In Situ, Protein-Mediated Generation of a Photochemically Active Chlorophyll Analogue in a Mutant Bacterial Photosynthetic Reaction Center.

All possible natural amino acids have been substituted for the native LeuL185 positioned near the B-side bacteriopheophytin (H) in the bacterial reaction center (RC) from . Additional mutations that enhance electron transfer to the normally inactive B-side cofactors are present. Approximately half of the isolated RCs with Glu at L185 contain a magnesium chlorin (C) in place of H. The chlorin is not the common BChl oxidation product 3-desvinyl-3-acetyl chlorophyll with a C-C bond in ring D and a C═C bond in ring B but has properties consistent with reversal of these bond orders, giving 17,18-didehydro BChl . In such RCs, charge-separated state PC forms in ∼5% yield. The other half of the GluL185-containing RCs have a bacteriochlorophyll (BChl ) denoted β in place of H. Residues His, Asp, Asn, and Gln at L185 yield RCs with ≥85% β in the H site, while most other amino acids result in RCs that retain H (≥95%). To the best of our knowledge, neither bacterial RCs that harbor five BChl molecules and one chlorophyll analogue nor those with six BChl molecules have been reported previously. The finding that altering the local environment within a cofactor binding site of a transmembrane complex leads to in situ generation of a photoactive chlorin with an unusual ring oxidation pattern suggests new strategies for amino acid control over pigment type at specific sites in photosynthetic proteins.