P-Polypeptides

The structure of the LH2 complex of R. acidophila is both simple and elegant (Figure 12.17). It is a ring of nine identical units, each containing an a and a P polypeptide of 53 and 41 residues, respectively, which both span the membrane once as a helices (Figure 12.18). The two polypeptides bind a total of three chlorophyll molecules and two carotenoids. The nine heterodimeric units form a hollow cylinder with the a chains forming the inner wall and the P chains the outer wall. The hole in the middle of the cylinder is empty, except for lipid molecules from the membrane. 

The light-harvesting complex LHl is directly associated with the reaction center in purple bacteria and is therefore referred to as the core or inner antenna, whereas LH2 is known as the peripheral antenna. Both are huilt up from hydrophohic a and p polypeptides of similar size and with low hut significant sequence similarity. The two histidines that hind to chlorophyll with absorption maxima at 850 nm in the periplasmic ring of LH2 are also present in LHl, but the sequence involved in binding the third chlorophyll in LH2 is quite different in LHl. Not surprisingly, the chlorophyll molecules of the periplasmic ring are present in LHl but the chlorophyll molecules with the 800 nm absorption maximum are absent. 

Fig. 1. Structure of (a) myoglobin and (b) hemoglobin, showing the a and p polypeptide chains.

Among the human prion diseases, Pi-p " polypeptide fragments can vary in number (typically 1 to 3), molecular weight, and the ratio of the three glycoforms. Based on these criteria, Prps<" is classified into three operational groups called type 1, type 2 (a and b), and GSS-type (Table 29.2, Figure... 

Tentative assignment for P (polypeptide) and C (carbohydrate) moieties of glycoproteins. 

The amino-acid sequences of the a- and p-polypeptides ofthe B870 and B800-850 complexes from a number of photosynthetic bacteria is shown in Fig. 3. Each ofthe a- and P-polypeptides contains about... 

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. ... 

In the model for B890, B850 or B1015, each ofthe a-helices binds aBChl-amolecule with the conserved histidine residue [shown as bold H in Fig. 4 (A)] near the C-terminus. In B800-850, each histidine near the C-terminus binds a BChl-a molecule that absorbs at 850 nm while a second conserved histidine near the N-terminus of the p-polypeptide presents one more binding site for yet another BChl a, the one absorbing at 800 nm. 

As shown in Fig. 5 (A), the 9 helical a-polypeptides are arranged in the form of an inner cylinder with a 36-A diameter, while the 9 P-polypeptides form a concentric outer cylinder with a 68-A diameter. Each a-polypeptides is parallel to the 9-fold axis to within an experimental error of 2°, whereas each p-polypeptides is inclined by 15° relative to this axis. The transmembrane helices formed by residues 11 to 36 [see Fig. 5 (C)] of both the a- and P-polypeptides (PP) are anchored to amphipathic helical tails at the N- and C-termini. The following hydrophobic residues are marked with solid dots in Fig. 5 (C) ... 

The 27 BChl-a molecules are distributed into two groups. The 18 BChl-a molecules of the first group, located near the periplasmic end of the complex, are ligated through their central Mg atoms to the conserved residues His-31 and His-30 of the a- and P-polypeptides, respectively. These a- and P-residues... 

The X-ray crystal-structure determination revealed a lack of any direct, helix-helix interaction between the a- and P-polypeptides within the hydrophobic core, as had been expected on the basis of earlier models [see Fig. 4 (B) and (C) above]. The association of the fMet-1 residue at the N-terminal of each a-polypeptide to a B800 BChl-a molecule prevents direct interaction between radially paired a-and P-helices. Interactions between the membrane-core helices are mediated only via pigment molecules or buried water molecules. The only polypeptide interaction occurs at the N- and C-terminal tails. In the C-terminal tail the large aromatic residues (a-Trp 40, a-Tyr 44, a-Trp 45 and P-Trp 39) contribute to binding between polypeptides via hydrogen bonds and hydrophobic interactions [see Fig. 5 (C)]. 

Sixteen BChl-a molecules, with their macrocycles oriented perpendicular to the membrane plane, form a ring near the periplasmic surface sandwiched between the a- and p-polypeptide cylinders. These 16 BChl-a molecules, which constitute B850, are situated in a non-polar environment, with the Mg... 

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