Membrane lipids transverse diffusion

NBD probes are often used to assay flip-flop. Flip-flop refers to the reversible transversal diffusion of lipids from one leaflet to the other leaflet of a lipid bilayer membrane. In intact membranes, this transversal diffusion is very slow (fi/2 on the order of hours to days). However, it can be accelerated by biological or synthetic flippases, which are a special class of membrane transporters related to ion carriers. Alternatively, micellar pores are synthetic ion channels and pores with flippase activity and can thus be identified with flip-flop assay (Figure 2 interfacial location of the transporter, as second distinctive characteristic of micellar pores, can be identified by fluorescence depth quenching experiments with DOXYL probes). 

The membranes of cells are generally asymmetric, in that the lipids and proteins that inhabit the membrane are not evenly distributed across both the leaflets of the bilayer. To maintain this necessary membrane asymmetry, transverse diffusion of phospholipids (flip-flop. Figure 6a) in cellular membranes is accelerated by translocase enzymes like the flippases. These enzymes overcome the energy barrier for the passage of polar headgroups through the apolar center of the membrane and maintain asymmetry by the consumption of adenosine triphosphate (ATP). ... 

To move through the membrane (change sides or transverse diffusion), a molecule must be able to pass through the hydrophobic portion of the lipid bilayer. For ions and proteins, this means that they must lose their interactions with water (desolvation). Because this is extremely difficult, ions and proteins do not move through membranes by themselves. Small molecules such as C02, NH3 (but not NH ). and water can diffuse through membranes however, most other small molecules pass through the lipid bilayer very slowly, if at all. This permeability barrier means that cells must develop mechanisms to move molecules from one side of the membrane to the other. 

Figure 12.31. Lipid Movement in Membranes. Lateral diffusion of lipids is much more rapid than transverse diffusion (flip-flop).

Membranes are structurally and functionally asymmetric, as exemplified by the restriction of sugar residues to the external surface of mammalian 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 (transverse diffusion, or flip-flop) is usually very slow. Proteins do not rotate across bilayers hence, membrane asymmetry can be preserved. The degree of fluidity of a... 

Biological membranes are fluid in nature. For example, when individual cells with different surface protein markers are fused, the initially separated proteins rapidly mix on the newly formed hybrid (Figure 10.11), This phenomenon is known as lateral diffusion, because molecules move laterally within the plane of the membrane. By contrast, in the much less frequent transverse diffusion, a molecule moves from one side of the lipid bilayer to the other. 

Statements (c) and (d) are correct Transverse diffusion is only rarely observed [statement (b)], and the term mosaic reier% to the pattern of distribution of proteins in the lipid bUayer [statement (e)]. Peripheral proteins are also considered part of the membrane [statement (a) ]. 

Describe the evidence for the lateral diffusion of membrane lipids and proteins. Contrast the rates for lateral diffusion with those for transverse diffusion. 

Early examples of synthetic flippases were lipidated polymers, which used bilayer distortion to bring about lipid flip-flop. In contrast to these mechanical flippases, synthetic species that apply the principles of molecular recognition to create phospholipid complexes capable of transverse diffusion have been shown to enhance lipid flip-flop in model membrane systems. Boon and Smith generated asymmetric bilayers by adding synthetic NBD phospholipids to the outer leaflet of POPC vesicles and then determined the rate of flip-flop to the inner leaflet... 

Lipids also undergo rapid lateral motion in membranes. A typical phospholipid can diffuse laterally in a membrane at a linear rate of several microns per second. At that rate, a phospholipid could travel from one end of a bacterial ceil to the other in less than a second or traverse a typical animal ceil in a few minutes. On the other hand, transverse movement of lipids (or proteins) from one face of the bilayer to the other is much slower (and much less likely). For example, it can take as long as several days for half the phospholipids in a bilayer vesicle to flip from one side of the bilayer to the other. 

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

Figure 16.13 Suggested mechanism for endosome formation of short DNA and CUV membranes, (a) DNA adsorption to the planar CUV membrane (dashed circles represents transverse sections of the DNA molecules), (b) Lateral diffusion and increase of the Sph+ concentration, decoupling of both monolayers, and external monolayer rolling up on the DNA molecules, (c) Topological transformation of the external lipid monolayers and encapsulation of DNA within a cylindrical inverted micellar structure. Membrane asymmetry is created (S xt < SJ. (d) Membrane invaginates at a scale of a few micrometers, (e) Formation of the endosome. Reprinted with kind permission of Springer Sciences-Business Media...

A cell membrane is illustrated in Fig. 6.1. It is built from a bilayer of lipids, usually phospholipids, associated with which are membrane proteins and polysaccharides. The antiparallel orientation of lipid layers in the bilayer is maintained due to the extremely slow flip-flop rate, i.e. the rate of diffusion transverse to the bilayer. The lipid bilayer is the structural foundation and the proteins and polysaccharides provide chemical functionality. The protein to lipid ratio shows a large variation depending on the cell, but proteins make up at least half of most cell membranes. A prominent exception is mammalian nerve cells which contain only 18 % protein (here also the lipids are sphingomyelins rather than phospholipids). Here, the primary requirement is that the cell membrane should be effective as an electrical... 

Diffusivities of binary, ternary and multi-component liquid crystalline mixtures, e.g. of soap (potassium laurate (PL), water [25, 58], and lipid (dipalmitoylphosphatidylcho-line (DPPC) [25, 59] systems in lamellar, hexagonal, cubic, nematic and micellar mesophases [25,60,61] have been studied extensively by pulsed-field-gradient NMR [25] and optical techniques [62], partly because of their intimate relation to the structure and dynamical performance of biological membranes [18]. The main distinction from thermotropic phases is that for layered structures a noticeable diffusion occurs only within the layers (i.e. lateral, frequently written as Dl, but in our notation DjJ, whereas it is negligibly small and difficult to detect across the layers [60-62] (transverse migration, for bilayers denoted by flip-flop ) so the mobility is essentially two dimensional, and the anisotropy ratio is so great that it is seldom specified explicit-... 

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