CELL MEMBRANE :CELL MEMBRANE :CHEMICAL COMPOSITION , CHEMICAL COMPOSITION ,

STRUCTURE AND MEMBRANE STRUCTURE AND MEMBRANE DYNAMICSDYNAMICS

Presenter: Dr. Dnyanesh Amle

Moderator: Dr. Smita Kaushik

• Boundaries of all the cells are defined by Boundaries of all the cells are defined by biological membranebiological membrane

• Barrier with selective permeabilityBarrier with selective permeability

COMMON PROPERTIES:COMMON PROPERTIES:

• Sheet like structures

• Contains mainly lipids and proteins

• Membrane lipids are small amphipathic molecules

• Specific proteins mediate distinctive function

COMMON PROPERTIES:COMMON PROPERTIES:

• Non covalent assemblies

• Asymmetric

• Fluid structures

• Electrically polarized

• FUNCTIONS:FUNCTIONS:

– Maintenance of cell shape

– Cellular movements

– Controls movement of molecules between inside and outside of the cell

– Cell-cell recognition and communication

• MAINLY COMPOSED OF :MAINLY COMPOSED OF :– Lipids

– Proteins

– Carbohydrates

• LIPIDS LIPIDS : – Phospholipids

– Glycolipids

– Cholesterol

1) PHOSPHOLIPIDS1) PHOSPHOLIPIDS

• Based on the platform:– Glycerophospholipids( Phosphoglycerides)

– Sphingolipids

PHOSPHOGLYCERIDESPHOSPHOGLYCERIDES

FATTY ACID

FATTY ACID

ALCOHOLPHOSPHATE

HYDROPHOBIC PART

HYDROPHILIIC PART

PHOSPHOGLYCERIDESPHOSPHOGLYCERIDES

• phosphatidate (diacylglycerol 3-phosphate), the simplest phosphoglyceride

• Major phosphoglycerides are derivatives of phosphatidate

• Phosphtidylinositol: – Golgi body

– Endosomes

– Plasma membrane

• Cardiolipin:Cardiolipin:– Inner mitochondrial membrane

• Phosphtidylcholine > Phosphtidylethanolamine

• Plasmalogens:Plasmalogens: – Nervous tissue

– Heart

SPHINGOLIPIDSSPHINGOLIPIDS

• Derived from sphingosine

• CeramideSSPHINGOSINE

FATTY ACID

ALCOHOLPHOSPHATE

SPHINGOSINE

2)GLYCOLIPIDS:2)GLYCOLIPIDS:• In animal cells: derived from sphingosine

• Sugar unit is attached to primary -OH group

• Simple glycolipid : cerebroside– Phrenosine Phrenosine

• Complex glycolipid: Ganglioside

• Galactocerebroside:– Brain and nervous tissue

• Glucocerebroside:– Non neural tissue

• Ganglioside:– 5-8% lipid in brain

3)CHOLESTEROL:3)CHOLESTEROL:• Third major membrane lipid

OH

• MEMBRANE LIPIDS : AMPHIPATHIC MOLECULES MEMBRANE LIPIDS : AMPHIPATHIC MOLECULES

Sphingolipids

glyceroPhospho

lipids

Cholesterol SHORTHAND DEPICTION

AMPHIPATHIC NATURE:AMPHIPATHIC NATURE:• Micelles

• Bilayer:

leaflets

PROPERTIES OF LIPID BILAYER:PROPERTIES OF LIPID BILAYER:• Formation in aqueous environment is rapid and

spontaneous• Hydrophobic interactions: major driving force• Other forces:– Van der waal’s attractive forces – Electrostatic– Hydrogen bonds

• Co-operative structures

HYDROPHOBIC INTERACTIONS:HYDROPHOBIC INTERACTIONS:• Inherent tendency to be extensive

• Tend to close on themselves: forms compartments

• Self sealing: As hole in lipid bilayer is energetically unfavorable

• Type of structures formed depends on:– Fatty acyl chains– Degree of saturation – Temperature

• LIPOSOMES:LIPOSOMES:

sonicating

Gel filtration

Uses– To study the effect of different fatty acids on

membranes – Drug delivery– Concentrate in regions of increased blood flow :

Cell gatingCell gating– Selective fusion

MEMBRANE PROTEINS:MEMBRANE PROTEINS:

• Integral • Peripheral • Amphitropic

Inside Outside

INTEGRAL MEMBRANE PROTEINS :INTEGRAL MEMBRANE PROTEINS :

+++++++ _ _ _ _ _Phosphtidyl-inositol

MembraneAnchored protein

Cytoplasmic side

PERIFERAL MEMBRANE PROTEINS :PERIFERAL MEMBRANE PROTEINS :

• Membrane proteins structure– Electron microscopy and X-ray crystallography

• Membrane spanning α helix

BACTERIORHODOPSIN

GLYCOPHORIN:GLYCOPHORIN:

• A protein containing single trans-membrane α helical strand

• Present in plasma membrane of human erythrocytes

• Amino terminus exterior to cell contains various oligosaccharide unit including ABO and MN blood group determinants

GLYCOPHORINE

74

95

• A channel can be formed from β strands

PORINE

Prostaglandin H2 synthase

PROSTAGLANDIN HPROSTAGLANDIN H22 SYNTHASE: SYNTHASE:

CARBOHYDRATES:CARBOHYDRATES:• Rarely exists as free component• Present as glycoprotein and glycolipid• Always present on the outer side

BIOLOGICAL MEMBRANES DIFFER IN BIOLOGICAL MEMBRANES DIFFER IN COMPOSITION:COMPOSITION:

• Myelin: 18% protein , ↑ glycosphingolipids

• Plasma membrane : 50% protein

• Inner mitochondrial membrane : 75% protein,

↑ cardiolipins

FLUID MOSAIC MODEL: FLUID MOSAIC MODEL: 1972

• S Jonathan Singer & Girth Nicolson

• Membranes are two dimensional solution of lipids and globular proteins

CARBOHYDRATES

LIPIDS

PERIFERAL PROTEINS

INTEGRAL PROTEINS

MEMBRANE DYNAMICSMEMBRANE DYNAMICS• ↓ physiological temp. : gel phase• ↑ physiological temp. : liquid-disordered

state• Intermediate temp. : liquid-ordered state• Unsaturated fatty acids• Cholesterol

LATERAL DIFFUSION:LATERAL DIFFUSION:• Biological membranes are not rigid structures• Lipids > proteins are constantly in a lateral

motion• Can be detected by FRAP

• S = (4Dt)1/2

• For lipid : D= 1µm2/s• S= 2 µm/S

• Proteins differ extremely in mobility– Rhodopsin : D=0.4 µm/s – Fibronectin : D = 10-2µm/s

• fluidity increases with increase in – No of short chain fatty acids– Unsaturated fatty acids– Temperature

• Cholesterol decreases fluidity at high temp• Increases fluidity at low temp

• Flip flop occurs once in several hours

• Flip flop of proteins have not been observed

• Thus proteins play important role in preserving the asymmetry of the membrane

• But sometimes Flip-Flop is needed

TRANSVERSE DIFFUSION:TRANSVERSE DIFFUSION:

Cytosolic leaflet

MEMBRANE FLUIDITY : CLINICAL CORRELATIONMEMBRANE FLUIDITY : CLINICAL CORRELATION• ↑cholesterol : alteration in membrane fluidity• Spur cell anemia• Alcohol intoxication • Abetalipoproteinemia: ↑ sphingomyeline

↓phosphatidylcholine• Lecithin cholesterol acyltransferase deficiency• Hypertension • Alzheimer’s

MEMBRANES : ASYMMETRIC STRUCTUREMEMBRANES : ASYMMETRIC STRUCTURE• Inside-outside asymmetry:– Phospholipids – Proteins– Carbohydrates

• Regional asymmetry

• Mechanism

MICRO DOMAINS OF LIPID PROTEIN COMPLEXMICRO DOMAINS OF LIPID PROTEIN COMPLEX• Micro domain called lipid raft contains

distinctly organized bilayer structures

• Enriched in sphingolipids and cholesterols

•Biological membranes are actually mosaic of Different micro-domains

• Outer leaflet : ceramid and glycosphogilipids with long chain fatty acids → thicker

• Inner leaflet ↑ saturated fatty acids → closed packing

• Function : to segregate and concentrate specific protein and to facilitate their activity

• Proteins are activated when – several rafts fuse together – Ligands binding which favors fusion of rafts

CAVEOLAE:CAVEOLAE:• Caveoline cholesterol binding integral

membrane protein• Forces bilayer to curve inwards forming

caveolae• Functions : membrane trafficking, signal

transduction

Caveoline dimer with six fatty acid moeitis

MEMBRANE CURAVATURE : MEMBRANE CURAVATURE : • Central to ability of membrane to undergo

fusion with other membrane

• Mechanisms– Intrinsically curved protein binding – Many subunits of scaffold protein into proteins

assembled into curved supra-molecular complexes

– May insert one or more hydrophobic helices into one face of bilayer

FUSION OF SYNAPTIC VESICLE:FUSION OF SYNAPTIC VESICLE:• v-SNARESv-SNARES• t-SNARESt-SNARES• SNAP-25SNAP-25• NSFNSF

V-SNARE

t-SNARE SNAP-25

Transferrin receptor cycle

TRANSFERRIN RECEPTOR CYCLE:TRANSFERRIN RECEPTOR CYCLE:

IN A NUTSHELL IN A NUTSHELL • Biological membranes define cellular

boundaries, divide cells into discrete compartments, organize complex reaction sequences, and act in signal reception and energy transformations.

• The lipid bilayer is the basic structural unit explained by Fluid-mosaic model.

• Membranes are structarally and functionally asymmetrical.

• Lipid > proteins are continuously in a state of motion in the plane of cell membrane called lateral diffusion

• But transverse diffusion or Flip-flop of lipids is very slow except when specifically catalyzed by flippases,floppases and scramblases.

• lipid rafts are rich in sphingolipids and cholesterol consist of membrane proteins that are GPI-linked

• Specific proteins mediate the fusion of two membranes, which accompanies processes such as viral invasion and endocytosis and exocytosis

THANK YOU

HYDROPATHY PLOT HYDROPATHY PLOT

GLYCOPHORINE

STEPS IN FUSION:STEPS IN FUSION:• Recognition • Close opposing• Local disruption• Fusion

• Fusion proteins

TIGHT JUNCTION:TIGHT JUNCTION:• Present between two cells that lies in close a

approximation• Forms narrow hydrophilic channels• Prevents the diffusion of macromolecules • Only three layers of plasma membrane are

presentDESMOSOMES:DESMOSOMES: provide attachment of cells to

the basal tissue • Mostly seen in epithelial cells