Bilayer spanning lipids

Langmuir films have been generated not only from phospholipids but also from tetraether lipids (Fig. 14b). Tetraether glycerophospho- and glycoUpids are typical for ar-chaea, where they may constitute the only polar lipids of the cell envelope [154,155]. Tetraether lipids are membrane-spanning lipids, a single monolayer has almost the same thickness as a phospholipid bilayer. 

Hennesthal, C., Steinem, C., Pore-spanning lipid bilayers visualized by scanning force microscopy, J. Am. Chem. Soc. 2000, 122, 8085-8086. 

The dynamics of molecules in a lipid bilayer is of fundamental importance in the role that the lipid bilayer plays in a biological membrane. Dynamics in a lipid bilayer spans a frequency scale from s (for the vibrational dynamics of single bonds... 

Figure 12.15. Permeability Coefficients (P) of Ions and Molecules in a Lipid Bilayer. The ability of molecules to cross a lipid bilayer spans a wide range of values.

At first it might seem contradictory that the addition of another macro-molecular component, namely, the lipid bilayer, could simplify understanding of the protein mechanism. Indeed, many membrane proteins have substantial mass that protrudes into the aqueous space and that is likely to be as complex in operation as soluble proteins. However, many integral membrane proteins share a common structural theme, namely, a core of almost parallel, bilayer-spanning helices or sheets. The structural order of these helices is a direct consequence of the amphiphilic environment introduced by the lipid bilayer. The discussion that follows will focus on mechanistic forces as they apply to this common core, and not to large aqueous protrusions. By contrast, aque-ously soluble proteins are structurally more diverse and, in the absence of a common structural theme, cannot be approached in the same manner. 

Our present view of the cell membrane structure is that it is analogous to a two-dimensional oriented solution of globular lipoproteins dispersed in a discontinuous fluid bilayer of lipid solvent [2]. Phospholipids constitute the bulk of the lipids. Both components of the membrane (proteins and lipid) are free to some degree to have lateral mobility in the plane of the membrane and in some cases are asymmetrically distributed across the two halves of the bilayer [2]. Some proteins are peripherally attached to either of the two faces while other polypeptide chains may span the whole thickness of the membrane [2]. These latter proteins probably correspond to... 

The predictable helical nature of many peptides leads to the possibility of using an a-helix as a scaffold to create chemical devices. One such device builds upon the molecular recognition properties of crown ethers discussed here. When every third amino acid in a 21 amino acid long peptide has an appended benzo-21-crown-7 entity, the crown ethers stack to form a column. This structure aligns within lipid bilayers spanning one side to another. The affinity of the crown ethers for K leads to conduction of this cation from one side of the membrane to the other via migration through the channel created by the stacked crowns. 

Hofer, L Steinem, C., A membrane fusion assay based on pore-spanning lipid bilayers. Soft Matter 7, 1644-1647. 

Figure 8 Configurations of lipid and water molecules spanning a 100 ps interval during an MD simulation of a DPPC bilayer. The two left-hand panels show 10 configurations of two different lipids and three of their associated water molecules (one N-bound, one P-bound, and one CO-bound). The right-hand panel shows 20 configurations of a bulk water molecule m the mterlamellar space of a bilayer stack. (From Ref. 55.)...

In contrast, the transmembrane helices observed in the reaction center are embedded in a hydrophobic surrounding and are built up from continuous regions of predominantly hydrophobic amino acids. To span the lipid bilayer, a minimum of about 20 amino acids are required. In the photosynthetic reaction center these a helices each comprise about 25 to 30 residues, some of which extend outside the hydrophobic part of the membrane. From the amino acid sequences of the polypeptide chains, the regions that comprise the transmembrane helices can be predicted with reasonable confidence. 

A. Side view of channel spanning the lipid layer of a planar lipid bilayer, The structure is comprised of two monomers, each in a left-handed, single stranded p -helical conformation, and joined together at the head or formyl end by means of six, intermolecular hydrogen bonds. The two formyl protons are seen at the center of the structure in this view. Replacement of these protons by methyls destabilizes the conducting dimer as shown with N-acetyl desformyl Gramicidin A (Fig. 3D). 

A family of related, membrane-spanning glycoproteins that catalyze the transport of glucose across a lipid bilayer of the plasma membrane along a concentration gradient. 

The nAChR is comprised of five subunits, each of which spans the lipid bilayer to create a water-filled pore or channel (Fig. la). Each subunit consists of four transmembrane segments, the second transmembrane segment (M2) lines the ion channel (Fig. lb). The extracellular N-terminal domain of every subunit... 

Figure 41-7. The fluid mosaic model of membrane structure. The membrane consists of a bimolecu-lar lipid layer with proteins inserted in it or bound to either surface. Integral membrane proteins are firmly embedded in the lipid layers. Some of these proteins completely span the bilayer and are called transmembrane proteins, while others are embedded in either the outer or inner leaflet of the lipid bilayer. Loosely bound to the outer or inner surface of the membrane are the peripheral proteins. Many of the proteins and lipids have externally exposed oligosaccharide chains. (Reproduced, with permission, from Junqueira LC, Carneiro J Basic Histology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

Fig. 3. (A) Model of the proposed pore forming part of K channel subunits. Segments S5 and S6 are possibly membrane-spanning helices. The helices are connected by a hydrophobic segment H5 which may be tucked into the lipid bilayer [48]. H5 is flanked by two proline residues P. Adjacent to these proline residues are amino acid side chains ( ) important for external TEA binding [45,46]. Approximately halfway between these two proline residues are amino acid side chains ( ) affecting internal TEA binding [46,47] and K channel selectivity [48]. (B) Mutations are indicated which affect in Shaker channels external TEA (TEAe) or internal TEA (TEA,) binding. Concentrations of TEA for half block of the wild-type and mutant K channels are given at the right-hand side of the corresponding sequence. Data have been compiled from [45-47].

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