Biochemistry, 4th Edition P30 pot

In addition to several trans-membrane helices that lie perpendicular to the trans-membrane plane like those of bac-teriorhodopsin, these structures each contain several long, severely ti

charged groups (amino or guanidinium) extending beyond to associate with

nega-tively charged phosphate groups This behavior, with the side chain pointing up out

of the membrane core, has been termed snorkeling (Figure 9.16) If a Phe residue

occurs near the lipid–water interface, it is typically arranged with the aromatic ring

oriented toward the membrane core This is termed antisnorkeling.

Membrane Protein Structures Show Many Variations on the Classical Themes

Although it revealed many insights of membrane protein structure,

bacterio-rhodopsin gave a relatively limited view of the structural landscape Many

mem-brane protein structures obtained since bacteriorhodopsin (and a few others) have

provided a vastly more complex picture to biochemists For example, the structures

of a homodimeric chloride ion transport protein and a glutamate transport protein

show several novel structural features (Figure 9.17) In addition to several

trans-membrane helices that lie perpendicular to the trans-membrane plane (like those of

bac-teriorhodopsin), these structures each contain several long, severely tilted helices that

span the membrane Both these proteins also contain several reentrant loops,

con-sisting of a pair of short α-helices and a connecting loop that together penetrate

part way into the membrane core There are also regions of nonhelical polypeptide

deep in the membrane core of these proteins, with helical segments on either side

that extend to the membrane surface (Figure 9.17)

Finally, most membrane protein structures are relatively stable; that is,

trans-membrane helices do not flip in and out of the trans-membrane, and they do not flip

across the lipid bilayer, inverting their orientation However, a few membrane

pro-teins can in fact change their membrane orientation Aquaporin-1 is a protein that

functions normally with six transmembrane -helices When this protein is first

in-serted into its membrane, it has only four transmembrane -helices (Figure 9.18a).

One of these, the third transmembrane helix (TM3), reorients across the

mem-brane, pulling helices 2 and 4 into the membrane Similarly, a glycoprotein of the

hepatitis B virus is initially inserted into the viral membrane with its N-terminal

do-main lying outside During the viral maturation process, about half of these

glyco-proteins rearrange (Figure 9.18b), with the N-terminal segment moving across the

membrane as TM4 creates a new transmembrane segment

Some Proteins Use ␤-Strands and ␤-Barrels To Span the Membrane The-helix

is not the only structural motif by which a protein can cross a membrane Some

in-tegral transmembrane proteins use structures built from -strands and -sheets to

diminish the polar character of the peptide backbone as it crosses the nonpolar

Lys111

Phe72 Snorkeling

Antisnorkeling

0 15

15

FIGURE 9.16 Snorkeling and antisnorkeling behavior in membrane proteins The SdhC subunit of succinate dehydrogenase (pdb id  1NEK) Lys 111 snorkels away from the membrane core and Phe 72 antisnorkels toward the membrane core (Adapted from Liang, J., Adamian, L., and Jackups, R., Jr., 2005 The membrane-water interface region of membrane proteins: Structural bias and the anti-snorkeling

effect Trends in Biochemical Sciences 30:355–357.)

FIGURE 9.17 Not all the embedded segments of membrane proteins are transmembrane and oriented

per-pendicular to the membrane plane (a) The glutamate transporter homolog (pdb id  1XFH).“Reentrant”

helices (orange) and interrupted helices (red) are shown Several of the transmembrane helices deviate

signifi-cantly from the perpendicular (b) The E coli ClC chloride transporter (pdb id  1KPK) Few of the

transmem-brane helices are perpendicular to the memtransmem-brane plane.

TM5 TM6 TM1 TM3

C

C N

N Pre-S

50%

N

N

TM2 TM3 TM4 TM5 TM6

C

TM3 TM2 TM1

C

TM4 TM2 TM1 TM4

(b) Hepatitis B virus

FIGURE 9.18 Dynamic insertion of helical segments of membrane proteins (a) Aquaporin-1 The second and

fourth transmembrane helices insert properly across the membrane only after reorientation of the third

trans-membrane helix (b) The large envelope glycoprotein of the hepatitis B virus The N-terminal “pre-S” domain

translocates across the endoplasmic reticulum membrane in a slow process in 50% of the molecules (Adapted

from von Heijne, G., 2006 Nature Reviews Molecular and Cell Biology 7:909–918.)

(g) (f)

(e) (d)

(c) (b)

(a)

FIGURE 9.19 Some proteins traverse the membrane with -barrel structures.

Several examples are shown, including (a) maltoporin from S typhimurium

(pdb id  2MPR), (b) ferric enterobactin receptor (pdb id  1FEP), (c) TolC,

an outer membrane protein from E coli (pdb id  1EK9), (d) the translocator

domain of the NalP autotransporter of N meningitides (pdb id  1UYN), (e) the

translocator domain of the Hia autotransporter from H influenzae (pdb id 

2GR8), (f) the outer membrane cobalamin transporter from E coli, in a complex

with the 100 Å coiled coil of colicin E3 (pdb id  1UJW), (g) the fatty acid

trans-porter FadL from E coli (pdb id  1T16).

membrane core These ␤-barrel structures (Figure 9.19) maximize hydrogen

bond-ing and are highly stable The barrel interior is large enough to accommodate

wa-ter molecules and often structures as large as peptide chains, and most barrels are

literally water filled

How does the -barrel structure tolerate water on one surface (the inside) and

the nonpolar membrane core on the other? In all transmembrane -barrels, polar

and nonpolar residues alternate along the -strands, with polar residues facing the

center of the barrel and nonpolar residues facing outward, where they can interact

with the hydrophobic lipid milieu of the membrane

bacte-ria such as Escherichia coli, and also in the outer mitochondbacte-rial membranes of

eu-karyotic cells, span their respective membranes with large -barrels A good

ex-ample is maltoporin, also known as LamB protein or lambda receptor, which

participates in the entry of maltose and maltodextrins into E coli Maltoporin is

active as a trimer The 421-residue monomer forms an 18-strand -barrel with

antiparallel-strands connected to their nearest neighbors either by long loops

or by -turns (Figure 9.20; see also Figure 9.19a) The long loops are found at

the end of the barrel that is exposed to the cell exterior, whereas the turns are

located on the intracellular face of the barrel Three of the loops fold into the

center of the barrel

-barrels can also be constructed from multiple subunits The -hemolysin toxin

(Figure 9.21) forms a 14-stranded -barrel with seven identical subunits that each

contribute two antiparallel -strands connected by a short loop Staphylococcus aureus

secretes monomers of this toxin, which bind to the plasma membranes of host

Cell

surface

Outer

membrane

Periplasmic space

FIGURE 9.20 The arrangement of the peptide chain in

maltoporin from E coli.

view

Axial view

ACTIVE FIGURE 9.21 The structure of

the heptameric channel formed by Staphylococcus

con-tributes a -sheet hairpin to the transmembrane

chan-nel (pdb id  7AHL) Test yourself on the concepts in

this figure at www.cengage.com/login.

Go to CengageNOW at www.cengage.com/login and click BiochemistryInteractive to discover how a

-sheet is expoilted by maltoporin.

ture The channel thus formed facilitates uncontrolled permeation of water, ions, and small molecules, destroying the host cell

Why have certain proteins evolved to use -strands instead of -helices as

membrane-crossing devices? Among other reasons, there is an advantage of genetic economy in the use of -strands to traverse the membrane instead of -helices An

-helix requires 21 to 25 amino acid residues to span a typical biological membrane; a

-strand can cross the same membrane with 9 to 11 residues Therefore, a given

amount of genetic information could encode a larger number of membrane-spanning segments using a -strand motif instead of -helical arrays.

Transmembrane Barrels Can also Be Formed with ␣-Helices Many bacteria,

in-cluding E coli, produce extracellular polysaccharides, some of which form a discrete

structural layer—the capsule, which shields the cell, allowing it to evade or

counter-act host immune systems In E coli, the components of this polysaccharide capsule are

synthesized inside the cell and then transported outward through an octameric outer

membrane protein called Wza To cross the outer membrane, Wza uses a novel

␣-helical barrel (Figure 9.22) Wza is composed of three novel domains that, with the

-helical barrel, form a large central cavity that accommodates the transported

poly-saccharides The transmembrane -helices of Wza are amphiphilic, with hydrophobic

outer surfaces that face the lipid bilayer and hydrophilic inner surfaces that face the water-filled pore

Lipid-Anchored Membrane Proteins Are Switching Devices

Certain proteins are found to be covalently linked to lipid molecules For many of these proteins, covalent attachment of lipid is required for association with a

mem-brane The lipid moieties can insert into the membrane bilayer, effectively

anchor-ingtheir linked proteins to the membrane Some proteins with covalently linked lipid normally behave as soluble proteins; others are integral membrane proteins and remain membrane associated even when the lipid is removed Covalently

Side

view

Cytosol

Axial view

Monomer

FIGURE 9.22 The structure of Wza, an octameric membrane protein that anchors the peptidoglycan layer and the outer membrane of Gram-negative bacteria The structure contains a central barrel constructed from

-helical segments (pdb id  2J58).

bound lipid in these latter proteins can play a role distinct from membrane

an-choring In many cases, attachment to the membrane via the lipid anchor serves to

modulate the activity of the protein

Another interesting facet of lipid anchors is that they are transient Lipid anchors

can be reversibly attached to and detached from proteins This provides a

“switch-ing device” for alter“switch-ing the affinity of a protein for the membrane Reversible lipid

anchoring is one factor in the control of signal transduction pathways in eukaryotic

cells (see Chapter 32)

Four different types of lipid-anchoring motifs have been found to date These are

amide-linked myristoyl anchors, thioester-linked fatty acyl anchors, thioether-linked

these anchoring motifs is used by a variety of membrane proteins, but each

nonetheless exhibits a characteristic pattern of structural requirements

Amide-Linked Myristoyl Anchors Myristic acid may be linked via an amide bond

to the -amino group of the N-terminal glycine residue of selected proteins (Figure

9.23a) The reaction is referred to as N-myristoylation and is catalyzed by

myristoyl–CoA⬊protein N-myristoyltransferase, known simply as NMT N-Myristoyl–

anchored proteins include the catalytic subunit of cAMP-dependent protein kinase, the

pp60 src tyrosine kinase, the phosphatase known as calcineurin B, the -subunit of G

teins (involved in GTP-dependent transmembrane signaling events), and the gag

pro-teins of certain retroviruses (including the HIV-1 virus that causes AIDS).

Thioester-Linked Fatty Acyl Anchors A variety of cellular and viral proteins contain

fatty acids covalently bound via ester linkages to the side chains of cysteine and

some-times to serine or threonine residues within a polypeptide chain (Figure 9.23b) This

type of fatty acyl chain linkage has a broader fatty acid specificity than N-myristoylation.

Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the C16

and C18chain lengths being most commonly found Proteins anchored to membranes

via fatty acyl thioesters include G-protein–coupled receptors, the surface glycoproteins of

sev-eral viruses, the reggie proteins of nerve axons, and the transferrin receptor protein.

Thioether-Linked Prenyl Anchors As noted in Chapter 8, polyprenyl (or simply

prenyl) groups are long-chain polyisoprenoid groups derived from isoprene units

Prenylation of proteins destined for membrane anchoring can involve either

far-nesyl or geranylgeranyl groups (Figure 9.23c and d) The addition of a prenyl group

A DEEPER LOOK

Exterminator Proteins—Biological Pest Control at the Membrane

Control of biological pests, including mosquitoes, houseflies, gnats,

and tree-consuming predators like the eastern tent caterpillar, is

frequently achieved through the use of microbial membrane

pro-teins For example, several varieties of Bacillus thuringiensis produce

proteins that bind to cell membranes in the digestive systems of

in-sects that consume them, creating transmembrane ion channels

Leakage of Na, K, and Hions through these membranes in the

insect gut destroys crucial ion gradients and interferes with

diges-tion of food Insects that ingest these toxins eventually die of

star-vation B thuringiensis toxins account for more than 90% of sales of

biological pest control agents

B thuringiensis is a common Gram-positive, spore-forming soil

bacterium that produces inclusion bodies, microcrystalline

clus-ters of many different proteins These crystalline proteins, called

-endotoxins, are the ion channel toxins that are sold

commer-cially for pest control Most such endotoxins are protoxins, which

are inactive until cleaved to smaller, active proteins by proteases in

the gut of a susceptible insect One such crystalline protoxin,

lethal to mosquitoes, is a 27-kD protein, which is cleaved to form the active 25-kD toxin in the mosquito This toxin has no effect on membranes at neutral pH, but at pH 9.5 (the pH of the mosquito gut) the toxin forms cation channels in the gut membranes This 25-kD protein is not toxic to tent caterpillars, but a larger,

130-kD protein in the B thuringiensis inclusion bodies is cleaved by

a caterpillar gut protease to produce a 55-kD toxin that is active in

the caterpillar Remarkably, the strain of B thuringiensis known as

azawai produces a protoxin with dual specificity: In the caterpillar gut, this 130-kD protein is cleaved to form a 55-kD toxin active in the caterpillar However, when the same 130-kD protoxin is con-sumed by mosquitoes or houseflies, it is cleaved to form a 53-kD protein (15 amino acid residues shorter than the caterpillar toxin) that is toxic to these latter organisms Understanding the molecu-lar basis of the toxicity and specificity of these proteins and the means by which they interact with membranes to form lethal ion channels is a fascinating biochemical challenge with far-reaching commercial implications

CH2 HN

C O

C O

C

CH2

–OOC

COO–

side

Cytoplasmic side

S

H2C

HC C O

HN

S

H2C

HC C O

O CH3 HN

(c) Farnesylation (d) Geranylgeranylation

O CH3

(e)

Vesicular stomatitis glycoprotein

E

P

M M M Gal

GN Gal

Gal Gal

I

P

Acetylcholinesterase

E

P

I

P

P

E

Thyroglobulin 1

E

P

I

P

E

P

I

P

Glycolipid A E

Gal

I

= Ethanolamine

= Galactose

= Mannose

= Glucosamine

= Inositol

Key:

M M

GN M

M M

GN M

M

GN M

M GN

FIGURE 9.23 Certain proteins are anchored to biological

membranes by lipid anchors Shown are (a) the

N-myristoyl motif, (b) the S-palmitoyl motif, (c) the

far-nesyl motif, (d) the geranylgeranyl motif, and (e) several

cases of the glycosyl phosphatidylinositol (GPI) motif.

HUMAN BIOCHEMISTRY

Prenylation Reactions as Possible Chemotherapy Targets

The protein called p21ras, or simply Ras, is a small GTP-binding

protein involved in cell signaling pathways that regulate growth

and cell division Mutant forms of Ras cause uncontrolled cell

growth, and Ras mutations are involved in one-third of all human

cancers Because the signaling activity of Ras is dependent on

prenylation, the prenylation reaction itself, as well as the

proteo-lysis of the -AAX motif and the methylation of the prenylated Cys

residue, have been considered targets for development of new

chemotherapy strategies

Farnesyl transferase from rat cells is a heterodimer consisting

of a 48-kD -subunit and a 46-kD -subunit In the structure shown

here, helices 2 to 15 of the -subunit are folded into seven short,

coiled coils that together form a crescent-shaped envelope

par-tially surrounding the -subunit Twelve helices of the -subunit

form a novel barrel motif that creates the active site of the enzyme

Farnesyl transferase inhibitors, one of which is shown here, are

po-tent suppressors of tumor growth in experimental animals

Mutations that inhibit prenyl transferases cause defective growth or death of cells, raising questions about the usefulness of prenyl transferase inhibitors in chemotherapy However, Victor Boyartchuk and his colleagues at the University of California, Berkeley, and Acacia Biosciences have shown that the protease that cleaves the -AAX motif from Ras following the prenylation re-action may be a better chemotherapeutic target They have

iden-tified two genes for the prenyl protein protease in the yeast

Sac-charomyces cerevisiae and have shown that deletion of these genes

results in loss of proteolytic processing of prenylated proteins, in-cluding Ras Interestingly, normal yeast cells are unaffected by this gene deletion However, in yeast cells that carry mutant forms of Ras and that display aberrant growth behaviors, deletion of the protease gene restores normal growth patterns If these remark-able results translate from yeast to human tumor cells, inhibitors

of CAAX proteases may be more valuable chemotherapeutic agents than prenyl transferase inhibitors

S

Ras CMSCKC COO–

Ras CMSCKCVLS COO–

Farnesyl pyrophosphate Additional

modification (methylation and palmitoylation)

Farnesyl transferase Ras CMSCKCVLS COO–

VLS

PPSMT

Ras CMSCKCVLS COO–

Ras CMSCKC C

S O OCH3

PPSEP

Endoplasmic reticulum membrane Plasma membrane

H

O

O

SO2CH3

OH N

O N

H

H2N

HS

2(S)-{(S)-[2(R)-amino-3-mercapto]propylamino-

3(S)-methyl}pentyloxy-3-phenylpropionyl-methioninesulfone methyl ester

䊱 The structure of the farnesyl transferase heterodimer (pdb id 

1JCQ) A novel barrel structure is formed from 12 helical segments

in the -subunit (yellow) The -subunit (green) consists largely of

seven successive pairs of -helices that form a series of right-handed

antiparallel coiled coils running along the bottom of the structure.

These “helical hairpins” are arranged in a double-layered,

right-handed superhelix resulting in a crescent-shaped subunit that

envelopes part of the subunit.

䊱 This substance, also known as I-739,749, is a farnesyl transferase

inhibitor that is a potent tumor growth suppressor.

䊱 The farnesylation and subsequent processing of the Ras protein

Follow-ing farnesylation by the FTase, the carboxy-terminal VLS peptide is removed

by a prenyl protein-specific endoprotease (PPSEP) in the ER; then a prenyl-protein-specific methyltransferase (PPSMT) donates a methyl group from

S - adenosylmethionine (SAM) to the carboxy-terminal S - farnesylated

cys-teine In addition, palmitates are added to cysteine residues near the C-terminus of the protein (not shown).

target protein, where C is cysteine, A is any aliphatic residue, and X can be any amino acid As shown in Figure 9.23c and d, the result is a thioether-linked farnesyl

or geranylgeranyl group Once the prenylation reaction has occurred, a specific protease cleaves the three carboxy-terminal residues, and the carboxyl group of the now terminal Cys is methylated to produce an ester All of these modifications ap-pear to be important for subsequent activity of the prenyl-anchored protein

Pro-teins anchored to membranes via prenyl groups include yeast mating factors, the p21 ras protein (the protein product of the ras oncogene; see Chapter 32), and the nu-clear lamins, structural components of the lamina of the inner nunu-clear membrane.

Glycosyl Phosphatidylinositol Anchors Glycosyl phosphatidylinositol, or GPI,

groups are structurally more elaborate membrane anchors than fatty acyl or prenyl groups GPI groups modify the carboxy-terminal amino acid of a target protein via an ethanolamine residue linked to an oligosaccharide, which is linked in turn to the in-ositol moiety of a phosphatidylinin-ositol (Figure 9.23e) The oligosaccharide typically consists of a conserved tetrasaccharide core of three mannose residues and a glu-cosamine, which can be altered by addition of galactosyl side chains of various sizes

and extra phosphoethanolamines, N-acetylgalactose, or mannosyl residues (Figure

9.23e) The inositol moiety can also be modified by an additional fatty acid, and a va-riety of fatty acyl groups are found linked to the glycerol group GPI groups anchor a

wide variety of surface antigens, adhesion molecules, and cell surface hydrolases to plasma

membranes in various eukaryotic organisms GPI anchors have not yet been observed

in prokaryotic organisms or plants

Membranes Are Asymmetric and Heterogeneous Structures Biological membranes are asymmetric and heterogeneous structures The two

mono-layers of the lipid bilayer have different lipid compositions and different comple-ments of proteins The membrane composition is also different from place to place across the plane of the membrane There are clusters of particular kinds of lipids, par-ticular kinds of proteins, and a variety of specific lipid-protein associations and aggregates, all of which serve the functional needs of the cell We say that both the lipids and the proteins of membranes exhibit lateral heterogeneity and transverse

asymmetry Lateral heterogeneity arises when lipids or proteins of particular types cluster in the plane of the membrane Transverse asymmetry refers to different lipid

or protein compositions in the two leaflets or monolayers of a bilayer membrane Many properties of a membrane depend on its two-sided nature Properties that are

a consequence of membrane “sidedness” include membrane transport, which is driven

in one direction only; the effects of hormones at the outsides of cells; and the immunological reactions that occur between cells (necessarily involving only the out-side surfaces of the cells) The proteins involved in these and other interactions must

be arranged asymmetrically in the membrane

Lipid transverse asymmetry can be seen in the typical animal cell, where the amine-containing phospholipids are enriched in the cytoplasmic leaflet of the plasma membrane, and the choline-containing phospholipids and sphingolipids are enriched in the outer leaflet (Figure 9.24) In the erythrocyte, for example, phosphatidylcholine (PC) comprises about 29% of the total phospholipid in the membrane Of this amount, 76% is found in the outer monolayer and 24% is found

in the inner monolayer

Asymmetric lipid distributions are important to cells in several ways The carbohy-drate groups of glycolipids (and of glycoproteins) always face the outside of plasma membranes, where they participate in cell recognition phenomena Asymmetric lipid distributions may also be important to various integral membrane proteins, which may prefer particular lipid classes in the inner and outer monolayers

Loss of transverse lipid asymmetry has dramatic (and often severe) consequences

for cells and organisms For example, appearance of PS in the outer leaflet of the

plasma membrane triggers apoptosis, the programmed death of the cell Similarly,

ag-ing erythrocytes and platelets slowly externalize PS, culminatag-ing in engulfment by

macrophages Many disease states, including diabetes and malaria, involve

microvas-cular occlusions that may result in part from alterations of transverse lipid asymmetry

Membrane Function?

Lipids and Proteins Undergo a Variety of Movements in Membranes

Motions of lipids and proteins in membranes underlie many cell functions Lipid

movements (Figure 9.25) range from bond vibrations (at 1012per sec), to bilayer

undulations (1 to 106per sec), to transverse motion—called “flip-flop” (roughly one

Gauche-trans

isomerization 10 10 /sec

Rotational diffusion 10 8 /sec

Lateral diffusion 107/sec

Undulations 1 10 6 /sec

Bond vibrations

1012/sec

Protrusion

10 9 /sec

Flip-flop

10–410 3 /sec

FIGURE 9.25 Lipid motions in the membrane and their characteristic frequencies (Adapted from Gawrisch, K., 2005.

The dynamics of membrane lipids In The Structure of Biological

Membranes, Chapter 4, Figure 4.1, Yeagle, P L., ed., 2005 Boca

Raton: CRC Press.)

0

100

100

Inner leaflet

SM PC PS PE PI PIP PIP2 PA

27 29 13 27

3

14%

44%

1%

45%

SM PC PE







sphingomyelin phosphatidylcholine

PS PI/PA





phosphatidylserine phosphatidylinositol/

phosphatidic acid phosphatidylethanolamine

FIGURE 9.24 Phospholipids are distributed asymmetrically in most membranes, including the human

erythro-cyte membrane, as shown here (a) The distribution of phospholipids across the inner and outer leaflets of

human erythrocytes The x-axis values show, for each lipid type, its percentage of the total phospholipid in the

membrane (b) The phospholipid compositions of the inner and outer leaflets All percentages in (a) and (b)

are weight percentages (Adapted from Zachowski, A., 1993 Phospholipids in animal eukaryotic membranes: Transverse

asymmetry and movement Biochemical Journal 294:1–14; and from Andreoli, T E., 1987 Membrane Physiology, 2nd ed Chapter

27, Table I New York: Springer.)

is rapid Adjacent lipids can change places with each other on the order of 107/sec Thus, a typical phospholipid can diffuse laterally in a membrane at a linear rate of several microns per second At that rate, a phospholipid molecule travels from one end of a bacterial cell to the other in less than a second or traverse a typical animal cell in a few minutes

Many membrane proteins move laterally (through the plane of the membrane)

at a rate of a few microns per minute On the other hand, some integral membrane proteins are more restricted in their lateral movement, with diffusion rates of about

10 nm per sec or even slower Slower protein motion is likely for proteins that asso-ciate and bind with each other and for proteins that are anchored to the cytoskele-ton, a complex latticelike structure that maintains the cell’s shape and assists in the controlled movement of various substances through the cell

Flippases, Floppases, and Scramblases: Proteins That Redistribute Lipids Across the Membrane Proteins that can “flip” and “flop” phospholipids from one side of

a bilayer to the other have also been identified in several tissues (Figure 9.26) Three classes of such proteins are known:

1 ATP-dependent flippases that transport PS, and to a lesser extent PE, from the

outer leaflet to the inner leaflet of the plasma membrane

2 ATP-dependent floppases that transport a variety of amphiphilic lipids, especially

cholesterol, PC, and sphigomyelin from the inner leaflet to the outer leaflet

3 bidirectional, Ca2+-activated (but ATP-independent) scramblases that function to

randomize lipids and thus degrade transverse asymmetry These proteins reduce the half-time for phospholipid movement across a mem-brane from days to a few minutes or less Approximately one ATP is consumed per lipid transported by flippases and floppases Energy-dependent lipid flippase activ-ity is essential for the creation and maintenance of transverse lipid asymmetries A number of diseases have been linked to defects in flippases and floppases Tangier disease causes accumulation of high concentrations of cholesterol in various tissues and leads to cardiovascular problems Infants with respiratory distress syndrome produce low amounts of lung surfactant (a mix of lipids) and typically die a few days after birth Both of these diseases involve flippase or floppase defects

Membrane Lipids Can Be Ordered to Different Extents

The phospholipids and sterols of membranes can adopt different structures de-pending on the exact lipid and protein composition of the membrane and on the

temperature At low temperatures, bilayer lipids are highly ordered, forming a gel

(Figure 9.27) In this state—called the solid-ordered state (or S o state)—the lipid chains are tightly packed and undergo relatively little motion The lipid chains are

in their fully extended conformation, the surface area per lipid is minimal, and the

Lipid molecule diffuses

to flippase protein

Flippase Floppase Scramblase

Ca 2+

Flippase protein

Flippase flips lipid

to opposite side

of bilayer

Lipid diffuses away from flippase

(b) (a)

ANIMATED FIGURE 9.26 (a) Phospholipids can be flipped, flopped, or scrambled across a

bilayer membrane by the action of flippase, floppase, and scramblase proteins (b) When, by normal diffusion

through the bilayer, the lipid encounters one of these proteins, it can be moved quickly to the other face of

the bilayer See this figure animated at www.cengage.com/login.