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Molecular Basis of Membrane Transport

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1 Molecular Basis of Membrane Transport
Manoj S. Nair, Ph.D Postodoctoral Fellow, Biochemistry 812 Biosciences bldg, 484 w. 12th ave Columbus, OH 43210

2 Outline of Talk Types of transport across membranes Passive transport
Carrier mediated Active transport Nernst equilibrium for ion transport Mechanism of ion transport (K-selectivity filter) Endocytosis of proteins/protein domains

3 A. Molecular Basis of Membrane Transport.
Introduction to Cellular Biophysics A. Molecular Basis of Membrane Transport. Essential Cell Biology Alberts, Bray, et al.

4 Fluid mosaic model of membranes

5 Dynamic diffusion of lipids in membranes

6 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.

7 Properties of “Active” membrane pumps Example of a Na+/K+ pump
ATPases (sometimes GTPases) 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.

8 a) Uniport: 1 type of solute is transported
Eg: Valinomycin (K+ transport)

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

10 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. 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.

11 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

12 Valinomycin is a passive carrier for 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.

13 b) Symport: 2 different solutes transported together in one direction
Eg: Glucose –Na+ tranporter in epithelial cells Lactose permease: H+ -lactose symport c)Antiport: 2 different solutes transport in opposite directions Eg: Adenine nucleotide translocase (ATP/ADP exchanger)

14 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


16 What is the mechanism for ion selectivity of channels
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 + + - - - + - + - + -

17 Electrochemical Gradient



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

21 KcsA Selectivity Filter

22 KvAP channel voltage sensing paddle

23 Avidin detection of voltage using biotinlyated KvAP

24 Receptor-activated gate
“Gated” channels i.e. channels that open in response to physiological stimuli Receptor-activated gate Essential Cell Biology Alberts, Bray, et al.

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