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Chlorophylls, ligands and assembly of light-harvesting complexes in chloroplasts.

Hoober JK, Eggink LL, Chen M - Photosyn. Res. (2007)

Bottom Line: Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains.The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group.These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

View Article: PubMed Central - PubMed

Affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA. khoober@asu.edu

ABSTRACT
Chlorophyll (Chl) b serves an essential function in accumulation of light-harvesting complexes (LHCs) in plants. In this article, this role of Chl b is explored by considering the properties of Chls and the ligands with which they interact in the complexes. The overall properties of the Chls, not only their spectral features, are altered as consequences of chemical modifications on the periphery of the molecules. Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains. These modifications influence formation of coordination bonds by which the central Mg atom, the Lewis acid, of Chl molecules interacts with amino acid sidechains, as the Lewis base, in proteins. Chl a is a versatile Lewis acid and interacts principally with imidazole groups but also with sidechain amides and water. The 7-formyl group on Chl b withdraws electron density toward the periphery of the molecule and consequently the positive Mg is less shielded by the molecular electron cloud than in Chl a. Chl b thus tends to form electrostatic bonds with Lewis bases with a fixed dipole, such as water and, in particular, peptide backbone carbonyl groups. The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group. These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

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Related in: MedlinePlus

Model of the association of Chls with Lhcb1. The arrangement of the protein in thylakoid membranes is illustrated according to Green and Durnford (1996). The symbols designating the chlorin rings of the six Chl b molecules are filled (green). A water ligand for four Chl molecules is indicated by a central blue dot. The Chl a molecules are numbered 1–8 and the Chl b molecules 9–14, as designated by Standfuss et al. (2005)
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Fig3: Model of the association of Chls with Lhcb1. The arrangement of the protein in thylakoid membranes is illustrated according to Green and Durnford (1996). The symbols designating the chlorin rings of the six Chl b molecules are filled (green). A water ligand for four Chl molecules is indicated by a central blue dot. The Chl a molecules are numbered 1–8 and the Chl b molecules 9–14, as designated by Standfuss et al. (2005)

Mentions: His120, at the lumenal end of helix-2 in LHCII (see Fig. 3), is not a ligand to Chl a, possibly because of its exposure to the thylakoid lumen where competition with water is greater than within the membrane. Also, the imidazole group may be protonated at the pH of the lumen during active photosynthesis, estimated to be near pH 5 (Kramer et al. 1999; Sacksteder et al. 2000). His212, a ligand to Chl a, is also near the lumenal surface of the membrane but is likely shielded from the aqueous lumen by helix-4 of the LHC protein.Fig. 3


Chlorophylls, ligands and assembly of light-harvesting complexes in chloroplasts.

Hoober JK, Eggink LL, Chen M - Photosyn. Res. (2007)

Model of the association of Chls with Lhcb1. The arrangement of the protein in thylakoid membranes is illustrated according to Green and Durnford (1996). The symbols designating the chlorin rings of the six Chl b molecules are filled (green). A water ligand for four Chl molecules is indicated by a central blue dot. The Chl a molecules are numbered 1–8 and the Chl b molecules 9–14, as designated by Standfuss et al. (2005)
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2117338&req=5

Fig3: Model of the association of Chls with Lhcb1. The arrangement of the protein in thylakoid membranes is illustrated according to Green and Durnford (1996). The symbols designating the chlorin rings of the six Chl b molecules are filled (green). A water ligand for four Chl molecules is indicated by a central blue dot. The Chl a molecules are numbered 1–8 and the Chl b molecules 9–14, as designated by Standfuss et al. (2005)
Mentions: His120, at the lumenal end of helix-2 in LHCII (see Fig. 3), is not a ligand to Chl a, possibly because of its exposure to the thylakoid lumen where competition with water is greater than within the membrane. Also, the imidazole group may be protonated at the pH of the lumen during active photosynthesis, estimated to be near pH 5 (Kramer et al. 1999; Sacksteder et al. 2000). His212, a ligand to Chl a, is also near the lumenal surface of the membrane but is likely shielded from the aqueous lumen by helix-4 of the LHC protein.Fig. 3

Bottom Line: Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains.The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group.These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

View Article: PubMed Central - PubMed

Affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA. khoober@asu.edu

ABSTRACT
Chlorophyll (Chl) b serves an essential function in accumulation of light-harvesting complexes (LHCs) in plants. In this article, this role of Chl b is explored by considering the properties of Chls and the ligands with which they interact in the complexes. The overall properties of the Chls, not only their spectral features, are altered as consequences of chemical modifications on the periphery of the molecules. Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains. These modifications influence formation of coordination bonds by which the central Mg atom, the Lewis acid, of Chl molecules interacts with amino acid sidechains, as the Lewis base, in proteins. Chl a is a versatile Lewis acid and interacts principally with imidazole groups but also with sidechain amides and water. The 7-formyl group on Chl b withdraws electron density toward the periphery of the molecule and consequently the positive Mg is less shielded by the molecular electron cloud than in Chl a. Chl b thus tends to form electrostatic bonds with Lewis bases with a fixed dipole, such as water and, in particular, peptide backbone carbonyl groups. The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group. These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

Show MeSH
Related in: MedlinePlus