1 / 21
Factor that determines ion selectivity is...
Click the card to flip 👆
Terms in this set (21)
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
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
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 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