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 microM. In the modified reaction centers, P+ was reduced by iron in the presence of sodium bicarbonate with dissociation constants of approximately 1 microM 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 approximately 1 microM 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.
Kálmán
L
L
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, USA.
LoBrutto
R
R
Williams
J C
JC
Allen
J P
JP
D001639:Bicarbonates / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D007501:Iron / Q000378:metabolism*; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*
D001639:Bicarbonates; D045322:Photosynthetic Reaction Center Complex Proteins; D007501:Iron