Light-harvesting bilayer membrane

Figure 23-30 Views of light-harvesting protein LHCII of green plants. (A) Side view indicating the approximate position in the lipid bilayer of the thylakoid membrane. Helices are labeled A-D. (B) Stereoscopic top view from the stromal side of the membrane. The structure, at 0.34 nm resolution, was determined by electron crystallography on highly ordered two-dimensional crystals. MolScript drawings from Kuhlbrandt et al.3W Courtesy of Werner Kiihlbrandt.

On the basis ofthe primary-structure information discussed above, and the known, 1 1 stoichiometry of the a- and p-polypeptides, Zuber developed a model for the light-harvesting BChl-protein complexes, as shown in Fig. 4 (A). In this model the two polypeptides span the chromatophore membrane with their hydrophobic segments consisting of 20 amino acids located in the hydrocarbon-tail region of the lipid-bilayer membrane. The polar N- and C-termini are located on the cytoplasmic and periplasmic sides, respectively. The hydrophobic amino acids in this model are present as an a-helix, in accord with the finding of a high helical content by circular dichroism. ... 

Fig. 4. (A) The side view of the LHC-II monomer in the membrane (lipid bilayer). The chlorophyll molecules are oriented nearly perpendicular to the membrane plane (the phytyl chains are omitted for clarity). Two lutein molecules form an internal X-shaped brace. (B) A sketch of the amino-acid sequence of the LHC-II polypeptide and a listing of the known Chl-residue ligation. White letters inside black circles indicate amino-acid ligands to the chlorophylls [also see legend on the right side of (B)]. Note that the model in (B) is rotated 90° with respect to that in (A) about an axis normal to the membrane. Figure source (A) Kuhibrandt, Wang and Fujiyoshi (1994) Atomic model of plant light-harvesting complex by electron crystallography. Nature 367 618 and 620.

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

Topographs of membrane proteins at subnanometer resolution were first acquired on highly ordered 2D reconstituted systems, that is, OmpF, bacteriorhodopsin (BR), water channels, potassium channels, halorhodopsin, outer membrane (OM) porins, adenosine triphosphate synthase (ATPase) " and light-harvesting (LH) complexes. " Target proteins were initially isolated from biological membranes and then reconstituted into lipid bilayers to form regular arrays. However, as described before, there is a certain... 

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