MCB 2210 001 1 30 2014 Membranes and Proteins cont Lipids placed in water can form two types of structures to get the hydrophobic tails of phospholipids away from water o Micelles small sphere with tails pointed in o Bilayers two layers of lipids with tails pointed inwards toward each other Close upon themselves to make a continuous surface interacting with water with no edges left exposed If it s damaged the cell has to reseal itself or it will die o Which structure forms depends on the chemistry of the lipid and its concentration o It s energetically unfavorable for hydrophobic tails to interact with water Biological Membrane Composition o Different cell membranes are composed of different compositions made of lipids cholesterol and proteins o Difference in composition is related to difference in function Lipid composition affects bilayer thickness and membrane curvature o Lipids with unsaturated tails have kinks in the chain So they aren t straight Form thinner membranes o Lipids with saturated tails are straight and thus for thicker membranes o Cholesterol can help unsaturated tails to straighten out to increase membrane thickness o Small hydrophilic head groups allow curvature of a membrane while larger ones form straighter membranes Membranes are dynamic structures o Lipids don t form individual covalent bonds to each other o Flexion a rapid movement of the tails within the membrane o Lateral Shift a lipid shifts moves on the same side of the bilayer w in its leaflet Not as quick as flexion Microscopy can be used to demonstrate this type of movement Fluorescent Recovery After Photobleaching FRAP o Label phospholipids with a fluorescent probe o Shine a bright laser on a small spot on the membrane Bleached that spot makes it no longer fluorescent o Measure how long it takes for the other fluorescent lipids to diffuse into the bleached region until it s as bright as the rest of the membrane o Transverse Diffusion a lipid moves from one side of the membrane to the other changes leaflets It s rare and a slow process because hydrophobic tails would become exposed to the aqueous solution around it which is thermodynamically unfavorable Membranes are not uniform they are asymmetric o The two leaflets sides of the membranes have different compositions Lipids are synthesized in the ER and inserted into one or the other faces of the bilayer Membrane proteins flipases transversely diffuse lipids back to their correct side of the membrane o Lipids aren t distributed randomly in the plane of the membrane Some lipids especially shingolipids like to cluster This can create microdomains with locally high concentrations of certain lipids and proteins called lipid rafts Membrane Proteins o Contribute to Selective permeability Signal transduction Biochemical reactions Cell to cell interactions Membrane properties o Proteins are the basic machinery of cells Enzymatic structural and regulatory Amino acid side chain properties drive protein structure and function chemically o There s a hierarchy to protein structure Primary structure sequence of amino acids Secondary structure local shape beta pleated sheets and alpha helices Hydrogen bonding determined by the primary amino acid sequence determines secondary structure Tertiary structure overall 3D shape Determined by o Noncovalent interactions Hydrogen bonding Hydrophobic interactions Ionic interactions Polar interactions Van der Waals forces o Covalent disulfide bonds How different amino acids interact with the others and the solution Quaternary structure multi subunit assemblies Formed by interactions between subunits to form a protein complex the same interactions that form tertiary structure but instead of between amino acids between subunits Homodimer two identical subunits Heterodimer two different subunits Can be stable or trigger by a signal to assemble o Protein Folding Can occur co translationally at the same time as translation The beginning part of a protein that s already been formed during translation can interact with the environment to cause folding o However because only the beginning portion Nterminus has been translated the C terminus end portion that usually interacts with it hasn t been translated yet So the protein may not fold correctly Some proteins can fold properly on their own other don t Chaperone and Chaperonins proteins that bind to proteins during or after synthesis and help them fold properly Hsp70 family proteins the major chaperones in organisms o heat shock proteins help refold proteins that have been denatured by heat o Requires ATP an energy input o 1 The substrate binding domain of the chaperon interacts with patches often hydrophobic on unfolded protein o 2 ATP is hydrolyzed to ADP causing a conformational change of the chaperone helping the folding of the protein o 3 An input of ATP leads to the chaperone going back to its original conformation o 4 The newly folded protein is released Some proteins need even more help from protein complexes called chaperonin o Disulfide bonds stabilize protein structures in oxidizing environments create a rigid structure The cytoplasm isn t an oxidizing environment so disulfide bonds are uncommon in cytoplasmic proteins The extracellular space is however to extracellular proteins can have disulfide bonds o Most membrane proteins and secreted proteins are glycosylated covalently bonded with a sugar via processing in the ER and Golgi apparatus Oligosaccharides are used as tags to mark the state of protein folding Glycoproteins orientated so that the carbohydrate chains are in the extracellular domain o Proteins are comprised of functional domains functional parts that do specific things A protein can have multiple domains w specific functions ATP binding sites Ca2 binding sites enzymatic activity and regulation via interactions with other proteins etc