BIOL 3090 Lecture 7 - Membranes 2024 PDF

BIOL 3090, Lecture 7 Membranes Membranes close off the the cell from its environment plasma membrane: membrane that encloses the cell  Membranes also separate different compartments within a cell there are many examples of membrane-encl...

BIOL 3090, Lecture 7 Membranes Membranes close off the the cell from its environment plasma membrane: membrane that encloses the cell  Membranes also separate different compartments within a cell there are many examples of membrane-enclosed organelles* plasma membrane: membrane that encloses the cell nucleus* nucleolus vacuole* lysosome* cytoplasm mitochondria* ER* Golgi* General structure: lipid bilayer & proteins held by noncovalent interactions lipid bilayer: two monolayers of lipids, arranged with tails to the center (impermeable to water soluble molecules) membrane proteins often span the bilayer and provide critical functions (transport, structural support, catalysis)  Most abundant type of lipid molecule in a membrane: phospholipids Lipids constitute about 50% of animal cell membrane mass (other half: protein) phospholipid characteristics: head group containing phosphate (here, a phosphoglyceride, due to glycerol backbone) two hydrocarbon tails, one of which is usually unsaturated (with 1 or more double bonds)  Different phospholipids differ in their head groups In phosphoglycerides, two carbons of the glycerol backbone bind to the two fatty acid tails (through ester bonds), and the third carbon binds to phosphate In turn, several types of head groups (choline, serine, ethanolamine) can be bound to the phosphate Note: head groups can have different properties (such as negative charge on phosphatidylserine)  Remember: despite these differences, phospholipids are all amphiphilic amphiphilic: hydrophilic (polar, water-loving) end, and hydrophobic (nonpolar, water-fearing) end hydrophilic heads on the outside; hydrophobic tails on the inside In addition, many membranes contain large amounts of cholesterol cholesterol: steroid alcohol common to eukaryotic plasma membranes (sometimes in a 1:1 ratio with phospholipids) Orients hydroxyl end towards to the polar head groups of phospholipids  Amphiphilic molecules (like lipids) undergo spontaneous packing in water Cone- shaped (big “head”, one tail) Cylinder- shaped (two tails, making a wider base) Amphiphillic molecules will “bury” their hydrophobic tails in the middle, away from water In three-dimensions, lipid bilayers form self-sealed compartments A free edge against water is energetically unfavorable (hydrophobic tails should not directly abut water molecules), so lipids will rearrange to eliminate a free edge The only way for a bilayer to prevent free edges is to form a sealed compartment (pictured to the right as a sphere)  The membranes of a cell can behave as a fluid optical tweezers: technique that uses a focused laser beam to provide an attractive force Lipids within a bilayer are not static – rather, they form a two-dimensional fluid A fluorescent dye (or gold particle) can be attached to the head of phospholipid in an isolated liposome (spherical vesicle) 1. Will it move around on one of the monolayers? 2. Will it flip from one monolayer to the other?  Lateral diffusion is fast, but flip flop rarely occurs 2. No 1. Yes lipids rarely flip from lipids can move one side to another laterally, rotate, and (polar head would flex need to move through center) Proteins called flippases allow for a lipid to flip from one side to another An inability to “flip” presents a hurdle for membrane synthesis Phospholipids are synthesized on one monolayer (the cytosolic side of the ER membrane) Proteins that “flip” the phospholipids from one side to the other must be present to “even out” the two sides of the bilayer  Two properties control fluidity of a lipid bilayer: temperature and composition In general, membranes are less fluid at lower temperature Short, unsaturated hydrocarbon tails oppose a phase transition (from liquid to gel) shorter tails reduce hydrocarbon interactions, and double bonds “kink” the structures lipids can be modified at different temperatures such that membranes stay fluid  Despite fluidity, lipid bilayers are not completely uniform at different sites Lipid Mixture 1 Lipid Mixture 2 (+ cholesterol) Red dye specific to one phase Phase segregation can be observed in liposomes of certain lipid mixtures  Lipid rafts show a specific molecular composition lipid rafts: specialized lipid domains, with a certain composition of lipid and protein Note the increased thickness (due to high levels of cholesterol): can help to preferentially recruit certain proteins to these regions  The two monolayers of each lipid bilayer are quite different in composition Example: human red blood cells (note different head groups on each side) glycolipids have sugars attached asymmetry in surface charge, due to phosphatidylserin  This asymmetry can allow an animal to differentiate live and dead cells 1. In a dying cell, phosphatidylserine (typically on the inner monolayer) will be translocated to the outside - can be due to overactivation of a scramblase (which randomly and nonspecifically transfers phospholipids in both directions) 2. This will serve as a signal for nearby macrophages to engulf the dead cell and digest it  A notable example of asymmetry: glycolipids on the non-cytosolic side Glycolipids are sugar-containing lipid molecules found exclusively on the non-cytosolic side (in both plasma membrane and intracellular membranes) - Membrane glycolipids protection - Cell-recognition processes - Interactions of a cell with its environment self-associate due to hydrogen bonds between sugars; common to lipid rafts as they have long hydrocarbon tails Remember: membranes are composed of both lipids and proteins Nearly half the mass of a membrane is attributable to proteins Many such proteins are transmembrane (i.e., they extend through the lipid bilayer) Thus, these proteins are commonly amphiphilic, too Transmembrane proteins often possess alpha-helices that span membranes 1. single-pass transmembrane protein: the polypeptide chain crosses the lipid bilayer only once 2. multipass transmembrane protein: the polypeptide chain crosses the lipid bilayer multiple times Note: transmembrane alpha-helices often interact by noncovalent interactions after being inserted sequentially Remember: hydrophobic side groups are oriented outwards Hydrophobic (nonpolar) side chains are oriented outwards from the alpha helix, such that they can interact with hydrophobic hydrocarbon chains of phospholipids Typically 20-30 amino acids regions exposed on the ends would be hydrophilic (again, remember these proteins are amphiphilic)  Hydropathy plots can help predict which amino acids span a membrane hydropathy plot: way to visualize hydrophobic/hydrophilic regions along a polypeptide (plots the amount of free energy needed to transfer a segment to water) Peaks in hydropathy correspond to hydrophobic segments single-pass multipass  Transmembrane proteins may also have strands arranged as a β-barrel Transmembrane strands arranged as a β-sheet that is rolled into a cylinder Very rigid, and can differ in number of strands Commonly form water-filled “pores” (polar amino acids line the aqueous channel, while nonpolar amino acid project away from the barrel) Other membrane proteins will be anchored by lipid attachments saturated 14-carbon saturated 16-carbon fatty acid that fatty acid that attaches to protein’s attaches to a cysteine N-terminus side chain Lastly, some membrane-associated proteins will rely on other proteins Can be attached by non-covalent bonds to polypeptide stretches on either side of the bilayer Due to these weak attachments, these proteins can be easily freed (such as by mild ionic treatments) and studied in isolation But, what about (integral) transmembrane proteins? How can they be isolated for study?  Detergents aid in the purification and study of membrane proteins detergents: small, amphiphilic molecules of variable structure, which are more soluble in water than lipids Solubilizing membrane proteins requires agents that disrupt hydrophobic associations among molecules in the lipid bilayer At high concentrations, detergents form aggregates called micelles, which can associate with (and protect hydrophobic regions of) membrane proteins  Remember: membranes are two- dimensional fluids This means that not only lipids, but also protein molecules, must also be able to move within the fluid-like membrane  Proteins of membranes can also diffuse in the plane of the membrane As with lipids, proteins can diffuse laterally, but rarely flip-flop heterokaryon from mouse- human cell fusion different antibodies recognize proteins of the different species A technique called FRAP can measure how fast proteins move in membranes Fluorescence Recovery After Photobleaching More mobile proteins: Faster, and larger, recovery Less mobile proteins: Slower, and smaller,  Like lipid subdomains, protein “corrals” can also form in membranes A sperm cell has a single membrane, A likely mechanism: but membrane tethering to other proteins do not molecules (such as the localize uniformly extracellular matrix, internal cytoskeleton, or lipid rafts)  Corrals can be demarcated by barriers made by the cortical cytoskeleton spectrin: flexible, rod-shaped cytoskeletal component that lines that intracellular surface of a cell and forms geometric arrangements As a result, the cytoskeleton can also restrict the diffusion of proteins that are not even directly linked to it Membrane proteins can help give a membrane its particular shape B. insert hydrophobic groups that increase the area of one monolayer, causing the bilayer to bend C. form rigid scaffolds that stabilize a bent conformation D. cause particular lipids to accumulate in a particular region (positive curvature can be induced by large head groups; removal of lipid heads can induce negative curvature) The inside and outside of a cell (or a compartment) show different concentrations of key molecules Membrane proteins can form conduits for the transport of molecules A lipid bilayer is highly impermeable to many types of molecules small nonpolar molecules (oxygen) diffuse rapidly small uncharged polar molecules (water) can diffuse, but more slowly large uncharged polar molecules have a harder time charged molecules cannot diffuse across, no matter how small they are  Two types of transport proteins: channels and transporters Channels interact with solute Transporters (aka carriers) much more weakly, and form bind a specific solute, then a continuous pore across the undergo a conformational bilayer (faster; active when change to release it to the open) other side  Channels and transporters can each allow the passive transport of molecules passive transport: “downhill” transport of solutes, without an input of energy for an uncharged small molecule, passive transport is driven by a molecule’s concentration gradient (difference in concentrations of the two  Electrical gradients combine with concentration gradients to define the driving force for transport Things are a little more complicated for a charged molecule The example on the far right is opposed despite the concentration gradient, because the ‘negative’ potential inside disfavors entry of negatively charged ions Electrochemical gradients take into consideration concentration and electrical gradients  Notably, only transporters allow for the active transport of molecules active transport: “uphill” transport, against electrochemical gradients Requires coupling to a source of metabolic energy, such as ATP hydrolysis  A summary for today Membranes are made of diverse lipids and proteins, but possess a general organization that defines a membrane’s properties Membranes exist as two-dimensional fluids, though a separation of phases can define unique lipid domains Membrane proteins are associated with lipid bilayers in a variety of ways, and are important for defining the concentrations of biological molecules both in and out of the cell