Molecular Basis of Membrane Transport

MOLECULAR BASIS OF MEMBRANE TRANSPORT

Manoj S. Nair, Ph.DPostodoctoral Fellow,

Biochemistry812 Biosciences bldg, 484 w.

12th aveColumbus, OH 43210

[email protected]

Outline of Talk Types of transport across membranes1. Passive transport2. Carrier mediated 3. Active transport Nernst equilibrium for ion transport Mechanism of ion transport (K-selectivity

filter) Endocytosis of proteins/protein domains

Introduction to Cellular Biophysics A. Molecular Basis of Membrane Transport.

Essential Cell Biology Alberts, Bray, et al.

Fluid mosaic model of membranes

Dynamic diffusion of lipids in membranes

Active transport

Transport up a concentration gradient

Uses energy (ATP) May also cause

charge gradient across the membrane causing the molecule to move against the membrane potential.

Properties of “Active” membrane pumpsATPases (sometimes GTPases)Example of a Na+/K+ pump

Na+/K+ pump uses 30% resting ATP

Active Pumps are used to transport materials against their electrochemical gradient

Essential Cell Biology Alberts, Bray, et al.

a) Uniport: 1 type of solute is transportedEg: Valinomycin (K+ transport)

Valinomycin is a carrier for K+. It is a circular molecule, made up of 3 repeats of the sequence shown above.

N C H C OHC

C HC H 3H 3 C

O

C N

C HC H 3H 3 C

OHC

C HC H 3H 3 C

C O C H

C H 3

C

O

H

O

H3

V a lin o m y c in

L -v a l in e D -h y d ro x y - D -v a l in e L - la c t ic i s o v a le r ic a c id a c id

Valinomycin is highly selective for K+ relative to Na+. The smaller Na+ ion cannot simultaneously interact with all 6 oxygen atoms within valinomycin. Thus it is energetically less favorable for Na+ to shed its waters of hydration to form a complex with valinomycin.

Valinomycin

O O O

O O

Hydrophobic

O

K+

Puckering of the ring, stabilized by H-bonds, allows valinomycin to closely surround a single unhydrated K+ ion. Six oxygen atoms of the ionophore interact with the bound K+, replacing O atoms of waters of hydration.

Whereas the interior of the valinomycin-K+ complex is polar, the surface of the complex is hydrophobic. This allows valinomycin to enter the lipid core of the bilayer, to solubilize K+ within this hydrophobic milieu.Crystal structure

Valinomycin

O O O

O O

Hydrophobic

O

K+

Valinomycin is a passive carrier for K+. It can bind or release K+ when it encounters the membrane surface.Valinomycin can catalyze net K+ transport because it can translocate either in the complexed or uncomplexed state.The direction of net flux depends on the electrochemical K+ gradient.

Val Val

Val-K+ Val-K+

K+

membrane

K+

b) Symport: 2 different solutes transported together in one directionEg: Glucose –Na+ tranporter in epithelial cellsLactose permease: H+ -lactose symport

c)Antiport: 2 different solutes transport in opposite directionsEg: Adenine nucleotide translocase (ATP/ADP exchanger)

Passive transport: Ion channels & Pores

Properties of transmembrane -helices:

Amphiphilic nature

Designer Peptides of Ser & Leu:Formed a hexamer channel in phospholipid membranes.

S.R.Goodman. 1998

What is the mechanism for ion selectivity of channels? This is a frontier of biophysics.

With Passive Channels, ions or other substances move DOWN their electrochemical gradient

+ +

+++

-- -

--

-

Electrochemical Gradient

Basic structure of the potassium channel. Doyle et al. Science, 1998. Nobel Prize in Chemistry in 2003

KcsA Selectivity Filter

KvAP channel voltage sensing paddle

Avidin detection of voltage using biotinlyated KvAP

Receptor-activated gate

• “Gated” channels i.e. channels that open in response to physiological stimuli

Essential Cell Biology Alberts, Bray, et al.