* To whom correspondence should be addressed.
Received August 22, 2011
The change in the dark reduction rate of photooxidized reaction centers (RC) of type II from three anoxygenic bacteria (Rhodobacter sphaeroides R-26, Chromatium minutissimum, and Chloroflexus aurantiacus) having different redox potentials of the P+/P pair and availability of RC for exogenous electron donors was investigated upon the addition of Mn2+ and HCO3–. It was found that the dark reduction of P870+ from Rb. sphaeroides R-26 is considerably accelerated upon the combined addition of 0.5 mM MnCl2 and 30-75 mM NaHCO3 (as a result of formation of “low-potential” complexes [Mn(HCO3)2]), while MnCl2 and NaHCO3 added separately had no such effect. The effect is not observed either in RC from Cf. aurantiacus (probably due to the low oxidation potential of the primary electron donor, P865, which results in thermodynamic difficulties of the redox interaction between P865+ and Mn2+) or in RC from Ch. minutissimum (apparently due to the presence of the RC-bound cytochrome preventing the direct interaction between P870+ and Mn2+). The absence of acceleration of the dark reduction of P870+ in the RC of Rb. sphaeroides R-26 when Mn2+ and HCO3– were replaced by Mg2+ or Ca2+ and by formate, oxalate, or acetate, respectively, reveals the specificity of the Mn2+-bicarbonate complexes for the redox interaction with P+. The results of this work might be considered as experimental evidence for the hypothesis of the participation of Mn2+ complexes in the evolutionary origin of the inorganic core of the water oxidizing complex of photosystem II.
KEY WORDS: Mn-bicarbonate complexes, reaction centers, purple bacteria, RC-bound cytochrome