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New Chapt 7-Molec struture of ion channels

Neuroscience, Fourth Edition

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Chapter 7 The Molecular Structure of Ion Channels In the previous sections, we showed how voltage gated Na+ and K+ channels work and how the cooperative operation of those channels generates and shapes the action potential. We showed that depolarization opens Na+ and K+ channels, but that Na+ channels automatically shut off, i.e., inactivate, while K+ channels do not, and only close when the cell repolarizes. These are the features that characterize ion channels as shown by electrophysiological studies. Electrophysiological studies, however, cannot show the structural underpinnings of the channel. In other words, voltage clamp and other electrophysiological studies show what the channel does and its electrical behavior, but they cannot reveal its structure nor how the particular structure it possesses produces each aspect of its behavior. The questions that we explore in this chapter concern the molecular structures of channels and how those structures impart activation, inactivation, and selectivity for passing only one type of ion. Since each channel is a protein, we will examine the molecular structure of a channel, its complement of amino acids, and how the arrangement of amino acids in different parts of the channel enables the channel to operate in a way consistent with its behavior revealed in voltage clamp studies. Before turning to the structures of ion channels, a brief review of amino acids and proteins will be helpful. A brief review of amino acids and proteins Individual amino acids are linked together to from polypeptide chains by peptide bonds in which the carboxyl group (COOH) of one amino acid is attached to the amino group (NH 2 ) of the next. Each polypeptide thus has a free NH 2 group at one end (the amino or N terminus) and a free COOH group at the other (the carboxyl or C terminus). The side groups of the amino acids (R1 and R2 in the example below) protrude from the peptide backbone and vary with the amino acid. Twenty amino acids are commonly used in the construction of proteins. Each amino acid within a polypeptide or protein is called a residue because amino acids lose a few atoms (usually hydrogen and oxygen ) when they are polymerized into a larger molecule . Figure 1. Amino acids are linked by peptide bonds to form polypeptides. Amino acids are connected by a peptide bond formed between the COOH - (carboxyl) group of one residue and the NH 2 + (amino) group of the next. R1 and R2 indicate the amino acid side chains. 66
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Some amino acids are acidic and positively charged, some are basic and negatively charged, some are polar but uncharged whereas others are non-polar. Non-polar residues are hydrophobic (water hating) and stretches of hydrophobic residues therefore tend to be found in the portions of ion channels that are embedded in the lipid membrane. The portions of the protein that are embedded in the membrane are called the transmembrane segments of the protein. The protein in this case is an ion channel. In
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