Reconstitution of 3-Acetyl Chlorophyll into Light-Harvesting Complex 2 from the Purple Photosynthetic Bacterium .

The manipulation of B800 bacteriochlorophyll (BChl) in light-harvesting complex 2 (LH2) from the purple photosynthetic bacterium (-LH2) provides insight for understanding the energy transfer mechanism and the binding of cyclic tetrapyrroles in LH2 proteins since -LH2 is one of the two LH2 proteins whose atomic-resolution structures have been determined and is a representative of type-2 LH2 proteins. However, there is no report on the substitution of B800 BChl in -LH2. We report the reconstitution of 3-acetyl chlorophyll (AcChl) , which has a 17,18-dihydroporphyrin skeleton, to the B800 site in -LH2. The 3-acetyl group in AcChl formed a hydrogen bond with β'-Thr23 in essentially the same manner as native B800 BChl , but this hydrogen bond was weaker than that of B800 BChl . This change can be rationalized by invoking a small distortion in the orientation of the 3-acetyl group in the B800 cavity by dehydrogenation in the B-ring from BChl . The energy transfer from AcChl in the B800 site to B850 BChl was about 5-fold slower than that from native B800 BChl by a decrease of the spectral overlap between energy-donating AcChl and energy-accepting B850 BChl .