TransportThrough Membranes
Necessity for Transport
• Plasma Membranes
• Intracellular Membranes (Organelles)
Biological Membranes
• Organized assemblies of lipids, proteins and small amounts of carbohydrates
• Regulate composition of intracellular medium by controlling flow of nutrients, waste products, ions, etc. in and out of cell
• Scaffolding– Oxidative phosphorylation– Photosynthesis– Nerve impulses– Hormone receptors
Membrane Lipids
Generally soluble in organic solvents
Types of Membrane Lipids
• Glycerophospholipids
• Sphingolipids• Cholesterol
Glycerophospholipids(Glycerol 3-phosphate)
H2C
CH
H2C OPO32-
OH
HOGlycerol
Phosphate
Phosphatidic Acid(1,2-diacylglycerol-3-P)
O
CH
H2C OPO32-
C R1H2C
O
OCR2
OFatty Acids
Membrane Glycerophospholipids
O
CH
H2C O
C R1H2C
O
P
O
O–
O R3 Alcohol
Fatty Acids
OCR2
O
Glycerol
Fatty Acids
• Saturated and unsaturated
• Amphiphilic
COO–
Glycerophospholipid Alcohols
• Ethanolamine
• Serine
• Choline
H O C H2
C H2
N H3
+
H O C H2
C H
N H3
+
C O O
–
H O C H2
C H2
N ( C H3
)3
+
Amphiphilicity
Nonpolar Tail
(Hydrophobic)
Polar Head
(Hydrophilic)
Alcohol
P
Glycerol
Sphingolipids(Sphingosine)
CH3(CH2)12 CH CH CH
OH
CH
CH2OH
NH2
Sphingolipids(Sphingomyelin)
C H3
( C H2
)1 2
C H C H C H
O H
C H
C H2
N H C R1
O
O P
O
O
–
O C H2
C H2
N ( C H3
)3
C h o l i n e
+
F a t t y A c i d
Myelin Sheath – defective in some metabolic diseases
Cholesterol
H O
C H3
C H3
C H
C H3
C H2
C H2
C H2
C H
C H3
C H3
F l e x i b l e H y d r o p h o b i c T a i lH y d r o p h i l i c
( P o l a r H e a d )
R i g i d F u s e d R i n g
Properties of Cholesterol
• Highly hydrophobic (low solubility)
• Lowers melting point of mixed lipids
• Stiffens surface proximal region of bilayer
Occurrence of Cholesterol
• Animal plasma membranes• Organelle membranes (some)• Precursor to steroid
hormones
Properties of Lipid Aggregates
Monolayers, Micelles and Bilayers
Monolayers(very dilute solutions)
Water
Polar Head Groups
Hydrophobic Tails
Air
Micelles(single-tailed lipids)
W e d g e - S h a p e d
( V a n d e r W a a l s e n v e l o p e )
Critical Micelle Concentration
Cylindrical Lipids
Nonpolar Tail
(Hydrophobic)
Polar Head
(Hydrophilic)
Alcohol
P
Glycerol
Bilayers
6 0 Å
O u t e r L e a f l e t
I n n e r L e a f l e t
H y d r o p h o b i c
T a i l s
H y d r o p h i l i c
H e a d s
A q u e o u s P h a s e
A q u e o u s P h a s e
Liposomes
P h o s p h o l i p i d B i l a y e r
Properties/Uses of Liposomes
Single Bilayer(inner and outer leaflets)
Delivery of Therapeutic Agents
Membrane Proteins
Peripheral or Extrinsic Proteins
Integral or Intrinsic Proteins
Peripheral or Extrinsic Proteins
• Easily dissociated– High ionic strength– pH chages
• Free of attached lipid (dissociated)
• Water-soluble (e.g. cytochrome c)
• Normal amino acid composition
Integral or Intrinsic Proteins
• Not easily dissociated• Retain associated lipid (dissociated)• >average hydrophobic amino acds• Significant number hydrophilic amino
acds• Asymmetrically oriented amphiphiles• Trans-membrane proteins
Asymmetric Orientation
Integral or Intrinsic Proteins
Membrane Carbohydrates
• Mostly oligosaccharides
• Variety of sugars• Often sialic acid• Glycolipids• Glycoproteins
Membrane Structure
Fluid Mosaic Model
Carbohydrate
Peripheral or
Extrinsic Protein
Integral
or
Intrinsic
Protein
Integral
or
Intrinsic
Protein
OUTSIDE
INSIDE
Lipid
Bilayer
Membrane Permeability(Impermeable to Most Polar Substrates)
Aqueous solutions
Water-soluble compound
Membrane
Thermodynamicsof
Transport
Move Toward Equilibrium
Move from Higher Concentration
toLower Concentration
No information about rate of movement!
Kinetics and Mechanisms
ofTransport
Types of Transport
• Nonmediated Transport (Diffusion)• Mediated Transport (Transport
Proteins)– Passive-mediated Transport
(facilitated diffusion)– Active Transport
Terminology
• Carriers
• Permeases
• Porters
• Translocases
• Translocators
• Transporters
Non-mediated Transport(Permeability Coefficient)
Rate
[A]out
– [A]in
Rate
[A]out
[A]out
> [A]in
[A]in
= Constant
Mediated Transport(Transport Proteins)
Rate
[A]
Mediated Transport Properties
• Saturation kinetics• Speed and specificity• Susceptibility to
competitive inhibition• Susceptibility to
chemical inactivation
Asymmetric Orientation
Types of Transporters
Ionophores
Valinomycin
Gramicidin
Stoichiometry
Electrical Character
• Electroneutral
• Electrogenic
Passive-MediatedGlucose Transport
Erythrocyte Glucose Transporter
Asymmetric Orientation
Conformational Change
Insulin Action(Muscle and Adipocytes)
ATP-Driven Active Transport
Against a concentration gradient
Often coupled to ATP hydrolysis
Na+-K+-ATPase of Plasma Membrane(Reaction)
3 Na+(in) + 2 K+(out) + ATP + H2O
3 Na+(out) + 2 K+(in) + ADP + Pi
Electrogenic Antiport
2 K+
3 Na+
Outside
Inside
Mechanism
2 K+
3 N a+
2 K+
3 N a+
3 N a+
3 N a+
3 N a+
2 K+ – P~ PA T P
M g2 +
M g2 +
A T PA D P3 N a
+
2 K+ H
2O
A T P
B i n d i n g
~ P
F o r m a t i o n
o f
A s p a r t y l ~ P
Pi
N a+
T r a n s p o r t
N a+
R e l e a s e
K+
B i n d i n g
P h o s p h a t e H y d r o l y s i s a n d K+
T r a n s p o r t
N a+
B i n d i n g
K+
R e l e a s e
I N S I D E
O U T S I D E
Mechanism
Mechanistic Steps
Na+-dependent ATP Phosphorylation of Aspartic Acid Side Chain
K+-dependent Hydrolysis
Mechanistic Evidence
C
HC
O
CH2
C
NH
O
OPO3
2+
Aspartyl Phosphate Residue
LiB H4
H
+ C
HC
O
CH2
C
NH
H
OH
H
Homoserine
Group Translocation
Simultaneous Transportand
Chemical Modification
PEP-dependent Phosphotransferase System
(PTS)
PEP
Pyruvate
EI
EI~P HPr
HPr~P EIIIg
EIIIg
~P
Glucose
Glucose-6-P
ATP
PPi
+ cAMP
Adenylate
Cyclase
Activation
EIIg
COOH
C
CH2
OP
Soluble
Cytoplasmic
Proteins
(Common)
EIIIg
Inhibits Lactose Permease
Inhibits Glycerol Kinase
Hpr: Histidine-containing phosphocarrier protein
C
HC
O
CH2
NH
N
N
P O
–
O
O
–
Phosphohistidine
Ion Gradient-DrivenActive Transport
Free Energy of Electrochemical Gradient
(Established by ion-pumping)Power Endergonic Physiological
Processes
Na+-Glucose Symport(Mechanism)
N a
+
+ G l u c o s e
N a
+
+ G l u c o s e
O u t s i d e
I n s i d e
N a
+
G l u c o s e
N a
+
G l u c o s e
C o n f o r m a t i o n a l C h a n g e
a n d
T r a n s p o r t R e l e a s e
C o n f o r m a t i o n a l C h a n g e s
Na+-Glucose Symport(Function)
Na
+
Na
+
K
+
K
+
Glucose
Glucose Glucose
Intestinal Lumen Capillaries
Brush
Border
Cell
Na
+
–glucose symport
Na
+
–K
+
–ATPase
Glucose uniport
Lactose Permease
H
+
+ L a c t o s e
H
+
+ L a c t o s e
O u t s i d e
I n s i d e
H
+
L a c t o s e
H
+
L a c t o s e
B i n d i n gT r a n s p o r t
R e l e a s e
R e c o v e r y
C o n f o r m a t i o n a l C h a n g e s
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