Iron as a bound secondary electron donor in modified bacterial reaction centers

The binding and oxidation of ferrous iron were studied in wild-type reaction centers and in mutants that have been modified to be both highly oxidizing and able to bind manganese [Thielges et al. (2005) Biochemistry 44, 7389-7394]. After illumination of wild-type reaction centers, steady-state optical spectroscopy showed that the oxidized bacteriochlorophyll dimer, P ⁺, could oxidize iron but only as a second-order reaction at iron concentrations above 100 μM. In the modified reaction centers, P⁺ was reduced by iron in the presence of sodium bicarbonate with dissociation constants of ∼1 μM for two mutants with different metal-binding sites. Transient optical spectroscopy showed that P⁺ was rapidly reduced with first-order rates of 170 and 275 s^-1 for the two mutants. The dependence of the amplitude of this rate on the iron concentration yielded a dissociation constant of ∼1 μM for both mutants, in agreement with the steady-state determination. The oxidation of bound iron by P⁺ was confirmed by the observation of a light-induced EPR signal centered at g values of 2.2 and 4.3 and attributed to high-spin Fe³⁺. Bicarbonate was required at pH 7 for low dissociation constants for both iron and manganese binding. The similarity between iron and manganese binding in these mutants provides insight into general properties of metal-binding sites in proteins.