title: Probing the bacteriochlorophyll binding site by reconstitution of the light-harvesting complex of Rhodospirillum rubrum with bacteriochlorophyll a analogues.
abstract: Structural features of bacteriochlorophyll (BChl) a that are required for binding to the light-harvesting proteins of Rhodospirillum rubrum were determined by testing for reconstitution of the B873 or B820 (structural subunit of B873) light-harvesting complexes with BChl a analogues. The results indicate that the binding site is very specific; of the analogues tested, only derivatives of BChl a with ethyl, phytyl, and geranylgeranyl esterifying alcohols and BChl b (phytyl) successfully reconstituted to form B820- and B873-type complexes. BChl analogues lacking magnesium, the C-3 acetyl group, or the C-13(2) carbomethoxy group did not reconstitute to form B820 or B873. Also unreactive were 13(2)-hydroxyBChl a and 3-acetylchlorophyll a. Competition experiments showed that several of these nonreconstituting analogues significantly slowed BChl a binding to form B820 and blocked BChl a-B873 formation, indicating that the analogues may competitively bind to the protein even though they do not form red-shifted complexes. With the R. rubrum polypeptides, BChl b formed complexes that were further red-shifted than those of BChl a; however, the energies of the red shifts, binding behavior, and circular dichroism (CD) spectra were similar. B873 complexes reconstituted with the geranylgeranyl BChl a derivative, which contains the native esterifying alcohol for R. rubrum, showed in-vivo-like CD features, but the phytyl and ethyl B873 complexes showed inverted CD features in the near infrared. The B820 complex with the ethyl derivative was about 30-fold less stable than the two longer esterifying alcohol derivatives, but all formed stable B873 complexes.
authors.affiliation: Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, Illinois 60208-3500.
authors.forename: P S
authors.initials: PS
authors.lastname: Parkes-Loach
authors.forename: T J
authors.initials: TJ
authors.lastname: Michalski
authors.forename: W J
authors.initials: WJ
authors.lastname: Bass
authors.forename: U
authors.initials: U
authors.lastname: Smith
authors.forename: P A
authors.initials: PA
authors.lastname: Loach
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D001429:Bacteriochlorophylls / Q000138:chemical synthesis / Q000378:metabolism*; D002734:Chlorophyll / Q000031:analogs & derivatives*; D002942:Circular Dichroism; D045342:Light-Harvesting Protein Complexes; D009682:Magnetic Resonance Spectroscopy; D045322:Photosynthetic Reaction Center Complex Proteins; D012247:Rhodospirillum rubrum / Q000378:metabolism*; D013053:Spectrophotometry; D013329:Structure-Activity Relationship
chemical_list: D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D002734:Chlorophyll