Membrane Biochemistry Lecture 10 - Transport Across Biological Membranes I

BIOC*4580 – Membrane Biochemistry Winter 2024 Module 4 - Transport Processes Overview + Diffusion & Facilitated Diffusion Size, polarity, charge govern rates of diffusion across lipid bilayers Factor More permeable Less permeable Permeability ratio Size – bilayer more H2O (water) permeable to smalle...

BIOC*4580 – Membrane Biochemistry Winter 2024 Module 4 - Transport Processes Overview + Diffusion & Facilitated Diffusion Size, polarity, charge govern rates of diffusion across lipid bilayers Factor More permeable Less permeable Permeability ratio Size – bilayer more H2O (water) permeable to smaller molecules H2N-CO-NH2 (urea) 100:1 Polarity – bilayer more permeable to nonpolar molecules CH3-CH2-CH2-OH (propanol) HO-CH2-CHOH-CH2OH (glycerol) 1000:1 Ionicity – bilayer highly impermeable to ions O2 (oxygen) OH- (hydroxide ion) 109:1 Pure lipid bilayers are practically impermeable to polar and charged species The Peptide Ionophore Valinomycin shuttles K+ cyclic structure of L-lactate, D-/L-valine, D-hydroxyisovalerate doughnut shaped - binds K+ tightly in central cavity by C=O coordination, shields charge ion-selective, binds K+ 1000× more tightly than Na+ (Na+=0.95 Å, K+=1.33 Å) Valinomycin contd., nonpolar groups contact acyl chains → lipid-soluble complex shuttles K+ to other side, by diffusing through bilayer → relatively slow rate of K+ transfer: 103 K+/s per molecule Ionophores can be antibiotics – kill other cells by dissipating their ion gradients ionophores (“ion bearers”) = compounds that shuttle ions across membranes in this way A DEPSIPEPTIDE Antibiotic – What is a depsipeptide? Gramicidin A forms a dimer of β-helices (not α-helices) & a 4-Å channel 15 aa hydrophobic peptide, alternating D- and L- residues forms a wide beta-6.3 (β6.3) helix in membrane central 0.4 nm pore, hydrophobic side chains face lipid two molecules assemble to form a head-to-head helical dimer monovalent cations (Na+, K+) pass through the channel no need for diffusion of the ionophore through bilayer → relatively fast rate of K+ transfer, 107 K+/s per channel Gramicidin A is a topical bactericidal agent (e.g., Neosporin) β6.3 helix is a tertiary structure Transporters can be described analogous to Enzymes Although transporters are not enzymes per se we can describe their activity in analogy to enzymes S Enzyme P Transporters do not bring about chemical changes in their substrates but move their “substrates” from one compartment to another. Transporter Sout Sin Activation Energy is an energy barrier to transport across membranes When moving a polar or charged solute across a membrane, energy is required to: strip the solute of its hydration layer move it across the non-polar membrane environment in which it is poorly soluble. Although the energy that is used is regained on the other side, this leads to a very high energy intermediate state similar to the transition state of an enzyme catalyzed reaction The energy input required is the activation energy ΔG‡ Activation energy and the rate of transport Rate of transport depends on the rate constant k and the [S] V = k [S] (1) Equation 6-1 page 182; (Lehninger 8ed) As can be seen by equation 2 (transition state theory), k is inversely and exponentially related to ΔG‡ (k=Boltzmann constant; h = Planck's constant) kT e -DG/RT k= h (2) Equation 6-6 page 182; (Lehninger 8ed) Lowering ΔG‡ increases the rate of transport Membrane proteins lower the activation energy and thereby increase the rate of transport Transporters bind solutes via weak, noncovalent interactions This – ve DGbinding counteracts the +ve ΔGdehydration Lowers the activation energy form a transmembrane pathway lined with polar amino acids Solutes do not have to dissolve in the bilayer, further lowering ΔG‡ Same as last slide –delete the previous slide Transporters and ion channels use different mechanisms Transporters: bind their “substrates” through non-covalent interactions are therefore saturable like enzymes – Rates will not increase significantly above a certain concentration of substrates Rates reached are much lower than for channels Have a gate on either side of the membrane and both are never open at the same time Ion Channels Provides an aqueous path through membranes where inorganic ions can diffuse. Most have a gate controlled by a biological signal. When the gate is open, ions move down its electrochemical gradient through the channel. Rates approach that of diffusion. Most show some specificity but are not saturable Transport stops when the gate closes (controlled by a biological signal) or when there is no longer an electrochemical gradient.