Ch 12 Lipids

what properties enable phospholipids to form membranes?

Membrane formation is a consequence of the amphipathic nature of the molecules

gangliosides

M ore-complex glycolipids, such as gangliosides, contain a branched chain of as many as seven sugar residues. Glycolipids are oriented in a completely asymmetric fashion with the sugar residues always on the extracellular side of the membrane.

periplasm

The inner membrane acts as the permeability barrier, and the outer membrane and the cell wall provide additional protection. The outer membrane is quite permeable to small molecules, owing to the presence of porins. The region between the two membranes containing the cell wall is called the periplasm [proks]

cerebroside

The simplest glycolipid, called a cerebroside, contains a single sugar residue, either glucose or galactose.

A channel protein can be formed from

beta strands

Thus, short chain length and unsaturation enhance the

the fluidity of fatty acids and of their derivative

glycerol

3 carbon alcohol, component of phospholipid

the protein components of a membrane can be readily visualized by

SDS-polyacrylamide gel electrophoresis. As stated earlier (p. 73), the electrophoretic mobility of many proteins in SDS-containing gels depends on the mass rather than on the net charge of the protein

12.2 There Are Three Common Types of Membrane Lipids

The major types of membrane lipids are phospholipids, glycolipids, and cholesterol. Phosphoglycerides, a type of phospholipid, consist of a glycerol backbone, two fatty acid chains, and a phosphorylated alcohol. Phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine are major phosphoglycerides. Sphingomyelin, a different type of phospholipid, contains a sphingosine backbone instead of glycerol. Glycolipids are sugar-containing lipids derived from sphingosine. Cholesterol, which modulates membrane fluidity, is constructed from a steroid nucleus. A common feature of these membrane lipids is that they are amphipathic molecules, having one hydrophobic and one hydrophilic end

integrin

a transmembrane protein that links the extracellular matrix to the cytoskeleton

sphingosine

alcohol that can be component of phospholipid

Proteins can span the membrane with

alpha helices . T he first membrane protein that we consider is the archaeal protein bacteriorhodopsin , shown in Figure 12.17. This protein uses light energy to transport protons from inside to outside the cell, generating a proton gradient used to form ATP.

most common structual motif in membrane proteins

alpha-helices

Transmembrane helices can be accurately predicted from

amino acid sequences

Phospholipids

are abundant in all biological membranes. A phospholipid molecule is constructed from four components: one or more fatty acids, a platform to which the fatty acids are attached, a phosphate, and an alcohol attached to the phosphate (Figure 12.3). The fatty acid components provide a hydrophobic barrier, whereas the remainder of the molecule has hydrophilic properties that enable interaction with the aqueous environment. T he platform on which phospholipids are built may be glycerol, a threecarbon alcohol, or sphingosine, a more complex alcohol. Phospholipids derived from glycerol are called phosphoglycerides. A phosphoglyceride consists of a glycerol backbone to which are attached two fatty acid chains and a phosphorylated alcohol.

lateral diffusion

biological membranes are not rigid, static structures. On the contrary, lipids and many membrane proteins are constantly in lateral motion, a process called lateral diffusion

fatty acid carbon atoms are numbered starting at

carboxyl terminus COO- > alpha carbon> beta carbon, methyle group at opposite end w carbon

Fatty acids vary in

chain length and degree of saturation atty acids in biological systems usually contain an even number of c arbon atoms, typically between 14 and 24 (Table 12.1). The 16- and 18-carbon fatty acids are most common. The dominance of fatty acid chains containing an even number of carbon atoms reflects the manner in which fatty acids are biosynthesized (Chapter 26). The hydrocarbon chain is almost invariably unbranched in animal fatty acids. The alkyl chain may be saturated or it may contain one or more double bonds. The configuration of the double bonds in most unsaturated fatty acids is cis. The double bonds in polyunsaturated fatty acids are separated by at least one methylene group.

Unsaturated fatty acids have lower melting points than

do saturated fatty acids of the same length.

Membrane fluidity is controlled by

fatty acid composition and cholesterol content

Fatty acids

fatty acid names are based on their parent hydrocarbons are long hydrocarbon chains of various lengths and degrees of unsaturation that terminate with carboxylic acid groups. The systematic name for a fatty acid is derived from the name of its parent hydrocarbon by the substitution of oic for the final e. For example, the C 18 saturated fatty acid is called octadecanoic acid because the parent hydrocarbon is octadecane. A C 18 fatty acid with one double bond is called octadec enoic acid; with two double bonds, octadeca dienoic acid; and with three double bonds, octadeca trienoic acid. The notation 18:0 denotes a C 18 fatty acid with no double bonds, whereas 18:2 signifies that there are two double bonds

Eukaryotic cells contain compartments bounded by

internal membranes

Major phosphoglycerides

phosphatidyl- Serine, ethanolamine, choline, glycerol, inositol

The three major kinds of membrane lipids

phospholipids, glycolipids, and cholesterol

the permeability of small molecules is correlated with their

solubility in a nonpolar solvent relative to their solubility in water, This relation suggests that a small molecule might traverse a lipid bilayer membrane in the following way: first, it sheds its solvation shell of water; then, it is dissolved in the hydrocarbon core of the membrane; and, finally, it diffuses through this core to the other side of the membrane, where it becomes resolvated by water. An ion such as Na 1 traverses membranes very slowly because the replacement of its coordination shell of polar water molecules by nonpolar interactions with the membrane interior is highly unfavorable energetically.

Membrane proteins can be classified as being either peripheral or integral on the basis of

this difference in dissociability

These hydrophobic interactions have three significant biological consequences:

(1) lipid bilayers have an inherent tendency to be extensive; (2) lipid bilayers will tend to close on themselves so that there are no edges with exposed hydrocarbon chains, and so they form compartments; and (3) lipid bilayers are self-sealing because a hole in a bilayer is energetically unfavorable.

8 common features of biological membranes

. Membranes are sheetlike structures, only two molecules thick, that form closed boundaries between different compartments. The thickness of most membranes is between 60 Å (6 nm) and 100 Å (10 nm). 2 . Membranes consist mainly of lipids and proteins. The mass ratio of lipids to proteins ranges from 1:4 to 4:1. Membranes also contain carbohydrates that are linked to lipids and proteins. 3 . Membrane lipids are small molecules that have both hydrophilic and hydrophobic moieties. These lipids spontaneously form closed bimolecular sheets in aqueous media. These lipid bilayers are barriers to the flow of polar molecules. 4. Specific proteins mediate distinctive functions of membranes. Proteins serve as pumps, channels, receptors, energy transducers, and enzymes. Membrane proteins are embedded in lipid bilayers, which create suitable environments for their action. 5. Membranes are noncovalent assemblies . The constituent protein and lipid molecules are held together by many noncovalent interactions, which act cooperatively. 6 . Membranes are asymmetric. The two faces of biological membranes always differ from each other. 7. Membranes are fluid structures . Lipid molecules diffuse rapidly in the plane of the membrane, as do proteins, unless they are anchored by specific interactions. In contrast, lipid molecules and proteins do not readily rotate across the membrane. Membranes can be regarded as two-dimensional solutions of oriented proteins and lipids . 8. Most cell membranes are electrically polarized, such that the inside is negative [typically 26 0 millivolts (mV)]. Membrane potential plays a key role in transport, energy conversion, and excitability (Chapter 13).

FRAP

. The rapid lateral movement of membrane proteins has been visualized by means of fluorescence microscopy using the technique of fluorescence recovery after photobleaching (FRAP; Figure 12.28). First, a cellsurface component is specifically labeled with a fluorescent chromophore. A small region of the cell surface (~3 mm 2 ) is viewed through a fluorescence microscope. The fluorescent molecules in this region are then destroyed (bleached) by a very intense light pulse from a laser, as indicated by the pale spot in Figure 12.28B. The fluorescence of this region is subsequently monitored as a function of time by using a light level sufficiently low to prevent further bleaching. If the labeled component is mobile, bleached molecules leave and unbleached molecules enter the illuminated region, resulting in an increase in the fluorescence intensity. The rate of recovery of fluorescence depends on the lateral mobility of the fluorescence-labeled component, which can be expressed in terms of a diffusion coefficient, D. The average distance S traversed in time t depends on D according to the expression S= (4Dt)^1/2 The diffusion coefficient of lipids in a variety of membranes is about 1 u m 2 s 2 1 . Thus, a phospholipid molecule diffuses an average distance of 2 u m in 1 s. This rate means that a lipid molecule can travel from one end of a bacterium to the other in a second. The magnitude of the observed diffusion coefficient indicates that the viscosity of the membrane is about 100 times that of water, rather like that of olive oil

12.6 Eukaryotic Cells Contain Compartments Bounded by Internal Membranes

A n extensive array of internal membranes in eukaryotes creates compartments within a cell for distinct biochemical functions. For instance, a double membrane surrounds the nucleus (the location of most of the cell's genetic material) and the mitochondria (the location of most ATP synthesis). A single membrane defines the other internal compartments, such as the endoplasmic reticulum. Receptor-mediated endocytosis enables the formation of intracellular vesicles when ligands bind to their corresponding receptor proteins in the plasma membrane. The reverse process—the fusion of a vesicle to a membrane—is a key step in the release of signaling molecules outside the cell.

12.1 Fatty Acids Are Key Constituents of Lipids

Biological membranes are sheetlike structures, typically from 60 to 100 Å thick, that are composed of protein and lipid molecules held together by noncovalent interactions. Membranes are highly selective permeability barriers. They create closed compartments, which may be entire cells or organelles within a cell. Proteins in membranes regulate the molecular and ionic compositions of these compartments. Membranes also control the flow of information between cells. 12.1 Fatty Acids Are Key Constituents of Lipids Fatty acids are hydrocarbon chains of various lengths and degrees of unsaturation that terminate with a carboxylic acid group. The fatty acid chains in membranes usually contain between 14 and 24 carbon atoms; they may be saturated or unsaturated. Short chain length and unsaturation enhance the fluidity of fatty acids and their derivatives by lowering the melting temperature.

two important features emerge from our examination of these three examples of membrane-protein structure.

First, the parts of the protein that interact with the hydrophobic parts of the membrane are coated with nonpolar amino acid side chains, whereas those parts that interact with the aqueous environment are much more hydrophilic. Second, the structures positioned within the membrane are quite regular and, in particular, all backbone hydrogen-bond donors and acceptors participate in hydrogen bonds. Breaking a hydrogen bond within a membrane is quite unfavorable, because little or no water is present to compete for the polar groups.

Eukaryotic organelle membranes

For example, peroxisomes, organelles that play a major role in the oxidation of fatty acids for energy conversion, are defined by a single membrane. Mitochondria, the organelles in which ATP is synthesized, are surrounded by two membranes. The nucleus is also surrounded by a double membrane, the nuclear envelope, that consists of a set of closed membranes that come together at structures called nuclear pores ( Figure 12.35) . These pores regulate transport into and out of the nucleus. The nuclear envelope is linked to another membrane-defined structure, the endoplasmic reticulum, which plays a host of cellular roles, including drug detoxification and the modification of proteins for secretion. Thus, a eukaryotic cell contains interacting compartments, and transport into and out of these compartments is essential to many biochemical processes.

plasma membrane

In addition to an external cell membrane (called the plasma membrane ), eukaryotic cells also contain internal membranes that form the boundaries of organelles such as mitochondria, chloroplasts, peroxisomes, and lysosomes

Key regulator of membrane fluidity in animals

In animals, cholesterol is the key regulator of membrane fluidity. Cholesterol contains a bulky steroid nucleus with a hydroxyl group at one end and a flexible hydrocarbon tail at the other end. Cholesterol inserts into bilayers with its long axis perpendicular to the plane of the membrane. The hydroxyl group of cholesterol forms a hydrogen bond with a carbonyl oxygen atom of a phospholipid head group, whereas the hydrocarbon tail of cholesterol is located in the nonpolar core of the bilayer. The different shape of cholesterol compared with that of phospholipids disrupts the regular interactions between fatty acid chains

let us consider one example of receptor-mediated endocytosis

Iron is a critical element for the function and structure of many proteins, including hemoglobin and myoglobin (Chapter 7). However, free iron ions are highly toxic to cells, owing to their ability to catalyze the formation of free radicals. Hence, the transport of iron atoms from the digestive tract to the cells where they are most needed must be tightly controlled. In the bloodstream, iron is bound very tightly by the protein transferrin, which can bind two Fe 3 1 ions with a dissociation constant of 10 2 23 M at neutral pH. Cells requiring iron express the transferrin receptor in their plasma membranes (Section 32.4). Formation of a complex between the transferrin receptor and iron-bound transferrin initiates receptor-mediated endocytosis, internalizing these complexes within vesicles called endosomes ( Figure 12.38). As the endosomes mature, proton pumps within the vesicle membrane lower the lumenal pH to about 5.5. Under these conditions, the affinity of iron ions for transferrin is reduced; these ions are released and are free to pass through channels in the endosomal membranes into the cytoplasm. The iron-free transferrin complex is recycled to the plasma membrane, where transferrin is released back into the bloodstream and the transferrin receptor can participate i lthough budding and fusion appear deceptively simple, the structures of the intermediates in these processes and the detailed mechanisms remain ongoing areas of investigation. These processes must be highly specific in order to prevent incorrect membrane fusion events. SNARE ( soluble N- ethylmaleimide-sensitive-factor attachment protein receptor) proteins from both membranes help draw appropriate lipid bilayers together by forming tightly coiled four-helical bundles (Figure 12.39). Once these membranes are in close apposition, the fusion process can proceed. SNARE proteins, encoded by gene families in all eukaryotic cells, largely determine the compartment with which a vesicle will fuse. The specificity of membrane fusion ensures the orderly trafficking of membrane vesicles and their cargos through eukaryotic cellsn another uptake cycle. A

12.3 Phospholipids and Glycolipids Readily Form Bimolecular Sheets in Aqueous Media

Membrane lipids spontaneously form extensive bimolecular sheets in aqueous solutions. The driving force for membrane formation is the hydrophobic interactions among the fatty acid tails of membrane lipids. The hydrophilic head groups interact with the aqueous medium. Lipid bilayers are cooperative structures, held together by many weak bonds. These lipid bilayers are highly impermeable to ions and most polar molecules, yet they are quite fluid, which enables them to act as a solvent for membrane proteins.

membranes differ in their protein content

Myelin, a membrane that serves as an electrical insulator around certain nerve fibers, has a low content of protein (18%). Relatively pure lipids are well suited for insulation. In contrast, the plasma membranes, or exterior membranes, of most other cells are much more metabolically active. They contain many pumps, channels, receptors, and enzymes. The protein content of these plasma membranes is typically 50%. Energy-transduction membranes, such as the internal membranes of mitochondria and chloroplasts, have the highest content of protein, around 75%. membranes performing different functions contain different repertoires of proteins

phosphoglycerides

Phospholipids derived from glycerol are called phosphoglycerides. A phosphoglyceride consists of a glycerol backbone to which are attached two fatty acid chains and a phosphorylated alcohol. In phosphoglycerides, the hydroxyl groups at C-1 and C-2 of glycerol are esterified to the carboxyl groups of the two fatty acid chains. The C-3 hydroxyl group of the glycerol backbone is esterified to phosphoric acid. When no further additions are made, the resulting compound is phosphatidate (diacylglycerol 3-phosphate), the simplest phosphoglyceride. Only small amounts of phosphatidate are present in membranes. However, the molecule is a key intermediate in the biosynthesis of the other phosphoglycerides he major phosphoglycerides are derived from phosphatidate by the formation of an ester bond between the phosphate group of phosphatidate and the hydroxyl group of one of several alcohols. The common alcohol moieties of phosphoglycerides are the amino acid serine, ethanolamine, choline, glycerol, and inositol.

12.4 Proteins Carry Out Most Membrane Processes

Specific proteins mediate distinctive membrane functions such as transport, communication, and energy transduction. Many integral membrane proteins span the lipid bilayer, whereas others are only partly embedded in the membrane. Peripheral membrane proteins are bound to membrane surfaces by electrostatic and hydrogen-bond interactions. Membrane-spanning proteins have regular structures, including b strands, although the a helix is the most common membrane-spanning structure. Sequences of 20 consecutive nonpolar amino acids can be diagnostic of a membrane-spanning a helical region of a protein.

Sphingomyelin

Sphingomyelin is a phospholipid found in membranes that is not derived from glycerol. Instead, the backbone in sphingomyelin is sphingosine, an amino alcohol that contains a long, unsaturated hydrocarbon chain (Figure 12.6). In sphingomyelin, the amino group of the sphingosine backbone is linked to a fatty acid by an amide bond. In addition, the primary hydroxyl group of sphingosine is esterified to phosphorylcholine.

biological membranes

Th e boundaries of all cells are defined by biological membranes (Figure 12.1), dynamic structures in which proteins float in a sea of lipids. The lipid component prevents molecules generated inside the cell from leaking out and unwanted molecules from diffusing in, while the protein components act as transport systems that allow the cell to take up specific molecules and remove unwanted ones biological membranes serve several additional functions indispensable for life, such as energy storage and information transduction, that are dictated by the proteins associated with them.

membrane asymmetry can be preserved for long periods.

The free-energy barriers to flip-f lopping are even larger for protein molecules than for lipids because proteins have moreextensive polar regions. In fact, the flip-flop of a protein molecule has not been observed. Hence, membrane asymmetry can be preserved for long periods

micelle

The polar head groups form the outside surface of the micelle, which is surrounded by water, and the hydrocarbon tails are sequestered inside, interacting with one another

The localization of prostaglandin H 2 synthase-l in the membrane is crucial to its function.

The substrate for this enzyme, arachidonic acid, is a hydrophobic molecule generated by the hydrolysis of membrane lipids. Arachidonic acid reaches the active site of the enzyme from the membrane without entering an aqueous environment by traveling through a hydrophobic channel in the protein (Figure 12.23). Indeed, nearly all of us have experienced the importance of this channel: drugs such as aspirin and ibuprofen block the channel and prevent prostaglandin synthesis by inhibiting the cyclooxygenase activity of the synthase. In particular, aspirin acts through the transfer of its acetyl group to a serine residue (Ser 530) that lies along the path to the active site (Figure 12.24).

Tm, the melting temperature

The transition from the rigid to the fluid state takes place abruptly as the temperature is raised above T m , the melting temperature (Figure 12.30). This transition temperature depends on the length of the fatty acid chains and on their degree of unsaturation (Table 12.3). The presence of saturated fatty acid residues favors the rigid state because their straight hydrocarbon chains interact very favorably with one another. On the other hand, a cis double bond produces a bend in the hydrocarbon chain. This bend interferes with a highly ordered packing of fatty acid chains, and so T m is lowered (Figure 12.31). The length of the fatty acid chain also affects the transition temperature. Long hydrocarbon chains interact more strongly than do short ones. Specifically, each additional OCH 2 O group makes a favorable contribution of about 2 2 kJ mol 2 1 ( 2 0.5 kcal mol 2 1 ) to the free energy of interaction of two adjacent hydrocarbon chains. Bacteria regulate the fluidity of their membranes by varying the number of double bonds and the length of their fatty acid chains

window

We can take the amino acid sequence of a protein and estimate the free-energy change that takes place when a hypothetical a helix formed of residues 1 through 20 is transferred from the membrane interior to water. The same calculation can be made for residues 2 through 21, 3 through 22, and so forth, until we reach the end of the sequence. The span of 20 residues chosen for this calculation is called a window. The free-energy change for each window is plotted against the first amino acid at the window to create a *hydropathy plot *

Porin

a protein from the outer membranes of bacteria such as E. coli and Rhodobacter capsulatus, represents a class of membrane proteins with a completely different type of structure. Structures of this type are built from b strands and contain essentially no a helices (Figure 12.19). The arrangement of b strands is quite simple: each strand is hydrogen bonded to its neighbor in an antiparallel arrangement, forming a single b sheet. The b sheet curls up to form a hollow cylinder that, as its name suggests, forms a pore, or channel, in the membrane. The outside surface of porin is appropriately nonpolar, given that it interacts with the hydrocarbon core of the membrane. In contrast, the inside of the channel is quite hydrophilic and is filled with water. This arrangement of nonpolar and polar surfaces is accomplished by the alternation of hydrophobic and hydrophilic amino acids along each b strand (Figure 12.20).

Lipid vesicles, or liposomes

are aqueous compartments enclosed by a lipid bilayer (Figure 12.11). These structures can be used to study membrane permeability or to deliver chemicals to cells. Liposomes are formed by suspending a suitable lipid, such as phosphatidylcholine, in an aqueous medium, and then sonicating (i.e., agitating by high-frequency sound waves) to give a dispersion of closed vesicles that are quite uniform in size. Vesicles formed by this method are nearly spherical and have a diameter of about 500 Å (50 nm). Larger vesicles (of the order of 1 m m or 10 4 Å in diameter) can be prepared by slowly evaporating the organic solvent from a suspension of phospholipid in a mixed-solvent system.

peripheral membrane proteins

are bound to membranes primarily by electrostatic and hydrogen-bond interactions with the head groups of lipids. These polar interactions can be disrupted by adding salts or by changing the pH. Many peripheral membrane proteins are bound to the surfaces of integral proteins, on either the cytoplasmic or the extracellular side of the membrane. Others are anchored to the lipid bilayer by a covalently attached hydrophobic chain, such as a fatty acid

Therapeutic applications for liposomes

are currently under active investigation. For example, liposomes containing drugs or DNA can be injected into patients. These liposomes fuse with the plasma membrane of many kinds of cells, introducing into the cells the molecules that they contain. Drug delivery with liposomes often lessens its toxicity. Less of the drug is distributed to normal tissues because long-circulating liposomes concentrate in regions of increased blood circulation, such as solid tumors and sites of inflammation. Moreover, the selective fusion of lipid vesicles with particular kinds of cells is a promising means of controlling the delivery of drugs to target cells. Another well-defined synthetic membrane is a planar bilayer membrane. This structure can be formed across a 1-mm hole in a partition between two aqueous compartments by dipping a fine paintbrush into a membraneforming solution, such as phosphatidylcholine in decane, and stroking the tip of the brush across the hole. The lipid film across the hole thins spontaneously into a lipid bilayer. The electrical conduction properties of this macroscopic bilayer membrane are readily studied by inserting electrodes into each aqueous compartment (Figure 12.13). For example, the permeability of the membrane to ions is determined by measuring the current across the membrane as a function of the applied voltage.

Lipid rafts

are highly dynamic complexes formed between cholesterol and specific lipids In addition to its nonspecific effects on membrane fluidity, cholesterol can form specific complexes with lipids that contain the sphingosine backbone, including sphingomyelin and certain glycolipids, and with GPI-anchored proteins. These complexes concentrate within small (10-200 nm) and highly dynamic regions within membranes. The resulting structures are often referred to as lipid rafts . One result of these interactions is the moderation of membrane fluidity, making membranes less fluid but at the same time less subject to phase transitions . The presence of lipid rafts thus represents a modification of the original fluid mosaic model for biological membranes. Although their small size and dynamic nature have made them very difficult to study, it appears that lipid rafts may play a role in concentrating proteins that participate in signal transduction pathways and may also serve to regulate membrane curvature and budding

lipids

are water-insoluble biomolecules that are highly soluble in organic solvents such as chloroform. Lipids have a variety of biological roles: they serve as fuel molecules, highly concentrated energy stores, signal molecules and messengers in signal-transduction pathways, and components of membranes.

All biological membranes are

asymmetric membranes are structurally and functionally asymmetric. The outer and inner surfaces of all known biological membranes have different components and different enzymatic activities. A clear-cut example is the pump that regulates the concentration of Na 1 and K 1 ions in cells (Figure 12.33). This transport protein is located in the plasma membrane of nearly all cells in higher organisms. The Na 1- K 1 pump is oriented so that it pumps Na 1 out of the cell and K 1 into it. Furthermore, ATP must be on the inside of the cell to drive the pump. Ouabain, a specific inhibitor of the pump, is effective only if it is located outside. We shall consider the mechanism of this important and fascinating family of pumps in Chapter 13

12.5 Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane

embranes 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 membrane depends on the chain length of its lipids and on the extent to which their constituent fatty acids are unsaturated. In animals, cholesterol content also regulates membrane fluidity.

receptor-mediated endocytosis

embranes must be able to separate or join together so that cells and compartments may take up, transport, and release molecules. Many cells take up molecules through the process of receptor-mediated endocytosis . Here, a protein or larger complex initially binds to a receptor on the cell surface. After the receptor is bound, specialized proteins act to cause the membrane in this region to invaginate. One of these specialized proteins is clathrin, which polymerizes into a lattice network around the growing membrane bud, often referred to as a clathrin-coated pit (Figure 12.36). The invaginated membrane eventually breaks off and fuses to form a vesicle. Various hormones, transport proteins, and antibodies employ receptormediated endocytosis to gain entry into a cell. A less-advantageous consequence is that this pathway is available to viruses and toxins as a means of invading cells. The reverse process—the fusion of a vesicle to a m embrane— is a key step in the release of neurotransmitters from a neuron into the synaptic cleft

some proteins associate with membranes through covalently attatched hydrophobic groups

he membrane proteins considered thus far associate with the membrane through surfaces generated by hydrophobic amino acid side chains. However, even otherwise soluble proteins can associate with membranes if hydrophobic groups are attached to the proteins. Three such groups are shown in Figure 12.25: (1) a palmitoyl group attached to a specific cysteine residue by a thioester bond, (2) a farnesyl group attached to a cysteine residue at the carboxyl terminus, and (3) a glycolipid structure termed a glycosylphosphatidylinositol (GPI) anchor attached to the carboxyl terminus. These modifications are attached by enzyme systems that recognize specific signal sequences near the site of attachment.

glycolipids

he second major class of membrane lipids, glycolipids, are sugar-c ontaining lipids. Like sphingomyelin, the glycolipids in animal cells are derived from sphingosine. The amino group of the sphingosine backbone is acylated by a fatty acid, as in sphingomyelin. Glycolipids differ from sphingomyelin in the identity of the unit that is linked to the primary hydroxyl group of the sphingosine backbone. In glycolipids, one or more sugars (rather than phosphorylcholine) are attached to this group. The simplest glycolipid, called a cerebroside, contains a single sugar residue, either glucose or galactose.

Integral membrane proteins

interact extensively with the hydrocarbon chains of membrane lipids, and they can be released only by agents that compete for these nonpolar interactions. In fact, most integral membrane proteins span the lipid bilayer.

Lipid bilayers are highly impermeable to

ions and most polar molecules Water is a conspicuous exception to this generalization;; it traverses such membranes relatively easily because of its low molecular weight, high concentration, and lack of a complete charge.

self-assembly process

ipid bilayers form spontaneously by a self-assembly process. In other words, the structure of a bimolecular sheet is inherent in the structure of the constituent lipid molecules. The growth of lipid bilayers from phospholipids is rapid and spontaneous in water. Hydrophobic interactions are the major driving force for the formation of lipid bilayers. Recall that hydrophobic interactions also play a dominant role in the stacking of bases in nucleic acids and in the folding of proteins (Sections 1.3 and 2.4). Water molecules are released from the hydrocarbon tails of membrane lipids as these tails become sequestered in the nonpolar interior of the bilayer. Furthermore, van der Waals attractive forces between the hydrocarbon tails favor close packing of the tails. Finally, there are electrostatic and hydrogen-bonding attractions between the polar head groups and water molecules. Thus, lipid bilayers are stabilized by the full array of forces that mediate molecular interactions in biological systems. Because lipid bilayers are held together by many reinforcing, noncovalent interactions (predominantly hydrophobic), they are cooperative structures

Embedding part of a protein in a membrane can

link the protein to the membrane surface The structure of the endoplasmic reticulum membrane-bound enzyme prostaglandin H 2 synthase-1 reveals a rather different role for a helices in protein-membrane associations. This enzyme catalyzes the conversion of arachidonic acid into prostaglandin H 2 in two steps: (1) a cyclooxygenase reaction and (2) a peroxidase reaction (Figure 12.21). Prostaglandin H 2 promotes inflammation and modulates gastric acid secretion. The enzyme that produces prostaglandin H 2 is a homodimer with a rather complicated structure consisting primarily of a helices. Unlike bacteriorhodopsin, this protein is not largely embedded in the membrane. Instead, it lies along the outer surface of the membrane, firmly bound by a set of a helices with hydrophobic surfaces that extend from the bottom of the protein into the membrane (Figure 12.22). This linkage is sufficiently strong that only the action of detergents can release the protein from the membrane. Thus, this enzyme is classified as an integral membrane protein, although it does not span the membrane.

lipid bilayer

lternatively, the strongly opposed preferences of the hydrophilic and hydrophobic moieties of membrane lipids can be satisfied by forming a lipid bilayer, composed of two lipid sheets (Figure 12.10). A lipid bilayer is also called a bimolecular sheet. The hydrophobic tails of each individual sheet interact with one another, forming a hydrophobic interior that acts as a permeability barrier. The hydrophilic head groups interact with the aqueous medium on each side of the bilayer. The two opposing sheets are called leaflets.

transverse diffusion or flip-flop

lthough the lateral diffusion of membrane components can be rapid, the spontaneous rotation of lipids from one face of a membrane to the other is a very slow process. The transition of a molecule from one membrane surface to the other is called transverse diffusion or flip-flop (Figure 12.29). The flip-flop of phospholipid molecules in phosphatidylcholine vesicles has been directly measured by electron spin resonance techniques, which show that a phospholipid molecule flip-flops once in several hours. Thus, a phospholipid molecule takes about 10 9 times as long to flip-flop across a membrane as it takes to diffuse a distance of 50 Å in the lateral direction.

fluid mosaic model

n the basis of the mobility of proteins in membranes, in 1972 S. Jonathan Singer and Garth Nicolson proposed a fluid mosaic model to describe the overall organization of biological membranes. The essence of their model is that membranes are two-dimensional solutions of oriented lipids and globular proteins. The lipid bilayer has a dual role: it is both a solvent for integral membrane proteins and a permeability barrier. Membrane proteins are free to diffuse laterally in the lipid matrix unless restricted by special interactions. A lthough the lateral diffusion of membrane components can be rapid, the spontaneous rotation of lipids from one face of a membrane to the other is a very slow process.

membranes are always synthesized by

the growth of preexisting membranes

Cholesterol

the third major type of membrane lipid, has a structure that is quite different from that of phospholipids. It is a steroid, built from four linked hydrocarbon rings. hydrocarbon tail is linked to the steroid at one end, and a hydroxyl group is attached at the other end. In membranes, the orientation of the molecule is parallel to the fatty acid chains of the phospholipids, and the hydroxyl group interacts with the nearby phospholipid head groups. Cholesterol is absent from prokaryotes but is found to varying degrees in virtually all animal membranes. It constitutes almost 25% of the membrane lipids in certain nerve cells but is essentially absent from some intracellular membranes.

The favored structure for most phospholipids and glycolipids in aqueous media is a bimolecular sheet rather than a micelle. The reason is that

the two fatty acid chains of a phospholipid or a glycolipid are too bulky to fit into the interior of a micelle. In contrast, salts of fatty acids (such as sodium palmitate, a constituent of soap) readily form micelles because they contain only one chain. The formation of bilayers instead of micelles by phospholipids is of critical biological importance. A micelle is a limited structure, usually less than 200 Å (20 nm) in diameter. In contrast, a bimolecular sheet can extend to macroscopic dimensions, as much as a millimeter (10 7 Å, or 10 6 nm) or more. Phospholipids and related molecules are important membrane constituents because they readily form extensive bimolecular sheets.

membrane lipids are amphipathic molecules (amphiphilic molecules), that is,

they contain both a hydrophilic and a hydrophobic moiety