membrane proteins ecb fig. 11-4. membrane proteins have a variety of functions
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Membrane proteins
ECB Fig. 11-4
Membrane proteins have a variety of functions
Association of proteins with membranes
Fig. 11-21
helixor barrel
-helix -helix transmembranetransmembrane
domaindomain
Transmembrane proteins span the bilayer
Hydrophobic R groups of a.a. interact with fatty acid chains
Nonpolar a.a.
Polar a.a.
Multiple transmembrane helices in one polypeptide
Hydrophilicpore
Membrane transporter for polar or charged molecules
Bleach with laser beamBleach with laser beam
If protein is mobile If protein is mobile then fluorescent then fluorescent
signal moves back into signal moves back into bleached areableached area
Mobility of transmembrane proteins
Recovery rate measuresmobility
ECB Fig. 11-36
QuickTime™ and a decompressor
are needed to see this picture.
Peripheral membrane proteins(associated with membrane, but not in bilayer)
Membrane Proteins
Enzyme mechanismsKinetic parameters of enzymes
Binding sitesFree energyActivation energy, enzyme function
Lecture 5 (cont’d)
Proteins as enzymes
Proteins as membrane transporters
Enzyme (protein)Enzyme (protein)
Binding siteBinding site
SubstrateSubstrate(ligand)(ligand) Non-covalent Non-covalent
interactionsinteractions
Enzymes bind substrates
ECB Fig. 4-30
Free energy is amount of useful energy available to do work
How do enzymes work?Start by considering free energy
In a chemical reactionG = Sheat; heat released is negativeS = entropy (randomness); increased
randomness is positive
Reactions occur spontaneously if G is negative
G (Delta G) = free energy change(Reactants - Products)
Enzymes lower activation energy but have
NO effect on G
GG
ActivationActivationenergyenergy
Energy of Energy of reactantsreactants
Energy Energy of of
productsproducts
Uncatalyzed reaction Catalyzed reaction
ECB Fig. 3-13ECB Fig. 3-13
Uncatalyzed reactionUncatalyzed reaction
XX YY
Enzyme catalyzed Enzyme catalyzed reactionreaction
XX YY
Enzymes accelerate reaction rates
ECB Fig. 3-26
Enzymes can holdsubstrates in positions
that encourage reactions to occur
Enzymes can change the ionic environment of substrates,accelerating the reaction
Enzymes can put physicalEnzymes can put physicalstress on substratesstress on substrates
Lower activation energyLower activation energy
How do enzymes accelerate reactions?
Adapted from ECB Fig. 4-35
YY
Solution: couple to reaction where G -
(Often hydrolysis of ATP)
Thermodynamically Unfavorable Reactions (G+)
G +
X Y G +
ATP ADP + PiG -
X + ATP Y + ADP + Pi + G -
Many reactions in cells have positive G:e.g. condensation reactions (forming polymersreduces randomness so S -, G +)G = S
Example of coupled reaction:synthesis of sucrose
ECB Panel 3-1
G values areadditive
ADP + PADP + Pii
+ energy+ energy
ATPATP
G of hydrolysis = -7.3 kcal/mole
(Nucleotide)
Binding in the activesite can prevent substrateinteraction
Enzymes can be regulated
Inhibitorscan bind to active site
Enzymes can be regulated at sites other than the active site
Example: phosphorylation
Fig. 5-36
ECB 4-41
Membrane Proteins
Proteins as membrane transporters (Ch 12 ECB)
Protein Secondary Structure
ChannelCarrier proteins
Facilitated diffusionActive transport
Lecture 5 OutlineLecture 5 Outline
Proteins as enzymes
Properties of a pure synthetic lipid bilayer
IONSH+, Na+, HCO3
-, K+, Ca2+, Cl-, Mg2+
Large, unchargedPolar molecules
Amino acids, glucose, nucleotides
Small Uncharged polar molecules
H2O, glycerol, ethanol
Small hydrophobicMolecules
O2, CO2, N2, benzene
Lipid Bilayer Permeability
ECB 12-2
Transmembrane proteins allow movement of molecules that cannot move through
bilayer
But it is not that simple……………
ECB 12-1
Charged molecules - transport influenced by concentration gradient and membrane potential (electrochemical (EC) gradient)
outout
inin
Concentration Concentration gradient onlygradient only
Membrane impermeability results in electrical and chemical gradients across membrane
Conc. Gradient withConc. Gradient withmembrane potential (-) membrane potential (-)
insideinside
ElectrochemicalElectrochemicalgradientgradient
ECB 12-8
Ion gradients across the plasma membrane
Different electrochemical gradient for each ion
pH 7.2* pH 7.4*
Electrical and concentration gradient can be opposite (e.g. K+)
- Need to get an impermeable molecule across the membrane - going WITH its electrochemical gradient
- Need to get a molecule (permeable or impermeable) across the membrane going AGAINST its electrochemical gradient
Solution -- specialized membrane proteins for transport functions.
Transport problems faced by cells:
Two broad classes of transmembrane proteins
A. channel proteinA. channel protein
B. carrier proteinsB. carrier proteins
Conformational change
ECB 12-3
Transport can be passive or active
electrochemical
ECB 12-4
Channels - Channels - Passive transport down
elecrochemical gradient
Channel Channel proteinprotein
Channel-mediatedChannel-mediateddiffusiondiffusion
(facilitated diffusion)(facilitated diffusion)
Impermeable
ECB 12-4
Channel structure
ECB 11-24
Aqueous pore due to polar and charged R groups
Always passive transport
Mechanism of K + channel selectivity
Slower than channels
Transfer across membrane driven by conformational change in transporter
Binds transported ligand - highly specific
Carrier Proteins:
Active transportActive transport(energy-driven)(energy-driven)
Transport against EC gradient
Carrier mediatedCarrier mediatedDiffusionDiffusion
(facilitated diffusion(facilitated diffusiondown EC gradient)down EC gradient)
ECB 12-7
Active transport - three types
-uses energy to drive transport against EC gradientthrough carrier protein
ECB 12-9
Antiport- move oppositeAntiport- move opposite
directionsdirections
CotransportedCotransportedMolecule Molecule
(against EC gradient) (against EC gradient) Down EC gradientDown EC gradient
Symport- move sameSymport- move samedirectiondirection
Coupled transport
ECB 12-13
Move glucose against its EC gradient, using the energy stored in the Na+
gradient.
Na-Glucose symporter
ECB 12-14
ATP-driven pumps
ATP
ADP + Pi
Move against EC gradient
Typically move ions generating EC gradientEC gradient can then be used in coupled transport
Na+/K+ pump in animal cells
ECB 12-10
Cyclic transport by Na+/K+ pump
Phosphoryation regulates the enzyme conformation
Conf. change 1
Low affinityNa binding sites
High affinityK binding sites
Conf. change 2
High affinityNa binding sites
Low affinityK+ binding sites
33
2
22
3
NaKATPase.avi
Chemiosmotic coupling of pumps and cotransport
HH++ transporters in transporters in vacuole and vacuole and lysosome are lysosome are similarsimilar
Osmosis
Osmosis: movement of water from region of low solute concentration to region of high solute concentration (or high water potential to low water potential)
How do cells prevent osmotic swelling?
ECB 12-17
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