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Publication Abstract from PubMed
The structural and functional consequences of changing the coordination state of one of the bacteriochlorophyll (BChl) cofactors in the purple bacterial reaction center have been explored. A combination of steady state spectroscopy and X-ray crystallography was used to demonstrate that mutagenesis of residue 181 of the L-polypeptide from Phe to Arg (FL181R) causes the BChl at the accessory (BB) position on the so-called inactive cofactor branch to become hexa-coordinated, with no significant changes to the structure of the surrounding protein. This change was accompanied by the appearance of a distinctive absorbance band at 631 nm in the room temperature absorbance spectrum. The ligand donor was not the Arg side chain but rather an intervening water molecule, and contrary to expectations the Mg of BB did not adopt a more in-plane geometry in response to hexa-coordination. The mutation caused a disturbance to the detailed conformation of the BChl macrocycle that manifested in a number of subtle changes to the resonance Raman spectrum. Hexa-coordination of BB produced a small increase in the lifetime of the excited electronic state of the primary donor bacteriochlorophylls (P*), indicating some disturbance to light-driven energy and/or electron transfer events on the time-scale of a few picoseconds after light excitation. The BB bacteriochlorophyll returned to a penta-coordinated state in a double mutant where the FL181R mutation was combined with removal of the native axial ligand through mutation of His M182 to Leu. Experimental evidence for hexa-coordinated bacteriochlorophylls in the literature on antenna proteins is considered, and possible reasons why hexa-coordinated bacteriochlorophylls and chlorophylls appear to be avoided in photosynthetic proteins are discussed.
Structural and spectroscopic consequences of hexa-coordination of a bacteriochlorophyll cofactor in the Rhodobacter sphaeroides reaction center.,Frolov D, Marsh M, Crouch LI, Fyfe PK, Robert B, van Grondelle R, Hadfield A, Jones MR Biochemistry. 2010 Jan 29. PMID:20112981[1]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
References
↑Frolov D, Marsh M, Crouch LI, Fyfe PK, Robert B, van Grondelle R, Hadfield A, Jones MR. Structural and spectroscopic consequences of hexa-coordination of a bacteriochlorophyll cofactor in the Rhodobacter sphaeroides reaction center. Biochemistry. 2010 Jan 29. PMID:20112981 doi:10.1021/bi901922t
