| Abstract
| - The X-ray crystal structure of a Rhodobacter sphaeroides reaction center with the mutationAla M260 to Trp (AM260W) has been determined. Diffraction data were collected that were 97.6%complete between 30.0 and 2.1 Å resolution. The electron density maps confirm the conclusions of aprevious spectroscopic study, that the QA ubiquinone is absent from the AM260W reaction center (Ridge,J. P., van Brederode, M. E., Goodwin, M. G., van Grondelle, R., and Jones, M. R. (1999) PhotosynthesisRes.59, 9−26). Exclusion of the QA ubiquinone caused by the AM260W mutation is accompanied by achange in the packing of amino acids in the vicinity of the QA site that form part of a loop that connectsthe DE and E helices of the M subunit. This repacking minimizes the volume of the cavity that resultsfrom the exclusion of the QA ubiquinone, and further space is taken up by a feature in the electron densitymaps that has been modeled as a chloride ion. An unexpected finding is that the occupancy of the QB siteby ubiquinone appears to be high in the AM260W crystals, and as a result the position of the QB ubiquinoneis well-defined. The high quality of the electron density maps also reveals more precise information onthe detailed conformation of the reaction center carotenoid, and we discuss the possibility of a bondinginteraction between the methoxy group of the carotenoid and residue Trp M75. The conformation of the2-acetyl carbonyl group in each of the reaction center bacteriochlorins is also discussed.
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