Presentation on theme: "Molecular Basis of Membrane Transport"— Presentation transcript:
1Molecular Basis of Membrane Transport Manoj S. Nair, Ph.DPostodoctoral Fellow, Biochemistry812 Biosciences bldg, 484 w. 12th aveColumbus, OH 43210
2Outline 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
3A. Molecular Basis of Membrane Transport. Introduction to Cellular BiophysicsA. Molecular Basis of Membrane Transport.Essential Cell Biology Alberts, Bray, et al.
6Active 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.
7Properties 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.
8a) Uniport: 1 type of solute is transported Eg: Valinomycin (K+ transport)
9Valinomycin is a carrier for K+. It is a circular molecule, made up of 3 repeats of the sequence shown above.
10Puckering 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.
11Whereas 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
12Valinomycin 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.
13b) 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)
14Passive 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
16What 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++---+-+-+-