Membrane Transport Proteins 2024 PDF

Cellular Biology & Homeostasis MEMBRANE TRANSPORT VP 2024 Clara Camargo, DVM LEARNING OBJECTIVES 1. List characteristics affecting permeability of molecules (i.e., polarity, size…) 2. Understand membrane potential and factors affecting it 3. Recognize membrane proteins and their topology 4. Provide...

Cellular Biology & Homeostasis MEMBRANE TRANSPORT VP 2024 Clara Camargo, DVM LEARNING OBJECTIVES 1. List characteristics affecting permeability of molecules (i.e., polarity, size…) 2. Understand membrane potential and factors affecting it 3. Recognize membrane proteins and their topology 4. Provide examples of common membrane transport proteins 5. List important differences between channels and transport proteins (carriers) 6. Define channels and describe their functioning (gated, leak, hormone-dependent) 7. Differentiate simple diffusion and facilitated diffusion across the plasma membrane 8. Explain active transport (primary, secondary, tertiary; endo/exocytosis) MEMBRANE TRANSPORT MEMBRANE PERMEABILITY AND TRANSPORT CYTOSOL (ICF): liquid matrix surrounding organelles CYTOPLASM: All the materials inside a cell except for the cell nucleus The interior of the lipid bilayer is hydrophobic, thus the passage of most polar molecules is restricted This allows cells to maintain concentrations of solutes in its cytosol that differ from those in the extracellular fluid and in each of the intracellular compartments 6 MEMBRANE TRANSPORT MEMBRANE PERMEABILITY AND TRANSPORT Cells must transfer molecules and ions across their membranes to maintain the homeostasis 15-30% of all membrane proteins are transport proteins Transport proteins in the cell membrane nature.com/scitable/content/transportproteins-in-the-cell-membrane-14704938/ The plasma membrane is permeable to specific molecules that a cell needs. Transport proteins in the cell membrane allow for selective passage of specific molecules from the external or internal environment. Each transport protein is specific to a certain molecule (indicated by matching colors). MEMBRANE TRANSPORT The smaller the molecule and the less strongly associated with water → the more rapidly the molecule diffuses across the membrane MEMBRANE POTENTIAL resting membrane potential = the membrane potential of an unstimulated cell Difference in the electrical charge on the two sides of a membrane due to a slight excess of positive ions over negative ones on one side and a slight deficit on the other ↑ [Na⁺] ↑ [K⁺] MEMBRANE POTENTIAL The resting membrane potential of cells is the result of an active transport (electrogenic) and a passive diffusion, as follows: Na +,K +-ATPase pumps Na+ out of the cell and draws K+ ions into the cell K+ tends to diffuse out of the cell through potassium channels to reach an equilibrium whereas negatively charged ions (phosphates and proteins) stay inside the cell The interior of the cell will turn more negative (-70 to -90 mV) MEMBRANE TRANSPORT The electrochemical gradient of a charged solute affects its transport (Electrochemical gradient = combination of membrane potential and concentration gradient of the solute) MEMBRANE TRANSPORT PROTEINS - TOPOLOGY Proteins can associate with the plasma membrane in different ways (proteins contain hydrophilic and hydrophobic regions) 1. 2. 3. 4. Single alpha helix; Multiple alpha helices Rolled-up beta sheet (beta barrel) Attached only to one layer (with one hydrophobic face) 5. Attached to the membrane by a covalently bound lipid chain 6. Via an oligosaccharide 7. 8. attached to other proteins TRANSPORT PROTEINS TRANSPORTERS SHARE COMMON STRUCTURAL FEATURES: They typically consist of 10 or more alpha helices that span the membrane (transmembrane domains) Substrate binding sites are located midway through the membrane They show two different states: inward-open OR outward-open conformation The binding sites are accessible by passageways from only one side of the membrane at one time They would be able to work in the reverse direction if ion and solute gradients were adjusted TRANSPORT PROTEINS EXAMPLES Absorption by enterocytes of the monosaccharide products of carbohydrate digestion. TRANSPORT PROTEINS Most membrane proteins cross the lipid bilayer in an alpha-helical conformation Na+/glucose cotransporter SGLT GLUT = glucose transporter, SGLT-1 = sodium (Na+)-dependent glucose cotransporter Glucose transporter GLUT 16 TRANSPORT PROTEINS  Is an antiporter membrane protein that removes calcium from cells Na+/Ca2+ exchanger (NCX)  It uses the energy that is stored in the electrochemical gradient of sodium (Na+) by Ca2+ out Bidirectional transporter → 1 x of the cell and 3 x Na+ into the cell. allowing Na+ to flow down its gradient across the plasma membrane in exchange for the counter transport of calcium ions (Ca2+) NCX exists in many different cell types and animal species is considered one of the most important cellular mechanisms for removing Ca2+ found in the plasma membranes, mitochondria and endoplasmic reticulum of excitable cells Na+/Ca2+ exchange inhibitors: a new class of calcium regulators https://pubmed.ncbi.nlm.nih.gov/17896959/ - FYI TRANSPORT PROTEINS NCKX2 Na+/Ca2+-K+ exchanger Located on neuronal cell membranes and constitutes a Ca2+ clearance mechanism, with key roles in synaptic plasticity It is associated with motor learning, memory, and cognitive functions MEMBRANE TRANSPORT The Na+, K+-ATPase is present in the plasma membrane of almost all animal cells and maintains Na+ and K+ concentration differences accross the plasma membrane From: Silbernagl and Despopoulos. Color Atlas of Physiology CHANNELS AND TRANSPORTERS TWO MAIN CLASSES OF MEMBRANE TRANSPORT PROTEINS: Channels: form pores for specific Transporters: solute (substrate) binds the solutes (ions, water, ammonia) specific transporter, which undergo a series  They interact with the solute much of conformational changes to transfer the more weakly compared to transporters  Transport will always be passive solute across the membrane  Transport can be passive or active TRANSPORTERS Each transporter can have one or more specific binding sites for its solute (substrate) Outward-open state: binding site for solutes is exposed to the outside Occluded state: binding sites are not accessible Inward-open state: binding sites exposed to the inside Transporters (carriers) can transfer solutes passively or actively GATED ION CHANNEL TYPES Examples: Channel/K+ Channel Nicotinic Ach-receptor Mechanosensitive channels Mechanosensitive channels: convert mechanical stimuli to chemical or electrical signals thereby modulating sensation. Skin: sensing vibration, pressure sensation, stretch, touch, and light touch Veins: blood pressure Cells: osmotic pressure + K LEAK CHANNEL The vestibule and the selectivity filter Leak channels are not gated, K⁺ flows downwards the concentration gradient (high to low concentration) In the vestibule (chamber), the ions are hydrated To pass the selectivity filter, ions dehydrate and interact with carbonyl groups of AA in the center of the filter Na+ is smaller than K+, it cannot be succesfully accomodated and will not be recognized in the filter K+ channel selective filter animation https://www.youtube.com/watch?v=Z1M8s9aLe4Q&t=100s AQUAPORINS: SPECIFIC WATER CHANNELS Abundant in: Facilitate osmotic water flow Cells that secrete high amounts of water  lining ducts of exocrine glands  mammary gland  sweat glands, eyes Cells that reabsorb high volumes of water  I.e., Kidneys (proximal tubules and collecting ducts)  express aquaporins on plasma membrane making water movement more efficient FYI In mammalian cells, more than 10 isoforms (AQP0-AQP10) have been identified so far https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3774486/ Aquaporins FYI Some aquaporins are hormone-responsive and play an important role in the formation of a concentrated urine in animals Anti-diuretic hormone (ADH) ↑ AQPs in the kidneys collecting ducts Water deficit  ↑ extracell. osmolarity  activation of osmoreceptors (hypothalamus) ADH secretion (post. pituitary)  ↑ AQP in plasma membrane ↑ water permeability in collecting ducts ADH or https://courses.washington.edu/pbio375/water/water-375.html TRANSPORT CAN BE PASSIVE or ACTIVE PASSIVE TRANSPORT → down a solute‘s concentration gradient  No energy required Types of passive transport Simple diffusion (solute pass through the plasma membrane) Channel-mediated transport Passive transporters PASSIVE- SIMPLE DIFFUSION Flow of solutes is always down their concentration gradient Small or non charged molecules simply diffuses through the phospholipid bilayer, crossing into the cytoplasm  No membrane proteins are involved  Direction of transport is determined by the relative concentrations of the molecule inside and outside of the cell  From a compartment with a high concentration to one with a lower concentration of the molecule PASSIVE - FACILITATED DIFFUSION It also involves the movement of molecules in the direction determined by their relative concentrations inside and outside of the cell Passage of solutes is mediated by proteins (channels or carriers) that enables large and polar (charged) molecules to cross the membrane without directly interacting with its hydrophobic interior No energy is required Solute travels across the transporter in the direction determined by  Solute’s concentration gradient  Electric potential across the membrane (in the case of charged molecules) FACILITATED DIFFUSION Is SGLT-1 working through facilitated diffusion? GLUT transporters GLUT 2 – Glucose, fructose, galactose GLUT 4 – insulin dependent GLUT 5 – Fructose Secondary active transport https://www.youtube.com/watch?v=nYC3_3hb54Q FACILITATED vs SIMPLE DIFFUSION KINETICS Simple diffusion and channel-mediated transport rates are directly proportional to the solute concentration Carrier-mediated transport is saturable recall: enzyme kinetics and order of reaction  hyperbolic curve → enzyme/transporter saturation  Reaction → first order and zero order TRANSPORT CAN BE PASSIVE or ACTIVE ACTIVE TRANSPORT → against solute‘s concentration gradient  Requires energy (usually from ATP)  It is always mediated by transporters (carrier) ACTIVE TRANSPORT Uses energy from ATP or a gradient generated by another active transporter  Active transporters can be classified according to  direction of transport  use of energy Direction Uniport Symport Antiport Energy Primary active Secondary active Tertiary active ACTIVE TRANSPORT ACCORDING TO THE TRANSPORT DIRECTION Uniporters: transport of only one molecule (i.e., H⁺ ATPase) Symporters: coupled transporters (cotransprosters) 2 molecules in the same direction (i.e., SGLT cotransporter) Antiporters: transport of 2 molecules in the opposite direction (exchangers) (i.e., Na⁺/Ca⁺² NCX exchanger; Na⁺/K⁺ ATPase) ACCORDING TO THE ENERGY SOURCE Primary active (Na+, K+ ATPase) Secondary active (Na⁺, H⁺ exchanger, NHE) Na+ Tertiary active (Proton/peptides co-transporter, PEPT) H+ Lumen Cytosol ATP ADP+Pi K+ Na+ Secondary active is driven by a gradient that was generated by a primary active transporter H+ Dipeptides Tripeptides Tertiary active is driven by a gradient that was generated by a secondary active transporter ENDOCYTOSIS Is an active transport in which molecules/substances are brought into the cell The material to be internalized is surrounded by an area of plasma membrane, which then buds off inside the cell to form a vesicle containing the ingested material.  Pinocytosis (cell drinking) Cell takes in fluids and small particles dissolved in it  Phagocytosis (cell eating) Cell engulfs a large particle, other cell, pathogen…  Phagosomes  Phagocyte https://jackwestin.com/resources/mcat-content/plasmamembrane/exocytosis-and-endocytosis EXOCYTOSIS Is a form of active and bulk transport (large quantities of molecules are released) in which a cell export molecules (e.g., neurotransmitters and proteins) Occurs via secretory portals at the cell plasma membrane called porosomes or via vesicle/membrane merging  Porosomes are permanent cup-shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell.