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