Biochemistry Lecture 15 Biological membranes_1.pptx

7/27/2019 Biochemistry Lecture 15 Biological membranes_1.pptx

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BIOLOGICAL

MEMBRANES


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Objectives

Importance

Composition & Models

Important properties

Specialized structures

RBC membrane

Biomembranes

Transport mechanisms


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Importance/ Biological role

Maintenance of shape

Control of movement of molecules across

Cell-cell recognition and communication

Cellular movement


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Chemical composition

Lipids Phospholipids: most predominant

Phosphatidylcholine (lecithin) : 40-50%

Sphingolipids:

Sphingomyelins Glycolipids: 2-10%

Cholesterol: free and esterified absent in prokaryotes

Most rigid lipid in membrane

Proteins Inner mitochondrial membrane contains highest proportion

of proteins

Glycoproteins Carbohydrates do not exist in free form


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Membrane proteins

Enzymatic activity:

Na+ - K+ ATPase pump

Carrier proteins:

Translocases Signal transduction:

IP3 and DAG

Interactions with ECM and cytoskeleton

Fibronectin, Spectrin, Ankyrin Regulation of permeability

Ion channels


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Bound to either face by

electrostatic interactions/

hydrogen bonds

Released by mild

treatment

Salt solution of different

ionic strength pH alteration

Usually enzymes

Removal does not damage

integrity

Embedded deeply

(transmembrane) by

hydrophobic bonds/ van

der Waals force

Removal requires use of

detergents or organic

solvents

Transport proteins

Removal causes

denaturation of protein andloss of function

Peripheral proteins Integral proteins

Membrane proteins


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Lipid bilayer: Models

Amphipathic molecules

Polar heads point outside, Non-polar tails point

inwards

Non-polar core acts as diffusion barrier:

impermeable to polar molecules and ions

1925, Gorter and Grendel proposed lipid

bilayer model 1935, Davson and Danielle suggested

phospholipids as major constituent


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Fluid mosaic model

1972, Singer and Nicholson

Intrinsic proteins immersed in protein bilayer

(60-100A)

Extrinsic proteins loosely attached to surface

of membrane

Charecteristics:

Icebergs in seaNo flip flop


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Fluid mosaic model


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Membrane asymmetry

Proteins are inserted in asymmetric fashion

Oligosaccharide units project towards exterior

Lipids are distributed asymmetrically:

Outer leaflet:

Phosphatidylcholine

Sphingolipids

Inner leaflet: Phosphatidylethanolamine

Phosphatidylserine


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Membrane fluidity

Temperature determine fluidity

Temp : fluidity

Phase transition / melting temperature (Tm)

Composition

Short chain FA fluidity

Cis- Unsaturated FA fluidity (more the no. of double

bonds, the Tm) Cholesterol has dual role

decreases fluidity above Tm

Increases fluidity below Tm by acting like impurity


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RBC membrane

Glycophorin

Integral glycoprotein with oligosaccharide units

facing exteriorly

Determine antigen specificityAnion exchanger

Integral glycoprotein

Extrusion of bicarbonate ions in exchange of

chloride

Ankyrin

Peripheral protein on cytoplasmic side

Cross-linked to spectrin and actin


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Other Biomembranes

Micelles: when critical

concentration of lipids is

present

Liposomes formed by sonication

spheres of lipid bilayers that

enclose aqueous medium.

Clinically useful

carriers of drugs in the

circulation

targeted therapy


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Aquasomes

Most recently developed delivery system

Used for proteins and peptides

Structure: nanoparticulate, three layered, self

assembled

Central solid nanocrystalline core coated with

polyhydroxy oligomers

Biochemically active molecules adsorbed to this core Core gives structural stability and stabilizes active

biological molecules

Used for: Insulin, Hemoglobin, Antigens,

Serratiopeptidase


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Transport across cell

membranes

Lipid bilayers are semipermeable

Non lipid-soluble molecules are handled bymembrane proteins:

Channels for ions/ small mols

Transporters for larger mols


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Lipid-protein association in

membrane

Membrane phospholipids are solvents for

membrane proteins

- helical structures in proteins minimize hydrophilic

character of peptide bonds Proteins are amphipathic

Hydropathy plot

Helps to predict whether a protein can have trans-membrane location


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Specialized membrane

structures

Lipid rafts

Exoplasmic location

Cholesterol, Sphingolipids and proteins

Caveolae

Caveolin-1 protein

Flask shaped indentations in cytosolic side

Tight junctions Prevent diffusion of macromolecules

Located below apical surfaces, between cells

Proteins: occludin, claudins


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Transport processes

Passive diffusion

Carrier mediatedtransport

Facilitated diffusion

Active transport

Primary activetransport

Secondary active

transport Exocytosis and

endocytosis

Transport through

gap junctions


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Passive diffusion

Gases: Highly permeable

Water: Moderately permeable

Ions and large polar molecules: Impermeable


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Carrier-mediated transport

Mediated through integral proteins

Permeases/ Porters/ translocases

Proteins are specific (GLUT)

Can be inhibited by structural analogs

1,5-anhydroglucitol

Uniport

Symport/ Antiport: obligatory simultaneouscotransport of another mol


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Facilitated diffusion

Along concentration gradient, no energy

consumed

Transport protein hastens the process

Mechanism:

oscillation between two conformations: ping-pong

Process is reversible

Kinetics follow Michelis - Menten rate law Egs.

Glucose transporters: GLUT

Chloride transporters: Cl-

/ HCO3-

antiport andc stic fibrosis transmembrane conductance


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Rate kinetics


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Active transport

Solute moves against concentration gradient

Expenditure of energy

Primary active transport:

Energy obtained from ATP hydrolysis: Na-K

ATPase

Secondary active transport:

Substrate molecule moves coupled to another iondown its concentration gradient: Na- glucose

symport

No ATP hydrolysis, energy derived from

electrochemical gradient of the primary ion


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Sodium Potassium

ATPaseSodium Glucose symport

Primary active

transport

Secondary active

transport


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Ion channels

Transmembrane proteins

Selective to certain ions

Allow ionic transport at high rates

Regulated

Types:

Voltage gated

Ligand gated

Mechanically gated

Affected by drugs

T b


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Trans-membrane pore

systems

Small cyclic organic

molecules Shuttles for ions

Eg. Valinomycin and

Gramicidin, used asantibiotics

Tetrameric

transmembraneproteins

Permit passage ofwater only

Mutations causenephrogenicdiabetes insipidus

Ionophores Aquaporins


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Segments of plasmamembraneinvaginate andenclose small vol ofECF

Phagocytosis andpinocytosis

Primary andsecondarylysosomes

LDL mol and

receptor internalized

Release

macromolecules

Involved in

membraneremodelling

Ca++ triggers

exocytosis

Endocytosis Exocytosis

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Biochemistry Lecture 15 Biological membranes_1.pptx