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

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.

Fig. 1

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)

Fig. 2

(A) The structure of the imidazole group of histidine and (B) its electronic charge density, determined by X-ray diffraction at 103 K for the projection in (A) (adapted from Epstein et al. 1982). In (A), R = remainder of the histidine molecule

Fig. 3

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)

Fig. 4

Model of LHCII assembly in the chloroplast envelope and the proposed role of Chl b. Several proposed intermediates are shown in the sequence, left to right. After synthesis in the cytosol, a LHCP precursor is imported sufficiently into the chloroplast stroma for removal of the transit sequence from the N-terminus and for the first membrane-spanning region to engage the inner membrane. Chl a (dark green rectangles) binds to ligands in the motif provided by the ion-pair of the sidechains of glutamate and arginine and the imidazole group of histidine (dotted line, a). However, binding to these sites is not sufficient to retain the protein in the envelope. Without Chl b the protein slips back into the cytosol for transfer to vacuoles and subsequent degradation. Chl b (light green rectangles) forms a strong coordination bond with the peptide bond carbonyl of Tyr24, near the N-terminus, and provides an additional hold on the protein (solid line, a + b). Along with the Chl a molecules that bind to the motifs in membrane-spanning helix 1, Chl b binds to Try24 and the peptide carbonyl of Val119 at the lumenal end of helix-2. These Chls retain the protein in the membrane sufficiently long for the remainder of the protein, including the conserved motif in membrane-spanning helix-3, to enter the membrane, bind additional Chl and xanthophylls molecules, and complete assembly (LHC). Other proteins in the membrane and stroma apparently assist assembly of the complete complex (see text)