Investigation of the redox interaction between Mn-bicarbonate complexes and reaction centers from Rhodobacter sphaeroides R-26, Chromatium minutissimum, and Chloroflexus aurantiacus

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.