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Lecture 3

Lecture 3 - Membrane and Membrane Structure Part 2 - January 15.docx

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BIOL 2021
Patricia Lakin- Thomas

January 8, 2012 Lecture 3: Membrane Structure, Part 2 Transmembrane Proteins  Always asymmetric o This is because of the asymmetry created when pattern is synthesized by ER  Only one orientation  Alpha helix o Helix that crosses the membrane o In order for stabilization, there are a lot of hydrophobic amino acids within the membrane o Single pass or multi pass transmembrane protein (1 vs. 3 on figure)  Beta-barrel o Beta sheet polypeptide that has been “rolled up” into a barrel o Ancient protein type o Special locations  Bacteria  Mitochondria  Chloroplasts o Form pores, important as transport proteins and act as receptors o H-bonds between strands (B-pleated sheets) o 8-20 strands passing through the system  Why are membrane-spanning protein segments almost always alpha-helicies or beta barrels and not random systems? o In both alpha-helicies and Beta-barrels, the polar H-bonding groups in the peptide bond make internal H bonds with other peptide bonds o In a disordered chain, the peptide polar groups are not bounded to each other or lipids o A disordered peptide chain, thus, increases the free Energy and is thus less favoured. Membrane Protein Modifications  Disulphide bonds on exterior chain o Cystiene S – S bonds o Always on the outside  Interior of the cell is a reducing environment, which will reduce the S – S o Bonds are made in the ER  Glycosylation o Just adding a sugar, only on the exterior o Often times it is oligosaccharides – they’re used as protection from crashing o Added during transport (Ch 12, 13)  Glycoproteins  mostly protein, Proteoglycan  mostly sugar  Which bonds are not found naturally? o NEVER ALLOWED TO FIND CARBS ON THE INSIDE o Cannot have a random protein just between the leaflets o All of the alpha helix must transfer across the cell Carbohydrate Layer  Cell coat  glycocalyx  Made of glycoproteins and proteoglycans and glycolipids Membrane Protein Movement  Proteins do NOT flip flop  Figure 10.39  They will rotate and diffuse o Measured via Fluorescence Recovery After Photobleaching o o Some proteins can diffuse, others cannot  Restrict Protein Movement o Lateral Motility is movement! o Main Methods:  Assemble into aggregates  Come together with similar proteins and make a patch/raft as a bunch of proteins that will stick together  Eg. Bacteriorhodopsin  Interact with the Extracellular matrix  Tight junctions will cause proteins to not be able to pass a connection place between two membranes o Basal Lateral vs. apical
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