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J. Biol. Chem., Vol. 281, Issue 15, 10626-10634, April 14, 2006

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Structural Role of (Bacterio)chlorophyll Ligated in the Energetically Unfavorable -Position^*

Adela Garcia-Martin, Lee Gyan Kwa, Brigitte Strohmann, Bruno Robert, Alfred R. Holzwarth^¶, and Paula Braun¹

From the Department I, Botanik, der Universität München, 80638 München, Germany, the Section de Biophysique des Fonctions Membranaires, DBJC/CEA and URA CNRS 2096, C.E. Saclay 91191 Gif sur Yvette Cedex, France, and the ^¶Max Planck-Institut für Bioanorganische Chemie, 45470 Mülheim an der Ruhr, Germany

Chlorophyll is attached to apoprotein in diastereotopically distinct ways, by - and -ligation. Both the - and -ligated chlorophylls of photosystem I are shown to have ample contacts to apoprotein within their proteinaceous binding sites, in particular, at C-13 of the isocyclic ring. The H-bonding patterns for the C-13¹ oxo groups, however, are clearly distinct for the -ligated and -ligated chlorophylls. The -ligated chlorophylls frequently employ their C-13¹ oxo in H-bonds to neighboring helices and subunits. In contrast, the C-13¹ oxo of -ligated chlorophylls are significantly less involved in H-bonding interactions, particularly to neighboring helices. Remarkably, in the peripheral antenna, light harvesting complex (LH2) from Rhodobacter sphaeroides, a single mutation in the -subunit, introduced to eliminate H-bonding to the -bacteriochlorophyll-B850, which is ligated in the "-position," results in significant thermal destabilization of the LH2 in the membrane. In addition, in comparison with wild type LH2, the expression level of the LH2 lacking this H-bond is significantly reduced. These findings show that H-bonding to the C-13¹ keto group of-ligated (bacterio)-chlorophyll is a key structural motif and significantly contributes to the stability of bacteriochlorophyll proteins in the native membrane. Our analysis of photosystem I and II suggests that this hitherto unrecognized motif involving H-bonding to -ligated chlorophylls may be equally critical for the stable assembly of the inner core antenna of these multicomponent chlorophyll proteins.

Received for publication, October 3, 2005 , and in revised form, February 15, 2006.

^* This work was supported by Grants BR1991/1-2 and SFB533 TPA13 from the Deutsche Forschungsgemeinschaft, Germany (to P. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 498917861232; Fax: 498917861185; E-mail: paula.braun{at}lrz.uni-muenchen.de.

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