MEMBRANE LIPIDS,
PROTEINS & CARBOHYDRATES
The nature of cell membranes
Lipids in membranes
Proteins in membranes
Lipid-protein interactions
Carbohydrate presence & roles
Cell membranes:
In the early part of the 20th century very little
was known about cell membranes either at
the cell surface (plasma membranes) or
within the “organs” of cells (sub-cellular
organelles).
Individuals who pioneered early work in this
area were: Overton, Langmuir, Gorter, Grendel,
Danielli and Davson.
Overton found (1895) that cell membranes were lipid in nature and not
easily penetrated. Langmuir developed a device (1917) to study lipid
layers spread out on thin films to examine their air-water interfaces.
Gorter & Grendel (1925) first proposed that membranes were made up
of lipid bilayers by using Langmuir’s device.
Hydrophilic
part
Hydrophobic
part
The bilipid model is often incorrectly attributed to Danielli & Davson.
The 1925 model (above) failed to account for any proteins associated with
the membranes. Danielli & Davson’s contributions will be shown later.
It is now confirmed
that the 1925 biliipid layer
hypothesis was correct.
The lipid portion of the
structure is composed
of two lipids facing foot-tofoot in which the “feet”
(red arrow) are the
hydrocarbon tails of the
fatty acids and the hydrophilic
“heads” (polar head groups)
are exposed to an aqueous
environment (blue arrows).
BILIPID LAYERS,IN BIOLOGICAL CELLS, ARE FORMED BY THEIR
ORIENTATION AGAINST AN AQUEOUS ENVIRONMENT. HOWEVER,
BILIPID LAYERS CAN ARRANGE THEMSELVES IN A NONPOLAR
ENVIRONMENT AS WELL. BELOW IS ANOTHER ORIENTATION.
AIR INSIDE
WALL
OF
SOAP
BUBBLE
AIR OUTSIDE
THE ARRANGEMENT OF THE BILIPID LAYER IS DOMINATED BY THE
PREVAILING NEED FOR THE MOST STABLE, LOW ENERGY ASSOCIATION
OF THE LIPIDS.
AMPHIPATHIC* LIPIDS SPONTANEOUSLY FORM
STRUCTURES IN WATER (BY HYDROPHOBIC BOND
FORMATION). THE ARRANGEMENT IS NOT
ALWAYS A BILIPID LAYER.
*LIPIDS WITH A CHARGE AT ONE END OF THE MOLECULE SUCH AS A
PHOSPHOLIPID OR A FATTY ACID.
MICELLES..ARE STRUCTURES THAT FORM FROM FATTY
ACIDS. THEY ARE SMALL AND HAVE ONLY NON-POLAR
LIPID INSIDE. DETERGENTS CAN ALSO FORM MICELLES.
BILIPID LAYERS..ARE STRUCTURES THAT FORM FROM
PHOSPHOLIPIDS. THEY ARE “LINEAR” AND REMAIN STABLE
EXCEPT AT THE EDGES. BECAUSE OF THE “END PROBLEM”,
THEY WILL READILY WRAP BACK ON THEMSELVES TO FORM
CLOSED VESICLES.
LIPOSOMES..ARE SPERICAL VESSELS OF BILIPID LAYERS.
THERE MAY BE ONE OR MORE BILIPID LAYERS. CELLS HAVE
ESSENTIALLY LIPOSOME COVERINGS WITH A SINGLE BILIPID
LAYER. ARTIFICIAL LIPOSOMES MAY BE USED TO DELIVER
DRUGS AND REAGENTS FOR DIAGNOSTIC PURPOSES. HOW?
WHAT ABOUT SPHINGOLIPIDS
AND CHOLESTEROL IN MEMBRANES?
AS PART OF A BILIPID LAYER,
SPHINGOLIPIDS FIT IN A
MEMBRANE JUST LIKE
PHOSOPHOLIPIDS, BUT
THEY ALSO CONTRIBUTE
DIFFERENT POLAR HEAD
GROUPS – ESPECIALLY THE
GANGLIOSIDES.
EXAMPLE: LACTOSYLCERAMIDE
THE CARBOHYDRATES
ACT AS CELL SURFACE
IMMUNOIDENTIFIERS
membrane
(outer) surface
USUAL PART OF THE
BILIPID LAYER
CYTOPLASMIC SIDE
CHOLESTEROL IS ACTUALLY
A RATHER FLAT MOLECULE
AND IS EASILY WEDGED INTO
THE ADJOINING PHOSPHOLIPIDS
OF A BILIPID LAYER.
PHOSPHOLIPID
½ OF BILIPID LAYER
WHEN THE LIPID COMPOSITION IS MODIFIED, THE APPEARANCE &
STRUCTURE OF SUCH LAYERS MAY OR MAY NOT BE CHANGED,
FOR EXAMPLE, WHEN
CHOLESTEROL IS
INCORPORATED INTO A
BILIPID LAYER NO CHANGE IS
OBVIOUS:
CHOLESTEROL
HOWEVER:
THE BILIPID LAYER BECOMES
MORE RIGID WITH GREATER
CHOLESTEROL INCORPORATION.
+ CHOLESTEROL
THE LENGTH AND UNSATURATION OF
FATTY ACIDS THAT MAKE UP MEMBRANE
PHOSPHOLIPIDS MAY HAVE MARKED
EFFECTS ON MEMBRANE STRUCTURE
AND PROPERTIES. HERE
IT IS APPARENT THAT
UNSATURATION OF FATTY
ACIDS CAUSES TWO EFFECTS:
MEMBRANE THINNING AND
DISORDER (=INCREASE IN
MEMBRANE FLUIDITY). THE
EFFECTS PRODUCED ARE ALSO
THE SAME WITH AN INCREASE IN
MEMBRANE TEMPERATURE. AN
INCREASE IN CHAIN LENGTH WOULD
HAVE THE OPPOSITE EFFECTS. HOWEVER,
ON THE AVERAGE, CHAIN LENGTHS IN
MEMBRANES ARE LESS VARIABLE THAN THE
DEGREE OF UNSATURATION. ALSO MEMBRANE
TEMPERATURES FOR MAMMALS TEND TO BE
STABLE. NOTE THAT MEMBRANE FLUIDITY IS AN
IMPORTANT PROPERTY IN MEMBRANE FUNCTIONS
SUCH AS TRANSPORT.
PROPERTIES OF TRANSITION IN BILIPID MEMBRANES:
STATE TRANSITION. This is the property of change in
the structure of a membrane. As learned before, phospholipids/
sphingolipids with greater unsaturation form more disorganized
bilipids and give a certain “liquidity” to a membrane. Phospholipids/sphingolipids at higher temperatures do the same. The
conversion of a membrane lipid to a more liquid state ( temp.)
occurs at its transition temperature (liquid crystal formation).
Liquid crystalline state
Paracrystalline or
gel state
MOTION TRANSITION Generally, the lipid components
of a membrane are under constant motion (vibration). The
amount of vibration depends on temperature and composition.
DIFFUSION – As a result the lipids tend to spread out and
form a uniform distribution on their half of the bilipid layer.
Usually they are confined to that layer. However, in more
recent studies, flippase enzymes have been described that
move phospholipids from one side of a membrane to another.
This seem to be required for limited applications such as
initial membrane formation or maintenance.
OTHER PROPERTIES THAT LIPIDS
IMPOSE ON MEMBRANES
ASSYMETRY: THE LIPID COMPOSITION OF THE TRANSVERSE
SECTIONS (ACROSS THE BILIPID LAYER) CAN VARY BETWEEN THE
OUTSIDE AND INSIDE OF A MEMBRANE. AN EXAMPLE IS SHOWN
FOR A RED BLOOD CELL (ERYTHROCYTE). SUCH ASSYMETRY CAN
IMPOSE SPECIFIC MEMBRANE CURVATURE OR IMMUNE SPECIFICITY
TO A CELL.
ASSYMETRY FROM MEMBRANE TO MEMBRANE IN A CELL:
LIPIDS VS.
MEMBRANE
LOCATION
Two extremes in composition of membranes are indicated for mitochondria
vs. plasma membranes. In mitochondria, the amount of cholesterol is quite
small while that of phosphatidyl ethanolamine is substantial (see arrows).
On the other hand, cholesterol is 7x higher in plasma membranes while its
phosphatidyl ethanolamine level is only about ½ that of mitochondria. These
components contribute to the need for a very fluid membrane in mitochondria (lack of a stiffening molecule [cholesterol] ) and availability of
greater amount of unsaturated fatty acids (found in PE).
THE ROLE OF PROTEINS IN
MEMBRANES?
AT THE TIME THAT DAVSON AND DANIELLI WERE WORKING
ON MEMBRANES (1930s – 1940s), THEY REALIZED THAT
IT WOULD BE NECESSARY TO EXPLAIN THE ROLE OF
PROTEINS IN MEMBRANES TO SHOW HOW –
SUBSTANCES WERE TRANSPORTED THROUGH
MEMBRANES
CELL FUNCTION AND SURVIVAL COULD NOT EXIST
WITHOUT SUCH TRANSPORT.
FLUID MOSAIC MODEL ACCOUNTS FOR:
1) 2 KINDS OF PROTEINS- EXTRINSIC & INTRINSIC
2) INCLUSION OF TRANSPORT
3) INTRODUCTION OF CARBOHYDRATES ON PROTEINS
INTRINSIC PROTEINS: ALSO
KNOWN AS INTEGRAL PROTEINS,
ARE FIRMLY ANCHORED IN BILIPID
LAYERS. THESE PROTEINS PASS
THROUGH THE MEMBRANE AND
ARE INVOLVED IN MEMBRANE
FUNCTIONS SUCH AS TRANSPORT
AND HORMONE RECEPTION.
THE EXAMPLE SHOWN IS GLYCOPHORIN (A RED BLOOD CELL
PROTEIN) – NOTE:
1)
ALPHA HELIX IN BILIPID LAYER (GREEN
ARROW)
2)
CARBOHYDRATES IN OUTER LAYER (RED
ARROW)
ABO/ MN BLOOD TYPE
MARKER PROTEINS
Na-K-ATPase IS AN INTRINSIC MEMBRANE
TRANSPORT ENZYME
MECHANISM: (shown at right)
(a) It loads up 3 Na+ ions inside the cell
(b) ATP causes a decrease in Na+ affinity
(c) (d) Na+ release, then K+ uptake occurs
(e) dephosphorylation
(f) K+ release inside the cell
GENERAL
DIAGRAM
OF
Na-K-ATPase
A FEW INTEGRAL (OR INTRINSIC) MEMBRANE PROTEINS EVEN USE
b-STRUCTURES (KNOWN AS STRANDS OR BARRELS) TO CROSS
CELLULAR MEMBRANES.
SOME OTHER EXAMPLES OF INTRINSIC MEMBRANE PROTEINS ARE:
SODIUM, POTASSIUM STIMULATED-ATPase (transport of Na and K ions)
HUMAN LEUKOCYTE ASSOCIATED (HLA) ANTIGENS (antigen presentation)
TolC (outer structure membrane protein of E. coli) [uses b-barrell structure]
AQUAPORINS (transport of water molecules)
TolC
HLA ANTIGEN
AQUAPORIN
NaK-ATPase
INTEGRAL MEMBRANE PROTEINS PASS THROUGH MEMBRANES
WITH ONE OR MORE a-HELICES (sometimes with b-barrels).
THOSE PROTEINS THAT ARE INVOLVED WITH TRANSCELLULAR
TRANSPORT HAVE BETWEEN 6 AND 12 TRANSCELLULAR HELICES.
A HYRODPATHY PLOT MAY BE MADE OF A MEMBRANE PROTEIN’S
AMINO ACID SEQUENCE VS. THE HYDROPATHY SCALE OF ITS
AMINO ACIDS (e.g. VAL= 4.2 vs. LYS= -3.9 where more positive numbers
are more hydrophobic) TO DETERMINE WHERE THE SEQUENCE
CROSSES THE CELL MEMBRANE. AS SHOWN, RHODOPSIN WOULD
HAVE SEVEN TRANSMEMBRANE a-HELICES.
EXTRINSIC PROTEINS.
ALSO CALLED PERIPHERAL
PROTEINS. THESE ASSOCIATE
WITH THE MEMBRANE BY
ELECTROSTATIC INTERACTIONS
AND HYDROGEN BONDING. THEY
ARE ISOLATED BY MILD TREATMENTS SUCH AS SALTING OUT.
(INTEGRAL PROTEIN ISOLATION??)
THE EXAMPLE IS CYTOCHROME C.
IT IS ONE OF THE PROTEINS THAT
IS INVOLVED IN OXIDATIONREDUCTIONS IN THE OX-PHOS
PATHWAY. IT IS ATTACHED TO THE
CRISTAE ON THE INTERMEMBRANE SIDE OF MITOCHONDRIA.
HOW ARE LIPIDS & PROTEINS “ASSOCIATED” IN MEMBRANES?
IT WAS KNOWN EARLY ON THAT EXTRINSIC MEMBRANE PROTEINS
COULD BE EASILY REMOVED FROM MEMBRANES BY EXPOSING THE
MEMBRANE TO SALT SOLUTIONS THAT WERE STRONGER THAN THE
IONIC (SALT) SURFACE FORCES ON THE INNER AND OUTER FACES OF
THE LIPIDS THEMSELVES. THESE “FORCES” ONLY SERVE THE ROLE
OF KEEPING THE PROTEINS IN PLACE IN A SOMEWHAT LOOSE MANNER.
THE EXTRINSIC PROTEINS ATTACH TO THE MEMBRANES BY: IONIC
INTERACTIONS WITH THE LIPIDS OR EVEN PARTS OF INTEGRAL
MEMBRANE PROTEINS THAT PROJECT OUT OF THE MEMBRANE. EVEN
SHORT LOOPS OF HYDROPHOBIC AMINO ACIDS THAT STICK INTO THE
LIPID ARE FOUND, BUT THESE ARE NOT SO PERMANENT THAT THEY
CAN’T BE REMOVED BY “SALTING OUT” THE OTHER PARTS OF THE
PROTEIN.
THE ATTACHMENTS OF INTEGRAL MEMBRANE PROTEINS WITHIN THE
LIPID PORTION OF THE MEMBRANE ARE MORE ROBUST (STRONGER)
THAN PERIPHERAL PROTEINS AND ARE OFTEN ASSOCIATED WITH
SOME MEMBRANE FUNCTION . IT HAS ALREADY BEEN SHOWN THAT
a-HELICES AND b-STRUCTURES (i. e., BARRELS AND STRANDS) OF
INTEGRAL MEMBRANE PROTEINS ARE USED JUST TO HAVE A PROTEIN
CROSS (PENETRATE) THE LIPID MEMBRANE.
IN ADDITION: ANCHORING (= POSITIONING) OF INTEGRAL PROTEINS
TO MEMBRANE LIPIDS MAY ALSO REQUIRE OR SUBSTITUTE SEPARATE
KINDS OF BONDS FOR PROTEIN FUNCTION TO OCCUR –
AMIDE-LINKED MYRISTOYL ANCHORS; THIOESTER-LINKED FATTY ACYL
ANCHORS; THIOETHER-LINKED PRENYL ANCHORS; AND GLYCOSYL
PHOSPHATIDYLINOSITOL ANCHORS.
MYRISTOYL = PROTEIN LINKED TO MYRISTIC ACID (C14 FATTY ACID)
FATTY ACYL = PROTEIN LINKED TO A FATTY ACID (NON-SPECIFIC)
PRENYL = PROTEIN LINKED TO A PRENYL GROUP: C-C=C-C-(CH3)2
HERE ARE TWO EXAMPLES: THE PROTEIN ON THE LEFT IS LINKED
TO MYRISTIC ACID THROUGH AN AMIDE GROUP. THE MYRISTIC ACID,
IN TURN IS PART OF THE LIPID BI-LAYER. THE PROTEIN ON THE RIGHT
IS LINKED TO PALMITIC ACID THROUGH A THIOESTER. THE PALMITIC
ACID IS PART OF THE LIPID BI-LAYER. HOWEVER, IN ADDITION, THE
PROTEIN TRAVERSES THE LIPID BILAYER WITH SEVERAL a-HELICES.
MYRISTIC
ACID
PALMITIC
ACID
PROTEIN
PROTEIN
A REAL EXAMPLE OF AN INTRINSIC PROTEIN WHOSE POLYPEPTIDE
CHAINS DO NOT ENTER THE BILIPID LAYER IS THE “REGGIE” PROTEIN.
REGGIE-1 IS HELD TO ITS
MEMBRANE BY MYRISTOLATION AND PALMITOLATION
NEAR THE N-TERMINAL
END OF THE PROTEIN.
THESE DOUBLE ANCHORS
DIRECT THE REMAINDER
OF THE MOLECULE TO
CARRY OUT ITS FUNCTIONS
ON THE INSIDE OF THE
CELL.
REGGIE-1 HAS BEEN
ASSOCIATED WITH THE
REFORMATION OF THE
CYTOSKELATIN, CELL
ADHESION, ENDOCYTOSIS
AND OTHER FUNCTIONS.
OUTSIDE OF CELL
THE CONCEPT OF A MEMBRANE “RAFT”
IT IS CURRENTLY HYPOTHESIZED (=THOUGHT) THAT THERE ARE
REGIONS ON A MEMBRANE BILIPID IN WHICH LATERAL DIFFUSION
(=MOVEMENT) IS HINDERED. THESE REGIONS ARE DESCRIBED AS
BEING RICH IN CHOLESTEROL AND GLYCOSPHINGOLIPIDS
(HAVING ACYL GROUPS THAT ARE LONG CHAINED AND SATURATED).
SUCH AREAS ARE ALSO KNOWN AS MICRODOMAINS OR MEMBRANE
RAFTS. THEY ARE CONSIDERED TO BE SMALL, MOBILE AND WITH
SHORT ½ LIVES. ONLY INDIRECT EVIDENCE HAS POINTED TO THEIR
EXISTENCE. THE ADVANTAGE OF A RAFT IS TO FACILITATE THE
FUNCTIONS OF SIGNALING (=RECEPTOR) PROTEINS.
GPI = GLYCOSYL PHOSPHOINOSITOL ANCHOR.
CAVEOLIN = A PROTEIN
INVOLVED IN SIGNALING &
ENDOCYTOSIS.
NOTE: ORANGE GLYCOSPHINGOLIPIDS AND
YELLOW CHOLESTEROL.
CARBOHYDRATES IN MEMBRANES
CARBOHYDRATES ARE, OF COURSE, NOT COMPATIBLE WITH THE
APOLAR NATURE OF A MEMBRANE. THEY ARE ASSOCIATED WITH
MEMBRANE LIPIDS UNDER THE GROUP KNOWN AS SPHINGOLIPIDS
(= GLYCOLIPIDS) AND THEY ARE ALSO COVALENTLY BOUND TO MANY
MEMBRANE PROTEINS (= GLYCOPROTEINS). THEY ARE POSITIONED ON
THE OUTER FACE OF A MEMBRANE (OFTEN ON THE OUTSIDE). THEY MAY
FORM A LOOSE ASSOCIATIONS WITH OTHER CARBOHYDRATES KNOWN
COLLECTIVELY AS THE GLYCOCALIX (LITERALLY SUGAR COAT) OF A CELL.
MANY CARBOHYDRATES ON MEMBRANES
SERVE THE ROLE OF IMMUNE IDENTIFIERS
(“FRIEND OR FOE?”) FOR THE ORGANISM.
OTHERS MAKE UP COMPLEX SYSTEMS THAT
COAT THE MEMBRANES OF MICROOGANISMS
AND SERVE AS PROTECTIVE DEVICES
FROM ATTACK OR ACT EVEN AS OSMOTIC
STABILIZERS FOR THE ORGANISMS’
MEMBRANE.
SYNDECAN (RIGHT) ACTS AS A MOLECULAR
GLUE FOR OTHER MOLECULES IN THE EXTRACELLULAR MATRIX.
A FEW OTHER FACTS ABOUT CARBOHYDRATES
THAT BELONG TO MEMBRANES:
-The glycocalyx may be involved in a process called
“lymphocyte homing” that helps guide white blood cells
to a cell requiring immunological intervention – that is
they serve as markers.
-Many of the oligosaccharides that make up a cell surface
glycocalyx end in sialic acid which has a negative charge.
This charge repels substances from approaching a cell
surface.
PUTTING IT ALL TOGETHER
MEMBRANES ARE COMPOSED OF LIPIDS,
PROTEINS AND CARBOHYDRATES FORMED
TO MAXIMIZE THE MEMBRANE’S FUNCTION
Remember that carbohydrates only are associated with membranes
by binding to certain lipids and proteins. They do not enter the bilipid layer.
WHAT IS ESSENTIAL TO UNDERSTAND:
1. The nature of a bilipid layer in a cell membrane and how phospholipids
form the double layer.
2. How does one distinguish between a micelle, a bilipid layer and a liposome?
3. How do sphingolipids and cholesterol contribute to the characteristics of a
membrane?
4. What is important about fatty acid chain length and degree of unsaturation
in membrane fluidity?
5. What is meant by “transition” in a membrane?
6. How are membranes asymetric and what does that impose on a membrane?
7. What was the contribution of Davson & Danielli to the understanding of
membrane structure?
8. How does one distinguish between an intrinsic (integral) and extrinsic
(peripheral) membrane protein? Give an example.
9. How MIGHT a hydropathy plot identify an intrinsic protein?
10. How do proteins and lipids associate (bind) together in a membrane?
11. How do intrinsic proteins bind to a lipid bilayer if they do not penetrate the
the membrane with alpha helices and what are membrane rafts?
12. How do carbohydrates associate with membrane bilayers?