Membrane trans-bilayer structure

Membranes are structurally and functionally asymmetric, as exemplified by the restriction of sugar residues to the external surface of mam malian plasma membranes. Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face of a membrane to the other (trans verse diffusion, or flip-flop) is usually very slow. I roteins do not rotate across bilayers hence, membrane asymmetry can be preserved. The degree of fluidity of a membrane partly depends on the chain length of its lipids and the extent to which their constituent fatty acids are unsaturated. In animals, cholesterol content also regulates membrane fluidity. 

Analysis of melittin-induced conductance fluctuations suggests perturbations of the lipid bilayer structure, and formation of structural channels. Addition of pronase to the trans-side in the presence of a trans-negatiwe membrane potential abolished the conductance as it did when added to the cis side. This implies that the conducting state of melittin requires a trans-membrane configuration. In a non-conducting absorbed state a hydrophobic loop, with the region from threonine-9 to proline-13 as the likely site of the turn, penetrates the lipid but does not extend across the membrane. In the conducting state the turn becomes extended as the peptide assumes a rra w-bilayer position. 

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

The answer is B. Saturated fatty acids and trans fatty acids are structurally similar their hydrocarbon tails are relatively linear. This allows them to pack tightly together in semi-crystalline arrays such as the membrane bilayer. Such arrays have similar biochemical properties in terms of melting temperature (fluidity). Although some of the other properties listed are also shared by saturated and trans fats, they are not thought to account for the tendency of these fats to contribute to atherosclerosis. 

The bradykinin receptor is a member of a family of receptors for which an intracellular interaction with a G-protein is a critical part of the signal transduction pathway following agonist binding. Structurally, these G-protein-coupled receptors extend from beyond the extracellular boundary of the cell membrane into the cytoplasm. The tertiary structure is such that the protein crosses the bilayer of the cell membrane seven times, thus forming three intracellular loops, three extracellular loops, and giving rise to cytoplasmic C-terminal and extra-cellular N-terminal strands. It is generally presumed that the transmembrane domains of these receptors exist as a bundle of helical strands. This assumption is derived primarily from the known structure of the trans-membrane portions of a structurally related protein, bacteriorhodopsin [40]. 

Figure 4 The modified stalk mechanism of membrane fusion and inverted phase formation, (a) planar lamellar (La) phase bilayers (b) the stalk intermediate the stalk is cylindrically-symmetrical about the dashed vertical axis (c) the TMC (trans monolayer contact) or hemifusion structure the TMC can rupture to form a fusion pore, referred to as interlamellar attachment, ILA (d) (e) If ILAs accumulate in large numbers, they can rearrange to form Qn phases, (f) For systems close to the La/H phase boundary, TMCs can also aggregate to form H precursors and assemble Into H domains. The balance between Qn and H phase formation Is dictated by the value of the Gaussian curvature elastic modulus of the bIlayer (reproduced from (25) with permission of the Biophysical Society) The stalk in (b) is structural unit of the rhombohedral phase (b ) electron density distribution for the stalk fragment of the rhombohedral phase, along with a cartoon of a stalk with two lipid monolayers merged to form a hourglass structure (reproduced from (26) with permission of the Biophysical Society).

A general feature of most of the known membrane protein structures is the occurrence of hydrophobic segments forming a-helices, which are buried in the bilayer. To span the lipid bilayer an a-helix needs about 20 residues. From the amino acid sequence it is therefore possible to predict trans-membrane a-helices. 

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