The normal pathway of electron transfer on the electron-acceptor side of photosystem II (PSII) involves electron transfer from quinone A, Q(A), to quinone B, Q(B). It is possible to redirect electrons from Q(A)(-) to water-soluble Co-III complexes, which opens a new avenue for harvesting electrons from water oxidation by immobilization of PSII on electrode surfaces. Herein, the kinetics of electron transfer from Q(A)(-) to [Co(III)(terpy)(2)](3+) (terpy = 2,2';6',2 ''-terpyridine) are investigated with a spectrophotometric assay revealing that the reaction follows Michaelis-Menten saturation kinetics, is inhibited by cations, and is not affected by variation of the Q(A) reduction potential. A negatively charged site on the stromal surface of the PSII protein complex, composed of glutamic acid residues near Q(A), is hypothesized to bind cations, especially divalent cations. The cations are proposed to tune the redox properties of Q(A) through electrostatic interactions. These observations may thus explain the molecular basis of the effect of divalent cations like Ca2+, Sr2+, Mg2+, and Zn2+ on the redox properties of the quinones in PRI, which has previously been attributed to long-range conformational changes propagated from divalent cations binding to the Ca(II)-binding site in the oxygen-evolving complex on the lumenal side of the PSII complex.