Development of Ideas about Membrane Structure

Our current understanding of membrane structure began with E. Overton’s proposal in 1895 that cellular membranes consist of lipid bilayers (Fig. 7-1A). Biochemical experiments in the 1920s supported the bilayer hypothesis. It was found that the lipids extracted from the plasma membrane of red blood cells spread out in a monolayer on the surface of a tray of water to cover an area sufficient to surround the cell twice. (Actually, offsetting errors—incomplete lipid extraction and an underestimation of the membrane area—led to the correct answer!) X-ray diffraction experiments in the early 1970s established definitely that membrane lipids are arranged in a bilayer.

Figure 7-1 development of concepts in membrane structure. A, Gorder and Grendel model from 1926. B, Davson and Danielli model from 1943. C, Singer and Nicholson fluid mosaic model from 1972. D, Contemporary model with peripheral and integral membrane proteins. The lipid bilayer shown here and used throughout the book is based on an atomic model (Fig. 7-5).

During the 1930s, cell physiologists realized that a simple lipid bilayer could not explain the mechanical properties of the plasma membrane, so they postulated a surface coating of proteins to reinforce the bilayer (Fig. 7-1B). Early electron micrographs strengthened this view, since when viewed in cross sections, all membranes appeared as a pair of dark lines (interpreted as surface proteins and carbohydrates) separated by a lucent area (interpreted as the lipid bilayer). By the early 1970s, two complementary approaches showed that proteins cross the lipid bilayer. First, electron micrographs of membranes that are split in two while frozen (a technique called freeze-fracturing; see Fig. 6-4D) revealed protein particles embedded in the lipid bilayer. Later, chemical labeling showed that many membrane proteins traverse the bilayer, exposing different regions of the polypeptide to the aqueous phase on the two sides. Light microscopy with fluorescent tags demonstrated that membrane lipids and some membrane proteins diffuse in the plane of the membrane. Quantitative spectroscopic studies showed that lateral diffusion of lipids is a rapid process but that flipping from one side of a bilayer to the other is a slow one. The fluid mosaic model of membranes (Fig. 7-1C) incorporated this information, showing transmembrane proteins floating in a fluid lipid bilayer. Subsequent work revealed structures of many proteins that span the lipid bilayer, the existence of lipid anchors on some membrane proteins, and a network of cytoplasmic proteins that restricts the motion of many integral membrane proteins (Fig. 7-1D).

Lipids

Lipids form the framework of biological membranes, anchor soluble proteins to the surfaces of membranes, store energy, and carry information as extracellular hormones and as intracellular second messengers. Lipids are organic molecules generally less than 1000 D in size that are much more soluble in organic solvents than in water. They consist predominantly of aliphatic or aromatic hydrocarbons.

This chapter concentrates on major lipids found in biological membranes. After an introduction to their structures, the following section explains how the hydrophobic effect drives lipids to self-assemble stable bilayers. Membranes also contain hundreds of minor lipids, some of which might have important biological functions that are not yet appreciated. For example, during the 1980s, a minor class of lipids with phosphorylated inositol head groups first attracted attention when investigators found that they had a major role in signaling (see Fig. 26-7).

Phosphoglycerides

Phosphoglycerides (also called glycerolphospholipids) are the main constituents of membrane bilayers (Fig. 7-2). (These lipids are often called phospholipids, an imprecise term, as other lipids contain phosphate.) Phosphoglycerides have three parts: a three-carbon back-bone of glycerol, two long-chain fatty acids esterified to carbons 1 and 2 (C₁ and C₂) of the glycerol, and phosphoric acid esterified to C₃ of the glycerol. Fatty acids have a carboxyl group at one end of an aliphatic chain of 13 to 19 additional carbons (Table 7-1). More than half of the fatty acids in membranes have one or more double bonds, which create a bend in the aliphatic chain. These bends contribute to the fluidity of the bilayer. Fatty acids and phosphoglycerides are amphiphilic, since they have both hydrophobic (fears water) and hydrophilic (loves water) parts. The aliphatic chains of fatty acids are hydrophobic. The carboxyl groups of fatty acids and the head groups of phosphoglycerides are hydrophilic. The cross-sectional areas of the head groups and the aliphatic tails are similar, so a phosphoglyceride is shaped approximately like a cylinder—an important factor in membrane structure. The hydrophobic effect (see Fig. 4-5) drives amphiphilic phosphoglycerides to assemble bilayers (see later).

Figure 7-2 structure and synthesis of phosphoglycerides. A, Stick figures and space-filling models of the alcohol head groups. B, Stick figures and space-filling models of a saturated and an unsaturated fatty acid. C, Combination of an alcohol, a glycerol, and two fatty acids to make a phosphoglyceride. In some cases CDP provides the phosphate linking glycerol to the alcohol. D, Diagram of the parts of a phosphoglyceride and a space-filling model of phosphatidylcholine.

Table 7-1 COMMON FATTY ACIDS OF MEMBRANE LIPIDS

Name	Carbons	Double Bonds (Positions)
Myristate	14	0
Palmitate	16	0
Palmitoleate	16	1 (Δ9)
Stearate	18	0
Oleate	18	1 (Δ9)
Linoleate	18	2 (Δ9, Δ12)
Linolenate	18	3 (Δ9, Δ12, Δ15)
Arachidonate	20	4 (Δ5, Δ8, Δ11, Δ14)

Cells make more than 100 major phosphoglycerides by using several different fatty acids and by esterifying one of five different alcohols to the phosphate. In general, the fatty acids on C₁ have no or one double bond, whereas the fatty acids on C₂ have two or more double bonds. Each double bond creates a permanent bend in the hydrocarbon chain. The alcohol head groups, rather than the fatty acids, give phosphoglycerides their names:

phosphatidic acid [PA] (no head group)

phosphatidylglycerol [PG] (glycerol head group)

phosphatidylethanolamine [PE] (ethanolamine head group)

phosphatidylcholine [PC] (choline head group)

phosphatidylserine [PS] (serine head group)

phosphatidylinositol [PI] (inositol head group)

The several head groups confer distinctive properties to the various phosphoglycerides. All have a negative charge on the phosphate esterified to glycerol. Neutral phosphoglycerides—PE and PC—have a positive charge on their nitrogens, giving them a net charge of zero. PS has extra positive and negative charges, giving it a net negative charge like the other acidic phosphoglycerides (PA, PG, and PI). PI can be modified by esterifying one to five phosphates to the hexane ring hydroxyls. These polyphosphoinositides are highly negatively charged.

The complicated metabolism of phosphoglyce-rides can be simplified as follows: Enzymes can interconvert all phosphoglyceride head groups and remod-el fatty acid chains. For example, three successive enzymatic methylation reactions convert PE to PC, whereas another enzyme exchanges serine for ethanolamine, converting PS to PE. Other enzymes exchange fatty acid chains after the initial synthesis of a phosphoglyceride. These enzymes are located on the cytoplasmic surface of the smooth endoplasmic reticulum. Biochemistry texts provide more details of these pathways.

Several minor membrane phospholipids are variations on this general theme. Plasmalogens have a fatty acid linked to carbon 1 of glycerol by an ether bond rather than an ester bond. They serve as sources of arachidonic acid for signaling reactions (see Fig. 26-9). Cardiolipin has two glycerols esterified to the phosphate of PA.

Sphingolipids

Most sugar-containing lipids of biological membranes are sphingolipids. Sphingolipids get their name from sphingosine, a nitrogen-containing base (Fig. 7-3) that is the structural counterpart of glycerol and one fatty acid of phosphoglycerides. Sphingosine carbons 1 to 3 have polar substituents. A double bond between C₄ and C₅ begins the hydrocarbon tail. Two variable features distinguish the various sphingolipids: the fatty acid (often lacking double bonds) attached by an amide bond to C₂ and the nature of the polar head groups esterified to the hydroxyl on C₁.

Figure 7-3 sphingolipids. A, Stick figure and space-filling model of sphingosine. B, Diagram of the parts of a glycosphingolipid. Ceramide has a fatty acid but no sugar. C, Stick figure and space-filling model of sphingomyelin.

The head groups of glycosphingolipids consist of one or more sugars. Some are neutral; others are negatively charged. Note the absence of phosphate. Sugar head groups of some glycosphingolipids serve as receptors for viruses. Alternatively, a phosphate ester can link a base to C₁. These so-called sphingomyelins have phosphorylcholine or phosphoethanolamine head groups just like PC and PE. Receptor-activated enzymes remove phosphorylcholine from sphingomyelin to produce the second messenger ceramide (see Fig. 26-11). Sphingolipids are much more abundant in the plasma membrane than in membranes inside cells. The hydrocarbon tails of sphingosine and the fatty acid contribute to the hydrophobic bilayer, and polar head groups are on the surface.

Sterols

Sterols are the third major class of membrane lipids. Cholesterol (Fig. 7-4) is the major sterol in animal plasma membranes, with lower concentrations in internal membranes. Plants, lower eukaryotes, and bacteria have other sterols in their membranes. The rigid four-ring structure of cholesterol is apolar, so it inserts into the core of bilayers with the hydroxyl on C₃ oriented toward the surface.