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BIO130 Readings Notes.docx

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University of Toronto St. George
Jennifer Mitchell

Key Words Lecture 1 Separates, boundaries p.617- plasma membrane defines the boundaries of the cell and separates the cytosol from the Ion gradients, solutes extracellular environment -specialized membrane proteins use ion gradients to Signals, change behv. synthesize ATP and drive solutes across the membrane -someproteins respond to various signals in the cell environment and change their behaviour. They’re known as receptors Transmembrane Proteins - Catalyze the synthesis of ATP to drive molecules across the membrane - Can act as a link between cytoskeleton of the cell and the extracell env or another cell - Can act as receptors -polar head, cis, saturated, packaging, fluidity p.618 -lipid molecules make up 50% of the membrane -most abundant lipids in membrane are phospholipids. Contain a polar head group + hydrocarbon tails. -one tail has cis bond (unsat.) other doesn’t (sat) -length of tails determines their packing in the -glycerol, ester bonds, head group membrane and fluidity -main phospholipids are phosphoglycerides. Contain 3 carbon glycerol linked via ester bonds to hydrocarbon tails. Also links to a phosphate which links to various head groups -sphingomyelin- amino group attached to fatty aid. Phosphocholine group attatched to terminal hydroxyl and there is a free hydroxyl which contributes to the polar prop of head group or can form H-bonds with another group p.620 Micelles, bilayer - hydrophobic molecules cause water to form cage like structures and prevents mixing in waterincreases energy -2 ways of organizing phospholipids 1. micelles: the tails are inward in a cone like shape 2. bilayers: tails sandwiched between polar heads. This is the most favoured. This protects the bilayer from Affects processes water edges Conc, temp p.622- fluidity of the bilayer needs to be regulated. Too much fluidity affects catalytic processes and transportation p623- fluidity of the bilayer depends on concentration Liquid-crystalline and temp. -change of bilayer(made from one type of phoslip) Hydrocarbons, cis from liquid to crystalline is a phase transition. Membrane harder to freeze ( lower fp) if hydrocarb. Chains are shorter or have double bonds. Want to still have fluidity at lower temps bc low temps decrease fluidity. - shorter tails = less interactions -cis double bonds= kinks= difficult to pack-fluidity at Permeability, insert, immobilize lower temps p.624 -Cholesterol affects the permeability of the bilayer. -inserts itself in the bilayer near the polar head grp and immobilizes the 1 few CH2`s of hydrocarb chains. -this decreases permeability in that region to small molecules -does not decrease fluidity. Prevent hysrocarb tails from crystallizing -bacterial membranes made of a single kind of phoslip and have no cholesterol -archea has prenyl chains rather than fatty acids -eukary. Membranes more complex- contain more cholesterol and different types of phoslip. Complexity in membrane is not only due to different phoslips, variation in head groups or varying lengths of hydrocarbon chains but also membranes contain structurally different inositol phospholipids. These minor phoslips regulate membrane traffic and cell signalling Membrane Proteins p.629 function, variety, num of proteins, -are proteins involved in the functions of the membrane -membranes vary in the num of proteins they have -PM contain 50% protein by mass -more lipid molec than proteins -diverse functions Parts, either side, amphiphilic -associate with bilayer in different ways -transmembrane proteins have parts on either side of their membrane. Hphobic parts remain on interior with the tails on phoslips while hphillic are exposed on Increasing hydrophobicity either side. -the covalent attachment of fatty acid to cytosolic monolayer increases hphobicity -different areas: Cytosolic side -some proteins are on the cytosolic side, attached via amphiphilic helix to cytosolic monolayer Lipid link -some attached to via fatty acid cov attached -some on the external env attached via a fatty acid link External env to phophatidyinositol in the lipid monolayer. GPI anchor -these proteins made in ER - transmembrane segment cleaved, GPI added, attached via GPI anchor -can be distinguished by enzyme PSPC P630 -some membrane proteins not bound directly to the hphobic interior at all. -interact with other proteins through noncov interact. -these proteins called peripheral membrane proteins: can be removed using solns that increase pH, high or low ion conc. Some not bound, peripheral, integral -integral membrane proteins cant be removed like this P631 -how a membrane protein is assoc with the bilayer determines its function -transmembrane proteins operate on either side of the bilayer-need to provide a path for the molec to pass through hphobic regions Modifications -Modifications of Src family kinases - the intercell signaling proteins that turn extracell Myristrolated, palmitic signals into intercell ones are bound to the cytosolic side 1. myristolated on the amino end of the protein 2. palmitic acid added to cysteine side chain -modifications are in response to extracell signal and brings other kinases P 632 Transmembrane bilayer crossing -transmembrane proteins are inserted in the bilayer in different orientations according to function and cytosolic, noncytosolic domains Domains separated by non polar -domains separated by segments of AAs that are non polar and interact with hphobic regions of bilayer -in absence of H20, polar peptide bonds make Hbonds with eachother maximized if segment is alpha helix Better for Hbonding -this is how they cross the bilayer - Beta barrel can also be formed for proteins to cross- better for Hbonding (porin proteins) -single pass/multipass -how the transmem prot. is associated with the bilayer is a function of its cytosolic and non cytosolic domains. -part of protein in membrane interacts with hphobic part of bilayer, has non polar AAs -water is absent so, peptide bonds form h-bonds with themselves -h-bonds increase if protein readily forms α helix as it crosses bilayer -pep.bonds can also form beta sheets in a barrel struc to increase hbonds. Eg porin (multipass) -transmembrane α helix if single pass m.proteins doesn’t aid in folding of domains on either side -Single pass membrane proteins - α helices form homodimers determined by the sequences of the AAs of the helix Multiple pass membrane proteins - α helices have certain positions in the folded protein of the bilayer which is determined by the interactions with neighbouring helices -interactions are important for structure and function of channels of transporters that move through bilayer -you can even cut a loop of transmem protein that hold the protein together, and you`ll still get linking of helices -can also express genes as separate parts of a protein and get them to work together -Xray crystallography can help to determine the 3D structure of transmembrane domains -It is possible to predict from the AA sequence which parts of the protein cross the bilayer -in multipass transmem proteins, other helices shield helices from lipid env. – function of how they were inserted by a protein translocator which after it leaves, each helix is surrounded by lipids -intr. With other helices shield protein lipid interc. Beta Barrel Proteins -found in the membranes of mitochondria, chloroplasts, and other bacteria -they are pore forming proteins and create channels within the membrane that allow for small hydrophilic solutes to pass -some loops extend out of the channel and narrow it so that only certain solutes can pass -eg. Porins-polar AAs on the inside and nonpolar AAs on outside to intr with lipid bilayer -beta barrels can be highly selective -some beta barrels are not transporters -some work as receptors or enzymes while the barrel hold the protein in the membrane allows the cytosolic loops to act as binding sites -where transmem helices can change conformations, beta barells do not Glycosyla
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