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

Abstract

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 helices 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.

Fine tuning of the spectral properties of LH2 by single amino acid residues

“…Intermolecular hydrogen bond between the peripheral C3 acetyl group and neighboring residues modulates the functional properties, especially the excitation energies of BChl in LH complexes (Fowler et al 1994;Sturgis and Robert 1997). Hydrogen bonding between the C13 1 keto carbonyl group of bacteriochlorophyll-B850 of the b-subunit to the hydroxyl group of serine(-4) of the a-subunit significantly contributes to the assembly and stability of LH2 without apparently affecting its spectral properties (Garcia-Martin et al 2006a;Kwa et al 2004).…”

Zn Pyropheophorbide a: A β‐Face Selective Nicotine Receptor

“…[1,3,5] Recent findings showed that the 13 1 -carbonyl of β-ligated chlorophylls in LH II has a strong tendency to H-bond with the neighbouring protein subunits, thus manifesting the key role of the ligation face in the antennae structure. [6] 4932 unit forms an ion pair with the chlorin acid group. The binding mechanism was further confirmed by association constants (K a ) obtained from titration experiments in CDCl It is very likely that selective chlorin β-face ligation has been formerly achieved by using a covalent approach by trivially replacing the phytyl group of chlorophyll with an N-propyl imidazole moiety.…”

Design principles for chlorophyll‐binding sites in helical proteins

“…Axial ligation to the central magnesium atom has long been recognized as critical for the binding of (B)Chl and thus for the assembly of (B)Chl proteins 6, 7. There are, however, numerous additional possibilities of interactions with the substituents; in particular, H‐bonding to the carbonyl groups seems to be widespread 8, 9. Additionally, there is the stereochemical aspect in (B)Chl ligation 10–12.…”

“…The two faces of the chlorin macrocycle are diastereotopic, and an additional chiral center is therefore generated by ligation of the central magnesium from either the top (α‐type) or the bottom face (β‐type) of the macrocycle [Scheme (C)]; these two ligation states should differ in their binding energies 13, 14. In natural (B)Chl proteins, the two types of ligation are unevenly distributed, and α‐type ligation of (B)Chls is predominant 9, 11. β‐Ligated (B)Chl has been proposed to play an important role in (B)Chl protein assembly 9.…”