On the role of Fe²⁺ in bacterial photosynthesis. The effect of biosynthetic substitution of Fe²⁺ by Mn²⁺ on the electron transfer step Q^-₁Q₂ → Q₁Q^-₂ in reaction centers

A test of the 'iron-wire' hypothesis for the role of Fe²⁺ in promoting the electron transfer between the primary (Q₁) and secondary (Q₂) quinones in bacterial reaction centers of Rhodopseudomonas sphaeroides strain R-26.1 has been conducted. Kinetics of this step, P⁺Q^-₁Q₂ → P⁺Q₁Q^-₂, and of recombination with the oxidized donor, P⁺Q^-₁ → PQ₁ and P⁺Q^-₂ → PQ₂, were followed optically at 4°C in normal iron-containing reaction centers and in reaction centers having 58% Mn²⁺, replacing Fe²⁺. This significant replacement is accomplished biosynthetically by control of the growth conditions, and so should preserve the native interactions between the cofactors. There are no significant differences observed in the recombination kinetics of the two types of reaction centers. The electron transfer between the quinones was observed to show apparent biphasic kinetics with major components of approx. 170 μs and 1.5 ms at 4°C and pH = 7.5. There is no statistically significant difference observed between the two types of reaction centers. This major change in the electronic structure of the metal and the unaltered kinetics discount the likelihood of any direct orbital participation of the metal in the electron transfer between the quinones.