Membrane Structure and Dynamics Membrane functions - physical barrier from entry and exit form cell and organelles What are membranes - Lipid bilayers with proteins imbedded or associated on either side of the membrane Ions and polar molecules basically impermeable to membrane - energy costs too high

Membrane components - 60 to 70% of mammalian lipids are phospholipids Bacteria have almost no PC and are mostly PE Neuronal tissue (myelin) PI > PC Alterations in lipid composition - permeability, fluidity, exocytosis, neural transmission and signaling potential

Membrane Asymmetry – P-ethanolamine and P-serine predominately faces inside of cell – P-choline faces outside of membrane and inside of organelles – carbohydrates of glycoproteins facing outside During apoptosis there is a re-arraignment of lipids where phosphatidyl serine moves to the exterior face of the membrane. One of the key signals of cell death

Membrane Fluidity - Singer and Nickolson fluid mosaic model - allows for dynamic nature of membrane - little transition of lipids can take place without specific enzymes to mediate transfer - flipase

Proteins - Add function and structure to membrane Extrinsic proteins (peripheral) –Loosely attached to membrane –ionic bonds with polar head groups and carbohydrates –hydrophobic bonds with lipid –proteins have lipids tails –easily displaced from membrane –salt, pH, sonication

Transmembrane portion often a helix takes about 20 aa to cross membrane many proteins cross many times odd # of transmembrane regions, why -COOH terminal usually cytosolic -NH 3 + terminal extracellular can be predicted by amino acid sequence high % of side chains will be hydrophobic Hydropathy scale used to predict free energy change - from organic to water long regions unusual in soluble proteins Non membrane sections often modified lipid, carbohydrate

Intrinsic proteins - tightly bound to membrane - span both sides Protein has both polar and hydrophobic sections removed only through disrupting membrane with detergents detergents disrupt lipid bilayer and incorporate proteins and some lipids into detergent micelles allows for purification of membrane proteins reconstitute into specific vesicles for study

Membrane associated proteins N or C terminal modifications Tightly associates protein to membrane Isoprenylated at C Terminus -Geranylgeranyl and farnesyl groups - from cholesterol biosynthesis - Lovastatin inhibits post-translational modification - deterimined for Ras and pancreatic cancer. -CAAX box - C = Cys A = aliphatic and X = various Last 4 aas are removed and new C-term is esterified with isoprenyl Other fatty acids can be modified at N terminus - Modification on amine or other amino acid residues - Myristoylation or Palmitoylation - usually occurs on Cys residues - highly reversible

Permeability - charged substances do not cross without help  measured by ability of small molecules to cross membranes Synthetic lipid vesicles formed by sonication Measure trapped ions that cross back out into solution Only charged molecule that can cross easily is water Movement slowed by transport though two environments Shed layers of hydration

Summary of membrane transport Three types of membrane transporters enhance the movement of solutes across plant cell membranes –Channels – passive transport –Carriers – passive transport –Pumps- active transport

Channels Transmembrane proteins that work as selective pores –Transport through these passive The size of the pore determines its transport specifity Movement down the gradient in electrochemical potential Unidirectional Very fast transport Limited to ions and water

Channels Sometimes channel transport involves transient binding of the solute to the channel protein Channel proteins have structures called gates. –Open and close pore in response to signals Light Hormone binding Only potassium can diffuse either inward or outward –All others must be expelled by active transport.

Remember the aquaporin channel protein? There is some diffusion of water directly across the bi- lipid membrane. Aquaporins: Integral membrane proteins that form water selective channels – allows water to diffuse faster –Facilitates water movement in plants Alters the rate of water flow across the plant cell membrane – NOT direction

Carriers Do not have pores that extend completely across membrane Substance being transported is initially bound to a specific site on the carrier protein –Carriers are specialized to carry a specific organic compound Binding of a molecule causes the carrier protein to change shape –This exposes the molecule to the solution on the other side of the membrane Transport complete after dissociation of molecule and carrier protein

Carriers Moderate speed –Slower than in a channel Binding to carrier protein is like enzyme binding site action Can be either active or passive Passive action is sometimes called facilitated diffusion Unidirectional

Active transport To carry out active transport: –The membrane transporter must couple the uphill transport of a molecule with an energy releasing event This is called Primary active transport –Energy source can be The electron transport chain of mitochondria The electron transport chain of chloroplasts Absorption of light by the membrane transporter Such membrane transporters are called PUMPS

Primary active transport- Pumps Movement against the electrochemical gradient Unidirectional Very slow Significant interaction with solute Direct energy expenditure

pump-mediated transport against the gradient (secondary active transport) Involves the coupling of the uphill transport of a molecule with the downhill transport of another (A) the initial conformation allows a proton from outside to bind to pump protein (B) Proton binding alters the shape of the protein to allow the molecule [S] to bind

pump-mediated transport against the gradient (secondary active transport) (C) The binding of the molecule [S] again alters the shape of the pump protein. This exposes the both binding sites, and the proton and molecule [S] to the inside of the cell (D) This release restores borh pump proteins to their original conformation and the cycle begins again

pump-mediated transport against the gradient (secondary active transport) Two types: (A) Symport : –Both substances move in the same direction across membrane (B) Antiport: –Coupled transport in which the downhill movement of a proton drives the active (uphill) movement of a molecule –In both cases this is against the concentration gradient of the molecule (active)

pump-mediated transport against the gradient (secondary active transport) The proton gradient required for secondary active transport is provided by the activity of the electrogenic pumps Membrane potential contributes to secondary active transport Passive transport with respect to H+ (proton)

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