1Biochemistry I Fall Term, 2004November 3, 2004Lecture 24: Biological MembranesAssigned reading in Campbell: Chapter 7.3-7.5Key Terms:Integral membrane proteinPeripheral membrane proteinCholesterolFluid mosaic modelElectrochemical potential ∆G = RTln([in]/[out]) ∆G = ZAF∆ΨReceptor proteinsLinks:(I) Review Quiz on Lecture 24 concepts(I) Membranes: Flash tutorial and key features(S) Examples of integral membrane proteins on Chime pages: Bacteriorhodopsin Maltoporin Hemolysin.Overview of Biological MembranesDifferent types of lipids – composition varies (type & acyl chain) by organism, tissue type, andfor prokaryotes, growth temperature.Defines cellular topology (i.e. an inside and an outside).Phospholipid bilayers are impermeable to molecules or ions without protein transporters.1. Transport of molecules and ions occurs across membranes.2. Signal transduction occurs across the membrane.3. Nerve conductance involves membrane potential changes.4. Generation and maintenance of an electrochemical potentialMembrane Energetics1. A concentration difference across the membrane can be used to generate free energy.ATP synthesis in the mitochondria utilizes the concentration gradient generated by pumpingprotons out. The free energy that results due to a concentration difference can be calculatedfrom: ∆G = RTln([in]/[out])For example, a concentration difference of 103 (mM versus µM) corresponds to 17 kJ/mol.22. The unequal distribution of ions across cellular membranes results in an electricpotential difference. Typically, the voltage difference is about -100 mV inside. The free energyavailable from the membrane potential for transport of ions is calculated from:∆G = ZAF∆Ψwhere ZA, is the charge on A; F is the Farraday constant, 96.5 kJ/volt-mol; and ∆Ψ is themembrane potential.Consequently, if a charged molecule or an ion (A) is transported into the cell, we mustconsider both the chemical and the electrical work required. At equilibrium, the electrochemicalpotential of A is∆G = RT1n([A]in/[A]out) + ZAF∆ΨCholesterolOH• About the same length as C16 fatty acid; therefore it reaches across half of the bilayer.• Required as a precursor for steroid hormone synthesis.• Used to make bile acids: a biological 'detergent' used to solubilize fats in the smallintestine.• Essential component of most mammalian membranes.• Destroys the steep phase transition of pure lipid membranes, thereby keeping themembranes fluid.• Synthesized in mammals, where the levels can be elevated by:• High intake.• Genetic deficiencies, e.g. in the LDL carrier proteinPeripheral Membrane Proteins• Loosely attached to membranes via electrostatic interactions – released with high salt.• Often involved in electron transport and, as specific binding proteins, sugar transportin bacteria.3Integral Membrane Proteins• Largely contained in the membrane (requires disruption of the membrane bydetergents for solubilization).• Stability energetics are similar to water soluble proteins, except that non-polar groupsinteract with acyl chains in the membrane. Surface residues are more hydrophobicthan the hydrophobic core.• Usually span the entire membrane.• Asymmetry is required for most functions:Cell surface markersTransport (e.g. of protons, metabolites, electrons)• Fluid mosaic model (Campbell, Fig. 7.18)Examples of Membrane Protein Structures and Functions1. Membrane receptors: the LDL receptor (Campbell, Fig. 7.27)The LDL receptor participates in endocytosis.Hormone receptors signal physiological conditions to the cell interior.2. Three examples of integral membrane proteins on Chime pages (Not in Campbell):Note some of these features in common:a) All have hydrophobic surfaces exposed to the bilayer.b) Lateral motion in the bilayer plane occurs, but not "flip-flop" across.c) Glycosylation occurs at residues on the outside surfaced) Most have two "girdles" of Trp residues at the inner and outer leaflet surfaces.Bacteriorhodopsina. The protein has seven α-helices that span the bilayer.b.The charged/polar residues are at the solvent interfaces and line the internalhydrophilic channel.Maltoporina. This trimeric bacterial protein has 18 β-strands in each β-barrel spanning thebilayer.b. Several aromatic side chains line the sugar binding site.4Hemolysina. Another all β-protein made up of seven subunits that assemble in themembrane.b. The 15 Å pore created in red blood cell membranes accounts for the cytolyticproperties of this bacterial toxin.Lipid-Linked Proteins• S-Farnesyl/geranylgeranyl cysteine methyl ester (anchor and trafficking)• Myristalization (attached to amino-terminal Glycine)• Attachment of palmitic acid to Cys.• GPI (glycosylphosphatidylinositol )