Anomalous acceleration of the photocycle in photosynthetic reaction centers inhibited on the acceptor side

The rate of the photocycle (quinone reduction cycle) was measured under continuous light excitation in an isolated reaction center protein of the photosynthetic bacterium Rhodobacter sphaeroides. The rate is determined by the slowest step of the photocycle, which could be the photochemistry (charge separation), the quinone/quinol and cytochrome c 2+ /c 3+ exchanges, or proton delivery to the secondary quinone. The photocycle was driven by high light intensity of a laser diode (5 W/cm 2 at 808 nm) to avoid light limitation of the observed rate. The fast turnover of the reaction center (up to 10 3 s –1 ) was slowed down by inhibition of the proton delivery to the secondary quinone by transition metal ions (Cd 2+ and Ni 2+ ), by mutation of a key protonatable group (L213Asp → Asn), or by use of low‐affinity ubiquinone (UQ 0 ) to the secondary quinone binding site. Although in all of these cases the rate of turnover was 2–3 orders of magnitude less than that of the primary photochemistry, marked light intensity dependence was observed. The rate of the photocycle increased from 7 s –1 (Ni 2+ , low light intensity) to 27 s –1 (high light intensity) at pH 8.4. The anomalous reacceleration is due to alternative events on the acceptor side induced by continuous excitation. We argue that the continuous excitation of the protein trapped in the reduced acceptor (Q A − Q B − ) state produces short‐lived reduced bacteriopheophytin (I − ) that delivers activation energy to anomalous changes on the acceptor side as second interquinone electron transfer before proton uptake or increase of the quinone dissociation constant. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004