Exam Review - Chapter 19

Under various growth conditions transbilayer movement

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Unformatted text preview: form a bilayer shell around the protein The topology of an integral membrane protein can sometimes be predicted from its sequence Presence of unbroken sequences of more than 20 hydrophobic residues suggest that these sequences traverse the bilayer Can predict secondary structure as well o Polypeptide chain surrounded by lipids, having no water molecules will form alpha helices and beta sheets Polarity of each amino acid can be determined by measuring the free- energy change accompanying the movement of that amino acid from hydrophobic solvent to water o Positive free energy change (endergonic): hydrophobic o Negative free energy change (exergonic): hydrophilic Overall hydropathy index of a sequence of amino acids is estimated by summing the free energies of transfer for the residues in the sequence Presence of Tyr and Trp residues serve as membrane interface anchors, able to interact simultaneously with central lipid phase and aqueous phase Positive- inside rule: positively charged Lys, His, and Arg residues of membrane proteins occur more commonly on the cytoplasmic face of membranes B- barrel: where 20 or more transmembrane segments for B sheets that line a cylinder Porins: proteins that allow certain polar solutes to cross the outer membrane Beta conformation are more extended than alpha helices Some membrane proteins contain one or more covalently attached lipids Attached lipid provides a hydrophobic anchor that inserts into the lipid biylaer and holds the protein at the membrane surface Proteins have more than one attached lipid moiety o – Membrane Dynamics Biological membranes are flexible o Change shape without losing their integrity and becoming leaky o Caused by the noncovalent interactions among lipids in the bilayer Phospholipid molecules have much freedom of motion, depending on temperature and lipid composition Liquid- ordered state: below normal physiological temperatures, the lipids in a bilayer form a semisolid in which all types of motion of lipid molecules are constrained Liquid- disordered state: Above physiological temperatures, the hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon- carbon bonds of the long acyl side chains and by lateral diffusion o...
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