Structure and Organization of Membranes

8/13/2019 Structure and Organization of Membranes

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Structure and Organization of Membranes

The first living cell probably came into being when a membrane formed,

separating that cells precious contents from the rest of the universe.Membranes define the external boundary of cells and regulate the molecular

traffic across that boundary. Membranes are tough but flexible, self-sealing,

and selectively permeable to polar solutes. Their flexibility permits the

shape changes that accompany cell growth and movement (such as

amoeboid movement). Their ability to seal over temporary breas in their

continuity allows two membranes to fuse, as in exocytosis, or a single

membrane-enclosed compartment to undergo fission, yielding two sealed

compartments, as in endocytosis or cell division, without creating gross

leas through the cell surface. !ecause membranes are selectively

permeable, they retain certain compounds and ions within cells and within

specific cellular compartments, and exclude others.

"ll biological membranes, whether from euaryotic or proaryotic cells,

have the same classes of chemical components, a similarity in structural

organi#ation, and a number of properties in common. There are ma$or

differences in the specific lipid, protein, and carbohydrate components but

not in the physicochemical interaction of these molecules in the membrane.

Membranes are not merely passive barriers. They include an array of

proteins speciali#ed for promoting or cataly#ing a variety of molecular

events. %umps move specific organic solutes and inorganic ions across themembrane against a concentration gradient& energy transducers convert one

form of energy into another& receptors on the plasma membrane sense

extracellular signals, converting them into molecular changes within the cell.

'ellular membranes control the composition of the space they enclose not

only by their ability to exclude a variety of molecules but also because of the

presence of selective transport systems permitting the movement of specific

molecules from one specific molecules from one side to the other. !y

controlling the translocation of substrates, cofactors, ions, and so on, from

one compartment to another, membranes modulate the concentration of

substances, thereby exerting an influence on metabolic pathways. The

plasma membrane of euaryotic cells also has a role in cell-cell recognition,

maintenance of the shape of the cell, and in cell locomotion. The site of

action of many hormone and metabolic regulators is on the plasma

membrane, where there are specific receptors, and in the information to be

imparted to the cell by the hormone or regulator is transmitted by the


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membrane component to the appropriate metabolic pathway by a series of

intracellular intermediates, termed second messengers.

Typical Structure of Biological Membranes

Typically, a biological membrane contains lipid, protein, and

carbohydrate in ratios varying with the source of the membrane. The

carbohydrate is covalently associated with protein (glycoproteins) or with

lipid (glycolipids and lipopolysaccharides). Thus the membrane can be

thought of as a lipid-protein matrixin which specific functionsare carried

out by proteins, while the permeability barrier and the structural

integrityof the membrane are provided by lipids.

The one membrane structure common to all cells is the plasma membrane.

This membrane encapsulates the cytoplasm and creates internal

compartments in which essential functions are carried out. n addition to itsrole as a physical barrier that maintains the integrity of the cell, the

plasma membrane provides functions necessary for the survival of a cell,

including exclusion of harmful substances, acquisition of nutrients and

energy sources, disposal of unusable and toxic materials, reproduction,

locomotion, and interaction ith components in the environment . "ll

these functions reuire coordination both for short-range processes, such as

sensation, and for long-range processes, such as growth and differentiation.

"lthough some characteristics of biological membranes can be explained by

the properties of membrane lipids in aueous solution, other characteristics,

especially the ability to perform function such as transport and en#ymatic

activities, depend on the presence of membrane-associated proteins.

The Molecular !onstituents of Membranes

*ne approach to understanding membrane function is to study membrane

composition-to determine, for example, which components are commonly

present in membranes and which are unique to membranes with specific

functions.

%roteins and polar lipids account for almost all of the mass of biologicalmembranes& the small amount of carbohydrate present is generally part of

glycoproteins or glycolipids. The relative proportions of protein and lipid

differ in different membranes, reflecting the diversity of biological roles.

The myelin sheath, which serves as a passive electrical insulator wrapped

around certain neurons, consists primarily of lipids, but the membranes of


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bacteria, mitochondria, and chlorophasts, in which many en#yme-cataly#ed

metabolic processes tae place, contain more protein than lipid.

The "ipid !onstituents of Biological Membranes

"ll biological membranes contain lipids as ma$or constituents. The molecule

that play the dominant roles in membrane formation all have highly polar

head groupsand, in most cases, to hydrocarbons tails. This composition

maes molecular sense as if a large head group is attached to a single

hydrocarbon chain, the molecule is wedge-shaped and will tend to form

spherical micelles. " double tail yields a roughly cylindrical molecule& such

cylindrical molecules can easily pac in parallel to form extended sheets of

bilayer membranes with the hydrophilic head groups facing outward into the

aueous regions on either side. The four ma#or classes of membrane-

forming lipids $ glycerophospholipids, sphingolipids,glycosphingolipids, and glycoglycerolipids $ share this type of

cylindrical molecular structure% They differ principally in the nature of

the head group%

&lycerophospholipids

+lycerophospholipids (also called phosphoglycerides) are the ma$or class of

naturally occurring phospholipids, lipids with phosphate-containing head

groups (ig). These compounds mae up a significant fraction of the

membrane lipids throughout the bacterial, plant, and animal ingdoms. "ll

glycerophospholipids can be considered to be derivatives of glycerol--

phosphate. 'arbon / in glycerol--phosphate is a chiral center, and the

ig. %hospholipids and membrane


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naturally occurring glycerophospholipids are derivatives of the 0

enantiomer.

The sterochemical configuration of the general structure of

glycerophospholipids is shown in ig /a. ig/b. shows the molecule in the

manner it will be generally used to represent membrane lipids, with the

hydrophilic tails drawn to the right and the hydrophilic head group to the

left.

1sually 2and 2/are acyl side chainsderived from the fatty acids& oftenone is saturated, the other unsaturated. The hydrophilic 2 group varies

greatly, and it is this that confers the greatest variation in properties among

the glycerophospholipids (ig.)

ig/3 +lycerophospholipid structure

ig. The hydrophilic groups (2in ig/ that distinguish common

glycerophospholipids). n addition

to this variation, there is also

variation in the hydrocarbon tails(2, 2/) in the structures shown in


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The simplest members of the group, phosphatidic acid, is only a minor

membrane constituent& its principal role is as an intermediate in the synthesis

of other glycerophospholipids. The names of glycerophospholipids are

derived from phosphatidic acid 3 phosphatidylcholine,

phosphatidylethanolamine, and so on. "s ig shows, the

glycerophospholipids have polar head groups, all carrying some charge.

!ecause the hydrocarbon tails are derived from the naturally occurring fattyacids in various combinations, an enormous variety of glycerophospholipids

exists. or example, the erythrocyte membrane contains molecules with

hydrocarbon chains of 4 to /5 carbons, with 6 to 4 double bonds. 7uch

variation in membrane composition allows 8fine-tuning9 of membrane

properties for the diverse functions that different membranes must perform.

Spingolipids and &lycosphingolipids

" second ma$or class of membrane constituents is built on the long-chainamino alcohol sphingosine, rather than on glycerol. f a fatty acid is lined

via an amide bond to the :;


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important example is sphingomyelin, in which a phosphocholine group is

attached to the '- hydroxyl.

n some of the membrane lipids built on sphingosine, the head group

contains saccharides. 0ipids containing saccharide groups go under the

general name of glycolipids. The glycosphingolipids constitute the third

ma$or class of membrane lipids. They include such molecules as the

cerebrosides (monoglycosyl ceramides) and gangliosides, anionic

glycosphingolipids containing one or more sialic acid residues. "s the

names of the compounds suggest, they are especially common in the

membranes of brain and nerve cells.

&lycoglycerolipids

"nother class of lipids, less common in animal membranes but widespread

in plant and bacterial membranes, are the glycoglycerolipids, exemplified by

monogalactosyl diglyceride. This compound may actually be the mostabundant of all polar lipids, for it constitutes about half the lipid in

chloroplast membranes. 7uch lipids are also abundant in archaebacteria,

where they are the ma$or membrane component.

!holesterol

*ne important lipid constituent of many membranes bears little superficial

resemblance to the compounds. This substance is cholesterol, which is a

member of a large group of substances called steroids. 7teroids include a

number of important hormones, among them the sex hormones of higher

animals. n fact, cholesterol is the precursor for the synthesis of many of

these substances.

'holesterol is a wea amphipathic substance, because of the hydroxyl group

at one end of molecule.


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hydroxyl group to a fatty acid. "s the conformational structure in ig===was

the fused cyclohexane rings in cholesterol are all in the chair conformation.

This maes cholesterol a buly, rigid structure as compared with other

hydrophobic membrane components such as the fatty acid tails. The

cholesterol molecule fits awwardly into membrane lipids and tends to

disrupts regularity in membrane structure. This property can have a ma$or

effect, because cholesterol constitutes />? or more of the lipid content in

some membranes. 'hanges in membrane regularity can have profound

effects on such properties as membrane stiffness and permeability.

The Structure and )roperties of Membranes and Membrane )roteins

The membranes of living cells are remarable bits of molecular architecture,

with many and varied functions. Much of our current understandingbiological membranes is based upon the fluid mosaic model proposed by 7.

@. 7inger and +.0. ;icholson in AB/. The fluid, asymmetric lipid bilayers

carries within it a host of proteins. 7ome of them, called peripheral

membrane proteins, are exposed at only one membrane face of the other.

They are held to the membrane by interaction with lipid heads or integral

membrane proteins. The integral membrane proteins are largely buried

within the membrane but are usually exposed on both faces. ntegral proteins

are freuently involved in transmitting either specific substances or chemical

signals through the membrane. The whole membrane is a mosaic of lipids

and proteins.

Membrane )roteins are *ntegral or )eripheral

Membrane proteins are classified as peripheral or integral. %eripheral

proteins are probably bound to the membrane as a result of specific

interactions with exposed, hydrophilic portions of integral membraneproteins. "s a conseuence they can be dissociated from isolated membranes

by agents that disrupt ionic or hydrogen bonds, such as high salt, CDT"

+which chelates divalent cations), or urea. n contrast, integral membrane

proteins appear to be deeply embedded in the membrane. They can be

released from the membrane only by disrupting the hydrophobic interactions

of membrane lipids with organic solvents or detergents. 7ignificant


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hydrophobic interactions with membrane lipids and proteins probably are

responsible for the interaction properties of integral proteins. Cven after

integral proteins have been solubili#ed, removal of the detergent may cause

the protein to precipitate as an insoluble aggregate. The insolubility of

integral membrane proteins results from the presence of domains rich in

hydrophobic amino acids& hydrophobic interactions between the protein and

the lipids of the membrane account for the firm attachment of the protein.

Some *ntegral )roteins ave ydrophobic Transmembrane nchors

ntegral membrane proteins generally have domains rich in hydrophobic

amino acids. n some proteins, there is a single hydrophobic seuence in the

middle of the protein (as in glycophorin) or at the amino or carboxyl

terminus. *ther membrane proteins have multiple hydrophobic seuences,

each long enough to span the lipid bilayer when in the -helical

conformation.


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5.


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7everal simple methods of analy#ing amino acid seuences have been found

to yield reasonably accurate predictions of secondary structure for

transmembrane proteins. The relative polarity of each of the /6 amino

acids has been determined experimentally by measuring the free-energy

change of moving a given residue from a hydrophobic solvent into water.

This free energy of transfer ranges from very exorgonic for changes or polar

residues to very endergonic for amino acids with aromatic or aliphatic

hydrocarbon side chains (Table ). To estimate the overall hydrophobicity of

a seuence of amino acids, one sums the free energies of transfer for those

residues, obtaining a hydropathy index for that region. To search a

seuence for potential membrane E spanning segments, one calculates the

hydropathy index for successive segments of a given si#e (a 8window9,

which may be from B to /6 residues). or a window of B residues, the

indexes for residues to B, / to F, to A, and so on, are plotted as in igure

. " region of /6 residues of high hydropathy index is presumed to be atransmembrane segment. Ghen the seuences of membrane proteins of

nown three-dimensional structure are scanned in this way, a reasonably

good correspondenceis found between predictedand nown membrane-

spanning segments.


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The first is the free energy difference between solution in water and association

with the interface (glycerol group) of a %*%' (palmitoyloleoylphosphocholine)

bilayer.

The second is the free energy difference between water and octanol, euivalent to

the environment inside a lipid bilayer.

2esidues which can be buried inside a lipid bilayer must be in a region of the

peptide where most residues show a free energy difference in favour of being in

an octanol environment or at least being in the lipidHwater interface region.

Ghite and Gimley (AAA) showed that a sliding window of either free energy

difference will indicate the location of probably transmembrane regions, but that

the best indicator is the difference between the two values, which is the free

energy difference between the interface and octanol environments.

The free energies are calculated over a sliding window of A residues, about thesi#e of a membrane spanning alphahelix. The energy values for each residue are

added over the window.

1atabase entry( ts(opsd2human

ID OPSD_HUMAN STANDARD; PRT; 348 AA.AC P08100; Q16414;DT 01-AUG-1988 (Rel. 08, Ce!"e#$DT 01-AUG-1988 (Rel. 08, %!&" &e'e)*e +#!"e$DT 1-U%-1999 (Rel. 38, %!&" !))"!"/) +#!"e$D RHODOPSIN.GN RHO.OS H &!+/e)& (H!)$.OC 2!"!; Me"!!; C5#!"!; C!)/!"!; e"e7!"!;M!!l/!;OC "5e/!; P/!"e&; C!"!5/)/; H/)/#!e; H.RN 1RP SQUNC :ROM N.A.R MD%IN; 84


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+/@e)"&!.>;R% H. M"!".


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2unning octanol with all three plots3 gives a graph with the water-interface and

water-octanol plots.

or those regions where the diference plot is close to #ero, both the other two

plots are above the line, showing a preference for either the octanol or theinterface membrane environments rather than water.

3eceptor-)rotein(

Membrane receptors consist of transmembrane domains and the ligand

domains, functional domains, which for many membrane receptors involve

protein inase activities. n additional, specific immunological domains

contain primary epitopes of antigenic regions.

-adrenergic receptor(

7everal membrane receptors have been cloned and studied with regard to

structure and function, including the -receptors ( and /), which recogni#e

catecholamines, principally norepinephrine, and stimulate adenylate cyclase.

- and /- receptors are subtypes that differ in affinities for synthetic

anatagonists. Thus, -adrenergic receptor binds norepinephrine with a high

affinity than epinephrine, whereas the order of affinities is reversed for the

/-adrenergic receptor. The drug isoproterenol has a greater affinity for both

receptors than the two hormones. n ig /, the amino acid seuence is shown

(with single letter abbreviations for amino acids& for the /-adrenergicreceptor. " polypeptide stretch extending from -helix extends to the

extracellular space. There are seven membrane-spanning domains and these

appear also in the receptor where there is extensive homology with the /receptor. 'ytoplasmic peptide regions extend to form loops from to ,

to K, K to K and an extended chain from K. The long extended chain

from K may contain sites of phosphorylation (serine residues) of the

receptor, which is part of the receptor regulation process involving receptor

desensiti#ation. 'ell exterior peptide loops extend from to , K to K,

and tae part in ligand binding. t appears that ligand binding may occur in a

pocet arranged by the location of the membranes-spanning cylinders -K,

ig . 2ecently, reported wor suggests that the sixth transmembrane

domain may play a role in the stimulation of the adenylate cyclase activity.

!y substitution of a specific cysteine residue in the sixth transmembrane

domain, a mutant was produced that displays normal ligand binding

properties but a decreased ability to stimulate the cyclase.


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Molecular mechanism of

Signal Transduction

!ellular signal transduction

is a to-step process3

. irst, a signaling

molecule is sensed by a

receptor at a target cell

4ig5()roposed

arrangement of the -

adrenergic receptor

helices in the membrane%

*t is also proposed thathelices *6, 6*, and 6**

reside in the membrane

so as to delineate a ligand

binding poc0et, ith helix

6** centrally located%

ig /3 %roposed model for the

insertion of the / adrenergic

receptor ("2) in the cellmembrane. This model is based

on hydropathy analysis of the

human /"2. The standard one

letter code for amino acid

residues is used.


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and then the receptor is activated. Membrane-bound receptors respond

to a large spectrum of extracellular signals. Cxternal signals range

from light and odours to hormones, growth factors, and cytoines.

/. Ghen the receptor sensing the signal is a catalyst, a inase, the

response is amplified. "s diversified as the signals, are the proteins,

which respond to them. n each case, binding of a signaling molecule

converts the dormant receptor to an activate state. " mechanism

involved in the transition of a receptor from its inactive to its active

state is receptor oligomeri#ation.

"ll receptors that transmit signals from the surface of the cell to the interior,

and finally to the nucleus and the genes, have two features in common3

() The signaling molecule binds to the extracellular domain of the

membrane-inserted receptor& and

(/) 0igand binding triggers, in a cooperative manner, a change in the

domain inside the cell.1pon binding the ligand and followed by activation, ligand-receptor

complexes are eventually internali#ed. nternali#ed ligand-receptor

complexes are dissociated in acidic endocyclic vesicles and the ligand is

degraded in lysosomes, whereas the receptor may be degraded or recycled

bac to the cell surface. 2eceptor-ligand complexes may be internali#ed

together with proteins, which regulate their endocytosis and degradation.

2eceptor desensiti#ation by removal from the membrane and endocytosis is

a feature shared by single-pass tyrosine inase receptors, and serpentine,

heptahelical +-protein-coupled receptors.

)hosphorylation

" process, which, in most cases, modulates receptor signaling is

phosphorylation. n the case of +-protein-coupled heptahelical receptors,

the interaction with specific inases is the first step in shutting off their

action. n other cases, binding of a growth factor to a receptor triggers the

intrinsic receptor inase activity and leads to autophosphorylation. The

important point is that the phosphates introduced in the receptor are essential

for recognition and binding of other proteins, adaptors and transducers,which are often cytosolic protein inases and phosphatases. 7ignaling

triggered by growth factor-receptor interactions leads to a response, which is

often of global nature, such as growth, proliferation, and differentiation of

cells. +rowth factors affect the cell cycle and the cell death programmes,

which determine the fate of the cell. "lthough, many processes vital for the

cell are affected, the main target is the genome. The essence of cellular


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signaling is the transmission of signals from the surface of the cell to the

nucleus and the subseuent expression of genes. Dysfunction of the

regulatory mechanisms controlling these processes can cause malignant

transformation of cells and other diseases.

3eceptors for .pinephrine Trigger !yclic M) )roduction

Sutherland7s Model(

The current understanding of the mechanism of epinephrine (and glucagons)

action originated in the wor of Carl G. 7utherland, @r., and his colleagues

in the early A>6s. These investigators showed that epinephrine stimulates

the activity of glycogen phosphorylase, which promotes the breadown of

glycogen to glucose--phosphate, the rate-limiting step in the conversion of

glycogen to glucose. 7utherlands laboratory identified adenosine ,>-

cyclic monophosphate (cyclic "M% or c"M%) as the intracellular messengerproduced in response to extracellular epinephrine. ig 5 schemati#es the

multistep path from the initial stimulus to the elevation of blood glucose.

7everal of these steps amplify the effect of hormone binding to the receptor,

so that a single molecule of hormone can change the catalytic activity of

thousands of en#yme molecules.

Cventually, five proteins essential to the epinephrine response were

identified and purified (ig>)3 () a hormone receptor in the plasma

membrane& (/) the en#yme adenylate cyclase, which cataly#es c"M%

formation& () +sprotein, which shuttles between the receptor and adenylate

cyclase, activating the cyclase when hormone is bound to the receptor& (5) a

c"M%-dependent protein inase, which phosphorylates target en#ymes

within the cell, altering their activities& and (>) cyclic nucleotide

phosphodiesterase, which degrades c"M% and thereby terminates the

intracellular signal.


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The .pinephrine--drenergic 3eceptor !omplex

ig 53 Cpinephrine triggers a series of reaction in

hepatocytes in which catalysts activate catalysts,

resulting in greater amplification of the signal.!inding of a small number of molecules of

epinephrine to specific receptors on the cell

surface activates adenylate cyclase. or

convenient, 56 molecules of c"M% are produced

by each molecule of adenylate cyclase. These 56

c"M% molecules activate 6 molecules of the

protein inase, each of which in turn activates 6

molecule of the next en#yme in the cascade. The

amplifications shown here for each step are

probably gross underestimates.

()

ig >3 The mechanism that couple binding of epinephrine (C) to its receptor (2ec) with

the activation of adenylate cyclase molecule in the plasma membrane may be regulated bya stimulatory + protein, +sas shown or an inhibitory + protein, +i(not shown). +sand +iare under the influence of different hormones.


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The action of epinephrine begins with the binding of the hormone to a

protein receptor in the plasma membrane of a hormone-sensitive cell, a

hepatocyte or myocyte, step (). The binding is tight but noncovalent, lie

the binding of an allosteric effector to an allosterically regulated en#yme.

The binding site on the receptor is stereospecific and will accommodate only

the natural hormone ligand or molecules with a closely similar three-

dimensional geometry. 7tructural analogs that bind to a receptor and mimic

the effects of its natural ligand are called agonists& antagonistsare analogs

that bind without triggering the normal effect, and thereby bloc the effects

of agonists.-adrenergic receptorsare integral membrane proteinswith amino acidseuences that contain seven hydrophobic regions of /6 to /F residues,

suggesting that the protein traverses the lipid bilayer seven times. The

binding site for epinephrine is on the outer face of the plasma membrane& thehormone causes an intracellular change without itself crossing the plasma

membrane. The binding of epinephrine apparently promotes a

conformational change in the receptor, including the receptor domain that

protrudes on the cytosolic face of the membrane. The first stage of hormone

action of an allosteric effector on an allosterically regulated en#yme. The

structural changes in the intracellular domain of the receptor allows its

interaction with the second protein in the signal transduction pathway, a

+T%-binding protein.

&T)-Binding )rotein and denylate !yclase

n the signal-transduction pathway, the next element is a protein called a

stimulatory + protein, or +s, located on the cytosolic face of the plasma

membrane. (+s taes its name from the fact that, when bound to +T%, it

stimulates the production of c"M% by adenylate cyclase, and en#yme of the

plasma membrane). +sis composed of three polypeptides, , , and . t is

one of a large family of guanosine nucleotide-binding proteins that mediate

a wide variety of signal transductions, including those triggered by many

other hormones as well as certain sensory stimuli.+s can exist in either of two forms. Ghen its nucleotide-binding site (on the

subunit) is occupied by +T%, +s is active and can interact with and

activate adenylate cyclase. Gith +D% bound to the site, + sis inactive and

incapable of activating adenylate cyclase. !inding of epinephrine cause the

receptor to cataly#e the displacement of the +D% bound to inactive + s by


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+T%& this converts +sto its active form, step (/). "s this occurs, the and

subunits dissociate from the subunit& +swith adenylate cyclase converts

the cyclase to its catalytically active form& the en#yme catalu#es the

production of c"M% from "T%, raising the cytosolic level of this second

messenger.

"ctivation of adenylate cyclase by +s is self-limiting& +s has a wea

+T%ase activity and turns itself off by converting its bound +T% to +D%

(ig 4). The now inactive +s dissociates from adenylate cyclase, thereby

inactivating it. "fter +s reassociate with the and subunits, +s again

becomes available for interaction with hormone-bound receptor.

7ignal transduction through adenylate cyclase involves two steps in

seuence that amplify the original hormonal signal. irst, one hormone

molecule bound to one receptor catalytically activates several +smolecules.7econd, by activating a molecule of adenylate cyclase, one active +smolecule leads to the catalytic synthesis of many molecules of c"M%. The

net effect of this cascade is a very significant amplification of the hormonal

signal, which accounts for the very low concentration of epinephrine (and of

other hormones) reuired for activity.

'yclic "M%, the intracellular second messenger in this system, is short-

lived, it is uicly degraded by cyclic nucleotide phosphodiesterase to >-

"M%, step (B), which is not active as a second messenger. The intracellular

signal therefore persists only as long as the hormone receptor remainsoccupied by epinephrine. Methyl xanthines such as theophylline (a

component of tea) inhibit the phosphodiesterase, potentiating the action of

agents that act through adenylate cyclase.


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ig 43 The protein +sacts a self-

inactivating switch.

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Structure and Organization of Membranes