Presentation on theme: "Molecular Basis of Membrane Transport"— Presentation transcript:
1 Molecular Basis of Membrane Transport Manoj S. Nair, Ph.DPostodoctoral Fellow, Biochemistry812 Biosciences bldg, 484 w. 12th aveColumbus, OH 43210
2 Outline of Talk Types of transport across membranes Passive transport Carrier mediatedActive transportNernst equilibrium for ion transportMechanism of ion transport (K-selectivity filter)Endocytosis of proteins/protein domains
3 A. Molecular Basis of Membrane Transport. Introduction to Cellular BiophysicsA. Molecular Basis of Membrane Transport.Essential Cell Biology Alberts, Bray, et al.
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 ATPActive Pumps are used to transport materials against their electrochemical gradientEssential 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 cellsLactose permease: H+ -lactose symportc)Antiport: 2 different solutes transport in opposite directionsEg: Adenine nucleotide translocase (ATP/ADP exchanger)
14 Passive transport: Ion channels & Pores Properties of transmembrane -helices:Amphiphilic natureDesigner 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++---+-+-+-