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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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Structures of the major Chls. Except for the oxidation of the 7-methyl group in Chl a to the formyl group in Chl b, Chls a and b are identical. Chl d contains a formyl group at position 3. Chls a, b, and d include the 20-carbon isoprene alcohol, phytol (Ph), esterified to the carboxyl group at position 173. This carboxyl group remains unesterified in Chl c, which also contains double bonds in the sidechain between positions 171 and 172 and in the macrocycle between carbons 17 and 18. These additional double bonds extend conjugation of the macrocyclic π system to the free carboxyl group. Chl c species differ at positions 7 and 8; c1: 7, –CH3, 8, –C2H5; c2: 7, –CH3, 8, –C2H3; c3: 7, –COOCH3, 8, –C2H3 (shown in figure)
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Fig1: Structures of the major Chls. Except for the oxidation of the 7-methyl group in Chl a to the formyl group in Chl b, Chls a and b are identical. Chl d contains a formyl group at position 3. Chls a, b, and d include the 20-carbon isoprene alcohol, phytol (Ph), esterified to the carboxyl group at position 173. This carboxyl group remains unesterified in Chl c, which also contains double bonds in the sidechain between positions 171 and 172 and in the macrocycle between carbons 17 and 18. These additional double bonds extend conjugation of the macrocyclic π system to the free carboxyl group. Chl c species differ at positions 7 and 8; c1: 7, –CH3, 8, –C2H5; c2: 7, –CH3, 8, –C2H3; c3: 7, –COOCH3, 8, –C2H3 (shown in figure)

Mentions: Conversion of 3,8-divinyl-Chl a to 3-monovinyl-Chl a by reduction of the 8-vinyl group to an ethyl group is the final step in Chl a biosynthesis and yields the predominant form of Chl a (Nagata et al. 2005). As a result, Chl a has electron-donating methyl and ethyl groups at positions 7 and 8, respectively (Fig. 1). Along with reduction of the C17–C18 double bond to a single bond, which converts the porphyrin precursor protochlorophyllide (Pchlide) to the chlorin ring system, these groups impose an electron density, from opposite sides of the molecule along the X axis, on the pyrrole nitrogens, which partially shields the positive charge of the central Mg atom. In addition, the 3-vinyl and 131-keto groups exert weak electron withdrawing effects on opposite ends of the Y axis.Fig. 1


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

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

Structures of the major Chls. Except for the oxidation of the 7-methyl group in Chl a to the formyl group in Chl b, Chls a and b are identical. Chl d contains a formyl group at position 3. Chls a, b, and d include the 20-carbon isoprene alcohol, phytol (Ph), esterified to the carboxyl group at position 173. This carboxyl group remains unesterified in Chl c, which also contains double bonds in the sidechain between positions 171 and 172 and in the macrocycle between carbons 17 and 18. These additional double bonds extend conjugation of the macrocyclic π system to the free carboxyl group. Chl c species differ at positions 7 and 8; c1: 7, –CH3, 8, –C2H5; c2: 7, –CH3, 8, –C2H3; c3: 7, –COOCH3, 8, –C2H3 (shown in figure)
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Related In: Results  -  Collection

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Fig1: Structures of the major Chls. Except for the oxidation of the 7-methyl group in Chl a to the formyl group in Chl b, Chls a and b are identical. Chl d contains a formyl group at position 3. Chls a, b, and d include the 20-carbon isoprene alcohol, phytol (Ph), esterified to the carboxyl group at position 173. This carboxyl group remains unesterified in Chl c, which also contains double bonds in the sidechain between positions 171 and 172 and in the macrocycle between carbons 17 and 18. These additional double bonds extend conjugation of the macrocyclic π system to the free carboxyl group. Chl c species differ at positions 7 and 8; c1: 7, –CH3, 8, –C2H5; c2: 7, –CH3, 8, –C2H3; c3: 7, –COOCH3, 8, –C2H3 (shown in figure)
Mentions: Conversion of 3,8-divinyl-Chl a to 3-monovinyl-Chl a by reduction of the 8-vinyl group to an ethyl group is the final step in Chl a biosynthesis and yields the predominant form of Chl a (Nagata et al. 2005). As a result, Chl a has electron-donating methyl and ethyl groups at positions 7 and 8, respectively (Fig. 1). Along with reduction of the C17–C18 double bond to a single bond, which converts the porphyrin precursor protochlorophyllide (Pchlide) to the chlorin ring system, these groups impose an electron density, from opposite sides of the molecule along the X axis, on the pyrrole nitrogens, which partially shields the positive charge of the central Mg atom. In addition, the 3-vinyl and 131-keto groups exert weak electron withdrawing effects on opposite ends of the Y axis.Fig. 1

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