Biol110-10-Lecture 4-Porins

Biol110-10-Lecture 4-Porins - Cellular membranes and their...

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Unformatted text preview: Cellular membranes and their proteins- Assigned Reading for next class: Chapter 12 (pg. 695 -703; 713-719) Proteins are embedded on membranes via hydrophobic surfacesHydrophobic tails Transmembrane domains Structure of an alpha helix usually 20 amino acids long Glycolipid anchor Fatty acid anchor Hydropathy plot Structure of transmembrane beta barrel. Its membrane domains cannot be predicted… Different types of membrane proteins- Term used for each protein in this intracellular membrane compartment: #1: lumenal soluble protein #2: lumenal peripheral membrane protein #3: transmembrane or integral membrane protein (single pass or multi-pass) #4: cytosolic monotopic-integral membrane protein #5: cytosolic peripheral membrane protein #6: cytosolic calcium-dependent peripheral membrane protein #7: cytosolic peripheral membrane protein #8: cytosolic lipid-anchored peripheral membrane protein Membrane proteins can be mobile as demonstrated using heterokaryons- Sendai virus induced fusion If you want to measure the mobility of a macromolecule use… FRAP (fluorescence recovery after photo bleaching) or FLIP (fluorescence loss in photo bleaching) Membrane proteins can be immobilizedA. patching B. physical barriers (or corraling) C. Anchoring Membrane proteins can be immobilized in “lipid rafts”- The lipids in the rafts are distinct from their surrounding lipids; they feature long, saturated fatty acid tails that attract certain kinds of membrane proteins Restricted diffusion of small molecules across the bilayerHow can cells exploit their semi-permeable membranes? Permeability coefficients Phospholipid bilayers are generally impermeable.Thus, transport of small molecules occurs via two types of proteins: channels and carrier proteins (permeases, transporters) Down a concentration gradient Against a concentration gradient pumps or facilitated diffusion Simple diffusion of gases across the erythrocyte membraneNo energy required. Proceeds down a concentration gradient Band 3 protein hemoglobin Carbonic anhydrase acts as a sink that promotes influx of carbon dioxide by converting carbon dioxide into bicarbonte Hemoglobin acts as a sink that promotes influx of oxygen hemoglobin Channel mediated passive transport> No stable binding to the walls of the channel > no energy consumed > proceeds down a concentration gradient. A channel created by a porin Many ion channels are gated Carrier protein Ionophores as tools in cell biology- Example: Gramicidin A allows monovalent cations through Example: Valinomycin- K+ and A23187- Ca2+ Made by microbes as defense against other organisms They operate by ‘shielding’ the charge of the solute The porin’s channel architecture features a vestibule and a selective filterpassive transport Water is removed by passage from the vestibule to the filter Selective filter Different types of gated channels for passive transport of ions- cytoskeleton Facilitated diffusion via carrier proteins- passive transport Carrier proteins contains binding sites for a specific solute(s) Undergo reversible conformational changes at random This passive transport is saturable and occurs down a concentration gradient, so no energy is required Three types of carrier-mediated transport channel carrier Conformational change Active transport via carrier proteinsMovement of solutes against a concentration gradient And the ABC transporters… MDR and cancer Coupled carriers use existing electrochemical gradients to “piggy-back” solutes A P-type proton pump: Some use energy in the form of ATP or photons Need to be coupled to energy consumption because they move ions against a concentration gradient F-type proton pumps work in reverse to generate ATP from a proton gradient: Example: acidification of lysosomes The plasma membrane sodium/potassium [ATPase] pump- Controls osmolarity in animal cells Ouabain is an inhibitor: Makes cells burst… why Epithelial cells use the energy stored in the Na+ gradient to transport glucose across an epithelial layer: transcellular transport Trans-cellular transport of glucose 1) Glucose pumped into cell by Na-powered symporter 2) Glucose passes out of the cell into the extracellular fluid and then onto the bloodstream down its concentration gradient by passive transport 3) The Na+ gradient driving the symporter is maintained by Na+ /K+ pumps on the basalateral surface. (keeps intracellular Na low) 6) Importance of the tight junction: keep the various carrier proteins segregated Ion channels are responsible for the ability of neurons to send and receive signals This voltage gated sodium channel undergoes conformational changes through an unstable intermediate Propagation of a signal in neurons involves sequential opening of voltage -gated sodium channels Carrier proteins can be purified and reconstituted into lipid vesicles to study their properties Tools in cell biology research- biochemical extraction of membrane proteins Subject membranes to centrifugation, which separates soluble (S) from insoluble (or membrane bound or membrane enclosed) material (P). ...
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