CHAPTER 8 - ION CHANNELS

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Factor that determines ion selectivity is...
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Factor that determines ion selectivity is...
size of an ion's shell of waters of hydration,

because the ease with which an ion moves in solution (its mobility) depends on the size of the ion together with the shell of water surrounding it.

THE SMALLER AN ION, THE MORE HIGHLY LOCALIZED IS ITS CHARGE AND THE STRONGER ITS ELECTRIC FIELD (((smaller ions attract water more strongly))
Figure 8-1 explains
model explains how a channel can select K+ and exclude Na+
The permeability of the cell membrane to ions is determined by
the interaction of ions with water, the membrane lipid bilayer, and ion channels.
Ion channels
are integral membrane proteins that span the lipid bilayer, providing a pathway for ions to cross the membrane. The channels are selective for specific ions.
Potassium Channels
have a narrow pore that excludes Na+. Although a Na+ ion is smaller than a K+ ion, in solution, the effective diameter of Na+ is larger because its local field strength is more intense, causing it to attract a larger cloud of water molecules. The K+ channel pore is too narrow for the hydrated Na+ ion to permeate
Sodium Channels
have a selectivity filter that weakly binds Na+ ions.

According to the hypothesis developed by Bertil Hille and colleagues, a Na+ ion binds transiently at an active site as it moves through the filter. At the binding site, the positive charge of the ion is stabilized by a negatively charged amino acid residue on the channel wall and also by a water molecule that is attracted to a second polar amino acid residue on the other side of the channel wall. It is thought that a K+ ion, because of its larger diameter, cannot be stabilized as effectively by the negative charge and therefore will be excluded from the filter
Describe the three physical Models for the opening and closing of ion channels
1. Conformational change in one region
localized conformational change occurs in
one region of the channel.

2. General Structure Change
A generalized structural change occurs along
the length of the channel

3. Blocking Particle
A blocking particle swings into and out of the
channel mouth
Types of Stimuli controlling the opening and closing of ion channels
A. Ligand gating
B. Phosphorylation gating
C. Voltage gating
D. Stretch or pressure gating
A ligand-gated channel
opens when a ligand binds a receptor site on the external surface of the channel protein. The energy from ligand binding drives the channel toward an open state.
Phosphorylation gating
Some channels are regulated by protein phosphorylation and dephosphorylation. The energy for channel opening comes from the transfer of the high-energy phosphate, P(subscript)i
Voltage-gated channelsopen and close with changes in the electrical potential difference across the membrane. The change in membrane potential causes a local conformational change by acting on a region of the channel that has a net charge.Stretch or Pressure GatingSome channels open and close in response to membrane stretch or pressure. The energy for gating may come from mechanical forces that are passed to the channel either directly by distortion of the membrane lipid bilayer or by protein filaments attached to the cytoskeleton or surrounding tissues.What can affect the gating control sites of an ion channel?Exogenous factors such as drugs and toxinsVoltage gated channels are inactivated by what two mechanisms ***A. Change in membrane potential B. Calcium BindingChange in membrane potential ***Many voltage-gated channels enter a refractory (inactivated) state after briefly opening in response to depolarization of the membrane. They recover from the refractory state and return to the resting state only after the membrane potential is restored to its resting value.Calcium binding ***Some voltage dependent Ca2+ channels become inactivated when the internal Ca2+ level increases following channel opening. The internal Ca2+ binds to calmodulin (CaM), a specific regulatory protein associated with the channelExogenous ligands, such as drugs, can bias an ion channel toward an open or closed state.A. In channels that are normally opened by the binding of an endogenous ligand (1), a drug or toxin may block the binding of the agonist by means of a reversible (2) or irreversible (3) reaction. B. Some exogenous agents can bias a channel toward the open state by binding to a regulatory site, distinct from the ligand-binding site that normally opens the channel.Three superfamilies of ion channels1. Ligand-gated channel (ACh receptor) 2. Gap-junction channel 3. Voltage-gated channel (Na+ channel)ligand-gated ion channelMembers of a large family of ligand gated channels, such as the acetylcholine receptor channel, are composed of five identical or closely related subunits, each of which contains four transmembrane α-helixes (M1-M4). Each cylinder in the figure represents a single transmembrane α-helix.gap junction channelThe gap-junction channel is formed from a pair of hemichannels, one each in the pre- and postsynaptic cell membranes, that join in the space between two cells. Each hemichannel is made of six identical subunits, each containing four transmembrane α-helixes. Gap junction channels serve as conduits between the cytoplasm of the pre- and postsynaptic cells at electrical synapsesVoltage-gated channelformed from a single polypeptide chain that contains four homologous domains or repeats (motifs I-IV), each with six membrane-spanning α-helixes (S1-S6). The S5 and S6 segments are connected by an extended strand of amino acids, the P-region, which dips into and out of the external surface of the membrane to form the selectivity filter of the pore. Voltage-gated Ca2+ channels share the same general structural pattern, although the amino sequences are different