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Manoj S. Nair, Ph.D Postodoctoral Fellow, Biochemistry 812 Biosciences bldg, 484 w. 12 th ave Columbus, OH
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
Introduction to Cellular Biophysics A. Molecular Basis of Membrane Transport. Essential Cell Biology Alberts, Bray, et al.
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 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.
a) Uniport: 1 type of solute is transported Eg: Valinomycin (K+ transport)
Valinomycin is a carrier for K +. It is a circular molecule, made up of 3 repeats of the sequence shown above.
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.
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 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.
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)
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
Basic structure of the potassium channel. Doyle et al. Science, Nobel Prize in Chemistry in 2003
KcsA Selectivity Filter
Receptor-activated gate Gated channels i.e. channels that open in response to physiological stimuli Essential Cell Biology Alberts, Bray, et al.