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lecture15 - LECTURE 15 28 September 2009(P J Hollenbeck...

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LECTURE 15 28 September 2009. (P. J. Hollenbeck) BIOL 231 Structure & function of membrane proteins Read: pp. 387-408; DVD animations: 12.1, 12.3-12.5; Problems 39-44; Ex II'04, #2; ECB Q11-4 Here we’ll consider integral membrane proteins (remember there are 3 classes - integral, peripheral, lipid-anchored; see fig. 11-21 to review). I. Integral membrane proteins A. Properties of membrane-spanning proteins (1) The portion of a transmembrane protein that actually spans the lipid bilayer is thought usually to be an alpha helix. Many types of transmembrane proteins span the membrane more than one time. There are families of membrane proteins that share similar arrangements and similar functions. Membrane-spanning regions of proteins are usually á -helices with mostly hydrophobic R-groups (left), although some proteins span the membrane with arrangements of â -strands called “ â barrels” (right). (2) Note that membrane protein structures we look at have often been indirectly determined, or even mainly predicted from the primary structure. (Why do you think that is?) (3) Trans-membrane (integral membrane) proteins serve various functions, and many have more than one of these functions: (a) receptors (d) regulatory proteins (b) transporters (passive & active) (e) junctions (c) enzymes <let’s consider some examples of different classes of integral membrane proteins> -1-
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This figure shows schematically the structure of the membrane protein that comprises a potassium channel. It has 6 membrane-spanning alpha-helices and a portion that dips into but does not span the membrane (H5). The inset in (a) shows the amino acid sequence of the membrane-spanning á -helix of a particular voltage-gated potassium channel – this is a subject we’ll return to in the next lecture. The diagram in (b) shows how several of the transmembrane proteins shown in (a) are actually arranged in the membrane to form a functional channel (4 E structure!). The sodium channel that we looked at in lecture has a very similar structure, but all the subunits of the channel are repeating parts of a single enormous, 24-span protein.
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