Arrangement in the membrane

Spectroscopic measurements show that the reaction center and LHl are tightly associated and therefore it is assumed that the ring of pigments in LHl surrounds the reaction center. Careful model building indicates that the hole in the middle of LHl is large enough to accommodate the whole reaction center molecule. We do not know exactly how the LH2 complexes are arranged in the membrane around the LHl-reaction center complex, but at least some of them should be in contact with the outer rim of LHl for efficient... 

Yang, J. and Weliky, D. P. (2003) Solid-state nuclear magnetic resonance evidence for parallel and antiparallel strand arrangements in the membrane-associated HIV-1 fusion peptide. Biochemistry, 42, 11879-11890. 

The nicotinic acetylcholine receptor is the most studied of this type and is considered to be typical. This receptor is found at the neuromuscular junction. It is a protein consisting of five subunits, two a and one each of P, y and 8, which are arranged in the membrane around the channel. (See Figure 3.4.)... 

There is also a wealth of structural information on how the actual conducting dimer is arranged in the membrane. It now seems almost certain that gramicidin forms a dimer in which the A -formyl groups are in the center of the membrane and... 

Muller DJ, Sass H-J, Muller SA, Biildt G, Engel A (1999) Surface stmctures of native bacteriorhodopsin depend on the molecular packing arrangement in the membrane. J Mol Biol 285 1903—1909... 

It has been known for a long time that membranes were composed basically of lipid and protein. The question was how were these constituents arranged in the membrane and how could their special properties be reconciled with their possible physiological functions. 

The second protein in the membrane of influenza vims, neuraminidase, does not belong to any of these three groups of barrel structures. Instead, it forms a propeller-like structure of 24 p strands, arranged in six similar motifs that form the six blades of the propeller. Each motif is a p sheet of 4 up-and-down-connected p strands. The enzyme active site is formed by loop regions on one side of the propeller. 

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

Membrane asymmetries in the transverse direction (from one side of the membrane to the other) can be anticipated when one considers that many properties of a membrane depend upon its two-sided nature. Properties that are a consequence of membrane sidedness include membrane transport, which is driven in one direction only, the effects of hormones at the outsides of cells, and the immunological reactions that occur between cells (necessarily involving only the outside surfaces of the cells). One would surmise that the proteins involved in these and other interactions must be arranged asymmetrically in the membrane. 

FIGURE 10.7 A model for the arrangement of the anion transport protein in the membrane, based on hydropathy analysis. 

Certain microbes synthesize small organic molecules, ionophores, that function as shuttles for the movement of ions across membranes. These ionophores contain hy-drophihc centers that bind specific ions and are surrounded by peripheral hydrophobic regions this arrangement allows the molecules to dissolve effectively in the membrane and diffuse transversely therein. Others, Hke the well-smdied polypeptide gramicidin, form channels. 

Fig. 3. Model for the two-dimensional arrangement of the human erythrocyte glucose transporter in the membrane. Amino acid residues are identified by their single letter code. Solid bars indicate the location of introns in the transporter gene. The regions coloured black are released from the membrane upon tryptic digestion. Shaded segments indicate the probable regions photolabelled by ATB-BMPA (helix 8) and by cytochalasin B (helix 11 and the loop connecting it to helix 10). The circles with heavy outlines indicate the region labelled by lAPS-forskolin (helix 10).

The information obtained from the experiments described above can be used to construct models for the three-dimensional arrangement of the membrane-spanning helices within the transport proteins. One such model, which takes the diameter of an a-helix as 1.1 nm and seems to fit the measured dimensions of lac permease quite nicely, is illustrated in Fig. 5, although it must be emphasized that this is only one of... 

Fig. 5. A speculative model for the arrangement of the helical regions of the sugar transporters in the membrane. The helices are numbered as shown in Fig. 4. The small circle labelled s represents a glucose molecule.

Liposomes, also known as lipid vesicles, are aqueous compartments enclosed by lipid bilayer membranes [56,57]. Figure 10.11 shows how lipid bilayers are arranged in the liposome and the lipid structures in large unilamellar vesicles and multilamellar vesicles. Lipids consist of two components ... 

The membrane establishes in and out. The membrane is asymmetric because the inner and outer leaflets can have a different lipid composition and contain different proteins (Fig. 3-3). Proteins can be associated with either side of the membrane, or they can pass through the membrane using membrane-spanning segments. The functional part of the protein can be on the cytosolic side, the external side, or even in the membrane itself. A common structure for spanning a membrane is an a-helix (but there are examples of sheets spanning a membrane). It takes about 20 amino acid residues arranged in a helix to span to a 30 A hydrophobic interior of the bilayer. 

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