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Manoj S. Nair, Ph.D Postodoctoral Fellow, Biochemistry 812 Biosciences bldg, 484 w. 12 th ave Columbus, OH 43210

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Presentation on theme: "Manoj S. Nair, Ph.D Postodoctoral Fellow, Biochemistry 812 Biosciences bldg, 484 w. 12 th ave Columbus, OH 43210"— Presentation transcript:

1 Manoj S. Nair, Ph.D Postodoctoral Fellow, Biochemistry 812 Biosciences bldg, 484 w. 12 th ave Columbus, OH

2 Types of transport across membranes 1. Passive transport 2. Carrier mediated 3. Active transport Nernst equilibrium for ion transport Mechanism of ion transport (K-selectivity filter) Endocytosis of proteins/protein domains

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

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6 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 ATPases (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.

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 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. 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.

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 +. 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 Properties of transmembrane - helices: Amphiphilic nature Designer Peptides of Ser & Leu: Formed a hexamer channel in phospholipid membranes. S.R.Goodman. 1998

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16 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

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

21 KcsA Selectivity Filter

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


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