Membrane Transport and Signal Transduction Biochem 4511 Figures Essential Biochemistry 3rd Ed Pratt and Cornely Principles of Biochemistry 5th Ed Moran et al Lehninger Principles of Biochemistry 5th Ed Nelson Cox Biochemistry 3rd Ed Voet Voet Fundamentals of Biochemistry 2nd Ed Voet Voet Pratt Membranes are selectively permeable barriers Diffusion across membranes dependent on hydrophobicity polarity and size of the molecule Hydrophobic molecules diffuse freely Water is able to diffuse but not well o Protein pores aquaporins assist in transport Small ions K Na Ca2 Mg2 Cl HCO3 have VERY low permeability o Membranes are good electric insulators Most Proteins also must be transported Permeabilities are correlated to the partition coefficient of the molecule or ion in organic solvents If the molecule readily dissolves in a nonpolar solvent it is more likely to pass through the hydrophobic barrier of the membrane Obviously the size of the molecule would play a part as well Hence charged ions like Na and K have low permeabilities Cl has a higher permeability since the charge density on the larger Cl anion is smaller Water has a surprisingly high permeability although it is polar It is small and can enter down deep into the head group region through sequential H bonding which must assist its transfer across the membrane Chemical Gradients in the Cell K 5 mM Na 140 mM Biological organisms are NOT at equilibrium with the environment Cells and organelles are not at equilibrium with other cells and organelles How are chemical gradients maintained in the cell Membranes are the most prominent and important barriers in our bodies Thermodynamics of Membrane Transport Charge imbalance between intracellular and extracellular generates a voltage across the membrane When a transport is independent of the effects of membrane potential When transport of an ion which is influenced by membrane potential and concentration gradient Dy RTZFlnCNa inside CNa outside 0 058logCNa inside CNa outside DG RTlnc inside c outside DG RTlnc inside c outside ZFDy Calculate the free energy change for the movement of Na into in cell when its concentration outside is 150mM and its cytosolic concentration is 10 mM Assume that T 20 C and Dy 50mV inside negative DG RTlnc inside c outside ZFDy 8 314 293 ln0 010 15 1 96485 0 05 11 6kJ mol 1 Modes of Membrane Transport Simple diffusion a substance moves through the membrane without any assistance limited to hydrophobic and small uncharged molecules CO2 O2 etc Facilitated transport a substance moves through the membrane with the aid of a channel pore or a carrier o Passive Transport a substances moves through the membrane down a concentration or charge gradient through a protein pore channel or passive carriers o Active transport a substances move through the membrane against a concentration gradient with the aid of an active carrier Movement against a gradient is thermodynamically unfavorable and typically requires energy Simple vs Facilitated Diffusion Polar molecules must shed their water shell and overcome high energy barrier to cross a hydrophobic bilayer Protein transporters can be considered catalysts that lower the activation energy barrier for transport across a membrane Classes of Transporters Transporters are classified by ligand and directionality Uniport moves one substance at a time Symport transports two different substances in the same direction Antiport moves two different substances in different directions across the membrane Passive Diffusion Channels pores are simultaneously open to both sides of the membrane Molecule movement is passive down the concentration gradient but can be selective and regulated High Concentration Low Concentration K and Na channels in neurons Propagation of a Nerve Impulse Propagation of a Nerve Impulse 2014 John Wiley Sons Inc All rights reserved Channels and Pores In general channels are weakly selective follow linear kinetics not saturated by a substrate Bacterial porins trimer of 16 strand barrel selective for small ions based on charge and size Aquaporin Selectivity Aquaporins water channels six TM helices highly selective for water while excluding protons Hydrophobic residues and two key Asn residues line aquaporin pores to prevent proton transport Bacterium K channel The potassium K selectivity filter is lined with carbonyl groups optimally positioned to coordinate a desolvated K ion This carrier is 10 000 times more permeable to K than the Na which is significantly smaller Carriers Carriers are open to only one side of the membrane at a time A conformational change attenuates accessibility May be passive or active Typically selective and regulated Gated channels undergo conformational changes 2014 John Wiley Sons Inc All rights reserved Active Transport Requires Energy Primary Active Transport Secondary Active Transport Energy of ATP Na K pump Energy of light bacterio Utilize gradients established by primary active transport rhodopsin Energy from red ox reactions E coli lactose transporter Uphill transport of one solute is coupled to downhill transport of another solute Sodium Potassium Pump Utilizes ATP Na K ATPase changes conformation as it pumps ions across the membrane The Structure of the Na K ATPase 2014 John Wiley Sons Inc All rights reserved Lactose Transport in E coli Lactose is pumped into E coli through a secondary active symporter Oxidation of a variety of substrates coupled to proton transfer generates a proton gradient which is coupled with lactose import Lactose permease works by a rocking mechanism Lactose analog in dark gray spheres 2014 John Wiley Sons Inc All rights reserved Glucose Transport is Coupled Energetically favorable Energetically unfavorable Glucose transport from intestinal space into the cell is coupled with energetically favorable transfer of Na High concentration of glucose in cell allows passive diffusion out of the cell into the bloodstream Classifying Transporters What class of transporters are demonstrated below Na K Transporter A Primary active uniporter B Secondary active symporter C Secondary active antiporter D Primary active antiporter Na Glucose Transporter A Primary active uniporter B Secondary active symporter C Secondary active antiporter D Primary active antiporter Neuron Synapse Regulated by Calcium Transport 1 Action potential triggers opening of voltage gated calcium channels 2 Influx of Ca2 induces fusion of vesicles with membrane releasing acetylcholine 3 Acetylcholine