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-13 1 oxo groups, however, are clearly distinct for the ␤-ligated and ␣-ligated chlorophylls. The ␤-ligated chlorophylls frequently employ their C-13 1 oxo in H-bonds to neighboring

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... lices and subunits. In contrast, the C-13 1 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-13 1 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.