Cation-induced increases in the rate of P700 recovery in photosystem I particles.

Divalent cations such as Ca²⁺ and Mg²⁺ ions increase the rate of the dark recovery of P700⁺ in the P700-chlorophyll a protein of Shiozawa et al. (J. A. Shiozawa, R. S. Alberte, and J. P. Thornber, 1974, Arch. Biochem. Biophys., 165, 388–397). Half-maximal increases were observed at 1.5 mm concentrations of Mg²⁺ and Ca²⁺ ions. This correlates very well with the concentrations required to cause conformational changes in the P700-chlorophyll a protein. Na⁺ and K⁺ ions were also effective but 16–22 mm concentrations were required for half-maximal effects. Addition of Triton X-100 at concentrations greater than 0.02% also increased the rate of the dark recovery of P700⁺. The increases in the rate of P700 recovery are caused by a structural change involving the disaggregation of the protein. Mg²⁺ ions increase the rate of recovery of P700⁺ when both negatively (ascorbate and dichlorophenol indophenol) and positively (tetramethyl phenylenediamine) charged electron donors are used. This rules out the possibility that cations simply change the net charge on the protein to increase the binding of negatively charged electron donors. Moreover, it appears that Mg²⁺ ions affect the electron transport step rather than the binding of the donors to the complex. In addition, Mg²⁺ ions affect only the linear electron transport process from donor to O₂, not the recombination of P700⁺ with the primary electron acceptor.