Paracrystalline State Of Bilayer Lipids

lipid moiety. Other interactions, such as ionic attractions between positively charged Lys residues in the protein and negatively charged lipid head groups, probably contribute to the stability of the attachment. The association of these lipid-linked proteins with the membrane is certainly weaker than that for integral membrane proteins and is, in at least some cases, reversible. But treatment with alkaline carbonate does not release GPI-linked proteins, which are therefore, by the working definition, integral proteins.

Beyond merely anchoring a protein to the membrane, the attached lipid may have a specific role. In the plasma membrane, proteins with GPI anchors are exclusively on the outer face and are confined within clusters, as we shall see below, whereas other types of lipid-linked proteins (with farnesyl or geranylgeranyl groups attached; Fig. 11-14) are exclusively on the inner face. Attachment of a specific lipid to a newly synthesized membrane protein therefore has a targeting function, directing the protein to its correct membrane location.

SUMMARY 11.1 The Composition and Architecture of Membranes

Biological membranes define cellular boundaries, divide cells into discrete compartments, organize complex reaction sequences, and act in signal reception and energy transformations.

Membranes are composed of lipids and proteins in varying combinations particular to each species, cell type, and organelle. The fluid mosaic model describes features common to all biological membranes. The lipid bilayer is the basic structural unit. Fatty acyl chains of phospholipids and the steroid nucleus of sterols are oriented toward the interior of the bilayer; their hydrophobic interactions stabilize the bilayer but give it flexibility.

Peripheral proteins are loosely associated with the membrane through electrostatic interactions and hydrogen bonds or by covalently attached lipid anchors. Integral proteins associate firmly with membranes by hydrophobic interactions between the lipid bilayer and their nonpolar amino acid side chains, which are oriented toward the outside of the protein molecule.

Some membrane proteins span the lipid bilayer several times, with hydrophobic sequences of about 20 amino acid residues forming transmembrane a helices. Detection of such hydrophobic sequences in proteins can be used to predict their secondary structure and transmembrane disposition. Multistranded 3 barrels are also common in integral membrane proteins. Tyr and Trp residues of transmembrane proteins are commonly found at the lipid-water interface.

The lipids and proteins of membranes are inserted into the bilayer with specific sidedness; thus membranes are structurally and functionally asymmetric. Many membrane proteins contain covalently attached oligosaccharides. Plasma membrane glycoproteins are always oriented with the carbohydrate-bearing domain on the extracellular surface.

11.2 Membrane Dynamics

One remarkable feature of all biological membranes is their flexibility—their ability to change shape without losing their integrity and becoming leaky. The basis for this property is the noncovalent interactions among lipids in the bilayer and the motions allowed to individual lipids because they are not covalently anchored to one another. We turn now to the dynamics of membranes:

the motions that occur and the transient structures allowed by these motions.

Acyl Groups in the Bilayer Interior Are Ordered to Varying Degrees

Although the lipid bilayer structure is quite stable, its individual phospholipid and sterol molecules have some freedom of motion (Fig. 11-15). The structure and flexibility of the lipid bilayer depend on temperature and on the kinds of lipids present. At relatively low temperatures, the lipids in a bilayer form a semisolid gel phase, in which all types of motion of individual lipid molecules are strongly constrained; the bilayer is paracrystalline (Fig. 11-15a). At relatively high temperatures, individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon-carbon bonds of the long acyl side chains. In this liquid-disordered state, or fluid state (Fig. 11-15b), the interior of the bilayer is more fluid than solid and the bilayer is like a sea of constantly moving lipid. At intermediate temperatures, the lipids exist in a liquid-ordered state; there is less thermal motion in the acyl chains of the lipid bi-layer, but lateral movement in the plane of the bilayer still takes place. These differences in bilayer state are easily observed in liposomes composed of a single lipid,

Phase Transition Temperature Lipid

FIGURE 11-15 Two states of bilayer lipids. (a) In the paracrystalline state, or gel phase, polar head groups are uniformly arrayed at the surface, and the acyl chains are nearly motionless and packed with regular geometry; (b) in the liquid disordered state, or fluid state, acyl chains undergo much thermal motion and have no regular organization. Intermediate between these extremes is the liquid-ordered state, in which individual phospholipid molecules can diffuse laterally but the acyl groups remain extended and more or less ordered.

FIGURE 11-15 Two states of bilayer lipids. (a) In the paracrystalline state, or gel phase, polar head groups are uniformly arrayed at the surface, and the acyl chains are nearly motionless and packed with regular geometry; (b) in the liquid disordered state, or fluid state, acyl chains undergo much thermal motion and have no regular organization. Intermediate between these extremes is the liquid-ordered state, in which individual phospholipid molecules can diffuse laterally but the acyl groups remain extended and more or less ordered.

but biological membranes contain many lipids with a variety of fatty acyl chains and thus do not show sharp phase changes with temperature.

At temperatures in the physiological range (about 20 to 40 °C), long-chain saturated fatty acids (such as 16:0 and 18:0) pack well into a liquid-ordered array, but the kinks in unsaturated fatty acids (see Fig. 10-1) interfere with this packing, favoring the liquid-disordered state. Shorter-chain fatty acyl groups have the same effect. The sterol content of a membrane (which varies greatly with organism and organelle; Table 11-1) is another important determinant of lipid state. The rigid planar structure of the steroid nucleus, inserted between fatty acyl side chains, reduces the freedom of neighboring fatty acyl chains to move by rotation about their carbon-carbon bonds, forcing acyl chains into their fully extended conformation. The presence of sterols therefore reduces the fluidity in the core of the bilayer, thus favoring the liquid-ordered phase, and increases the thickness of the lipid leaflet (as described below).

Cells regulate their lipid composition to achieve a constant membrane fluidity under various growth conditions. For example, bacteria synthesize more un-saturated fatty acids and fewer saturated ones when cultured at low temperatures than when cultured at higher temperatures (Table 11-2). As a result of this adjustment in lipid composition, membranes of bacteria cultured at high or low temperatures have about the same degree of fluidity.

Transbilayer Movement of Lipids Requires Catalysis

At physiological temperature, transbilayer—or "flip-flop"—diffusion of a lipid molecule from one leaflet of the bilayer to the other (Fig. 11-16a) occurs very slowly if at all in most membranes. Transbilayer movement requires that a polar or charged head group leave its

TABLE 11-2 Fatty Acid Composition of E. coli Cells Cultured at Different Temperatures

Percentage of total fatty acids*

TABLE 11-2 Fatty Acid Composition of E. coli Cells Cultured at Different Temperatures

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