion is supported by the effects of ATP on electron flow and ΔpH as well as the effect of external pH and light intensity on the reactivity of coupling factor 1 to N-ethylmaleimide.