Bidirectional Photoinduced Electron Transfer in Ruthenium(II)-Tris-bipyridyl-Modified PpcA, a Multi-heme c-Type Cytochrome from Geobacter sulfurreducens.
PpcA, a tri-heme cytochrome c7 from Geobacter sulfurreducens, was investigated as a model for photosensitizer-initiated electron transfer within a multi-heme "molecular wire" protein architecture. Escherichia coli expression of PpcA was found to be tolerant of cysteine site-directed mutagenesis, demonstrated by the successful expression of natively folded proteins bearing cysteine mutations at a series of sites selected to vary characteristically with respect to the three -CXXCH- heme binding domains. The introduced cysteines readily reacted with Ru(II)-(2,2'-bpy)2(4-bromomethyl-4'-methyl-2,2'-bipyridine) to form covalently linked constructs that support both photo-oxidative and photo-reductive quenching of the photosensitizer excited state, depending upon the initial heme redox state. Excited-state electron-transfer times were found to vary from 6 × 10(-12) to 4 × 10(-8) s, correlated with the distance and pathways for electron transfer. The fastest rate is more than 10(3)-fold faster than previously reported for photosensitizer-redox protein constructs using amino acid residue linking. Clear evidence for inter-heme electron transfer within the multi-heme protein is not detected within the lifetimes of the charge-separated states. These results demonstrate an opportunity to develop multi-heme c-cytochromes for investigation of electron transfer in protein "molecular wires" and to serve as frameworks for metalloprotein designs that support multiple-electron-transfer redox chemistry.
†Chemical Sciences and Engineering Division and ‡Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
Oleksandr
O
Kokhan
†Chemical Sciences and Engineering Division and ‡Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
Nina S
NS
Ponomarenko
†Chemical Sciences and Engineering Division and ‡Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
P Raj
PR
Pokkuluri
†Chemical Sciences and Engineering Division and ‡Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
Marianne
M
Schiffer
†Chemical Sciences and Engineering Division and ‡Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
Karen L
KL
Mulfort
†Chemical Sciences and Engineering Division and ‡Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
David M
DM
Tiede
D015082:2,2'-Dipyridyl / Q000737:chemistry* / Q000378:metabolism; D003574:Cytochrome c Group / Q000737:chemistry* / Q000378:metabolism; D004579:Electron Transport; D045302:Geobacter / Q000201:enzymology*; D008958:Models, Molecular; D055668:Photochemical Processes; D012428:Ruthenium / Q000737:chemistry* / Q000378:metabolism
D003574:Cytochrome c Group; D015082:2,2'-Dipyridyl; D012428:Ruthenium