Iron-depleted reaction centers from Rhodopseudomonas sphaeroides R-26.1: characterization and reconstitution with Fe2+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2+

Abstract

Reaction centers (RCs) from the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26.1 were depleted of Fe by a simple procedure involving reversible dissociation of the H subunit. The resulting intact Fe-depleted RCs contained 0.1-0.2 Fe per RC as determined from atomic absorption and electron paramagnetic resonance (EPR) spectroscopy. Fe-depleted RCs that have no metal ion occupying the Fe site differed from native RCs in the following respects: (1) the rate of electron transfer from QA- to QB exhibited nonexponential kinetics with the majority of RCs having a rate constant slower by only a factor of approximately 2, (2) the efficiency of light-induced charge separation (DQA----D+QA-) produced by a saturating flash decreased to 63%, and (3) QA appeared readily reducible to QA2-. Various divalent metal ions were subsequently incorporated into the Fe site. The electron transfer characteristics of Fe-depleted RCs reconstituted with Fe2+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2+ were essentially the same as those of native RCs. These results demonstrate that neither Fe2+ nor any divalent metal ion is required for rapid electron transfer from QA- to QB. However, the presence of a metal ion in the Fe site is necessary to establish the characteristic, native, electron-transfer properties of QA. The lack of a dominant role of Fe2+ or other divalent metals in the observed rate of electron transfer from QA- to QB suggests that a rate-limiting step (for example, a protonation event or a light-induced structural change) precedes electron transfer.