Chapter 9 - Lipids and Biological Membranes

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Another example of a membrane protein is bacteriorhodopsin, a 247-residue proteinconsisting of a single polypeptide chain. The membrane-spanning domain of Bacteriorhodopsinconsists of seven alpha helical segments (See Figure 9-22). Bacteriorhodopsin is involved in atransduction event in which absorbed light is converted to the chemical energy of ATP. Theprotein is essentially a light-driven proton pump. The light-absorbing pigment is retinal, derivedfrom vitamin A, and is the same light-sensitive element in vision. The structure of retinal,. Anisoprenoid, is shown below:CH 3H 3 C CH 3CH 3 CH 3CHOIf the CHO group is reducedto CH 2 OH, the resulting form is retinol, or vitamin A, which is derived from beta carotene.Notice the presence of a cis double bond between carbons 11 and 12. All the others (excludingthat in the ring) are trans. The absorbtion of light induces a cis to trans conversion, resulting in aconformational change in the protein to which retinal is bound that leads to the pumping ofprotons in the case of bacteriorhodopsin and vision in the case of rhodopsin (in our eyes). Inbacteriorhodopsin, retinal is bound to a lysine residue via a Shiff’s base.Figure 9-24 (a) (see page 256) illustrates the transmembrane domain of consisting of an
antiparallel beta sheet rolled up into a barrel (beta barrel, p. 256). Note that this arrangement, aswell as alpha helices, satisfies the hydrogen-bonding requirement of transmembrane segments ofmembrane proteins. In this case hydrogen bonds form between strands. Beta barrels occur inporins, channel-forming proteins in the outer membrane of gram-negative bacteria, and are alsofound in the outer membranes of eukaryotic mitochondria.Lipid-Linked ProteinsWe’ve seen in previous chapters that carbohydrate can be covalently linked to proteins(via O- or N-linkages) to proteins to form glyoproteins, and that carbohydrate can be covalentlylinked to lipids to form glycolipids, such as in gangliosides (O-linkage). Lipids can also becovalently linked to proteins to form lipid-linked proteins (as opposed to lipoproteins, which willbe discussed later in the chapter). Cysteine residues are involved in attachment of lipids derivedfrom isoprene units (via thioether linkages) to form prenylated proteins, and also in theattachment of fatty acids (either myristate, 14:0, or palmitate, 16:0) via amide (myristate) orthioester (palmitate) linkages.
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Page 3 and 4: HCC CH 2 ONO 2-H 3+SerineOHOH OHOHO
Page 5 and 6: SphingolipidsThese phospholipids ar
Page 7 and 8: the metabolic precursor of steroid
Page 9 and 10: Other LipidsIsoprenoids are built f
Page 11 and 12: Phospholipidbilayers are noncovalen
Page 13: portion of glycophorin (Figure 9-20
Page 17 and 18: Membrane StructureThe technique of
Page 19 and 20: Spectrin, comprised of two similar

Chapter 9 - Lipids and Biological Membranes

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