In bacterial reaction centers, a key residue suppresses mutational blockage of two different proton transfer steps.
In reaction centers of Rhodobacter (Rb.) capsulatus, the M43Asn --> Asp substitution is capable of restoring rapid rates for delivery of the second proton to QB in a mutant that lacks L212Glu. Flash-induced absorbance spectroscopy was used to show a nearly native rate for transfer of the second proton to QB (approximately 700 s-1) in the L212Gln+M43Asp double-mutant reaction center; this rate was shown to decrease more than 1000-fold in the photoincompetent L212Glu --> Gln mutant [Miksovska, J., Kálmán, L., Maróti, P., Schiffer, M., Sebban, P., and Hanson, D.K. (1997) Biochemistry 36, 12216-12226]. In Rb. sphaeroides, the equivalent M44Asn --> Asp mutation was reported to restore the rate of transfer of the first proton to a wild-type level when it is added to the L213Asp --> Asn photoincompetent mutant [Rongey, S.H., Paddock, M.L., Feher, G., and Okamura, M.Y. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 1325-1329]. It is remarkable that the same second-site mutation can compensate for both of these mutations which severely impair reaction center function by blocking two different proton-transfer reactions. We suggest that residue M43Asp is situated in a key position which can link pathways for delivery of both the first and second protons (involving structured water molecules) to QB.
Miksovska
J
J
Centre de Génétique Moléculaire, CNRS, Gif sur Yvette, France.
Valerio-Lepiniec
M
M
Schiffer
M
M
Hanson
D K
DK
Sebban
P
P
D003574:Cytochrome c Group / Q000378:metabolism / Q000528:radiation effects; D004563:Electrochemistry; D004579:Electron Transport; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017354:Point Mutation; D011487:Protein Conformation; D011522:Protons; D016354:Rhodobacter capsulatus / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects
D003574:Cytochrome c Group; D045322:Photosynthetic Reaction Center Complex Proteins; D011522:Protons