3. Ion Channels
Ion channels are a special class of proteins conducting small ions such as Na+, K+, Ca2+ or Cl-. The macroscopic ion current discussed in Chapter 2 is a summation of many single channel currents, arising from the passage of Na+ ions through Na+-selective channels or K+ ions through K+-selective channels. Both Na+ and K+ channels involved in the generation of nerve impulses are sensitive to membrane voltage. At the resting potential, most of these channels are closed. Their open probability increases with increasing depolarization, as reflected in the macroscopic ion conductance.
In contrast to the voltage-sensitive ion channels, some channels are activated by the binding of specific molecules (the ligands) such as AMPA receptor and NMDA receptor, which play crucial roles in learning and memory. How the membrane depolarization or ligand binding opens the channel is not well understood, but the mystery of ion selectivity has been solved.
By definition, a potassium channel conducts mainly K+ ions while excluding Na+ and Ca2+ ions. We note that a Na+ ion is smaller than a K+ ion. How can a channel conduct K+ ions while excluding smaller ions? The answer lies in the ion's hydration energy.
The diameter of the potassium channel's selectivity filter (the narrowest region of its channel pore) should be about the same as the size of a Cs+ ion (diameter = 3.3Å) because Cs+ is the largest ion which can barely pass through the potassium channel. In a solution, ions are surrounded by water molecules. The first hydration shell of a K+ or Na+ ion contains six water molecules. The diameters of unhydrated Na+ and K+ ions are 1.96Å and 2.66Å, respectively. The effective diameter of a water molecule in a hydrated ion is larger than 2Å. Therefore, to pass through the selectivity filter, the K+ or Na+ ion must remove four water molecules from its first hydration shell, leaving only two (one at the front and another at the back). The smaller Na+ ion requires greater dehydration energy than the K+ ion, because its nucleus has shorter distance with surrounding water molecules and thus interacting more strongly. As a result, the Na+ ion is harder to pass through the potassium channel than the K+ ion. The observation that lithium (Li+) is excluded from the potassium channel can also be explained by the same energetic consideration. Divalent cations such as Ca2+ should also be excluded because their dehydration energies are much higher than monovalent cations.
In the sodium channel, the Na+ ion is more permeable than the K+ ion. This is because the selectivity filter of the sodium channel is slightly larger than that of the potassium channel. It is large enough to accommodate a Na+ ion attached with three water molecules, but not enough for a K+ ion attached with three water molecules. Therefore, to pass through the sodium channel, the Na+ ion needs to remove only three, but the K+ ion has to remove four, water molecules from its first hydration shell. The required dehydration energy for the K+ ion is greater than the Na+ ion.
In calcium channels, the permeability of monovalent cations (Na+ and K+) is about three orders of magnitude smaller than the Ca2+ permeability. This ion selectivity does not seem to involve hydration, because Ca2+ is more heavily hydrated than Na+, and the unhydrated diameters of Ca2+ and Na+ are almost identical. Then, how can calcium channels select Ca2+ over Na+?
Although the permeability of monovalent cations in the calcium channel is quite small at normal ionic concentrations, large monovalent cationic current can be observed in the absence of Ca2+ and other divalent cations. This suggests that the calcium channel is basically permeable to both divalent and monovalent cations, but the selectivity arises from competition between ions. The calcium channel may contain a negatively charged binding site to facilitate ion conduction. The monovalent cations simply cannot compete with Ca2+ for this binding site. This idea has been confirmed experimentally. In the calcium channel, if a negatively charged glutamate residue in the pore-lining region is mutated into a positively charged lysine, the calcium channel becomes more permeable to Na+ than Ba2+ (Tang et al., 1993). Conversely, in the sodium channel, mutation of a pore-lining lysine residue into glutamate transforms the channel from a Na+-selective to a Ca2+-selective channel (Heinemann et al., 1992).
There are many types of potassium channels. The one involved in the generation of action potentials is composed of four subunits, each is homologous to the Shaker protein (Figure 3-2). The hydrophobicity profile indicates that it contains six hydrophobic segments, designated as S1 - S6. These segments are likely to be the transmembrane domains (Figure 3-3). The selectivity filter is located in the P-region which contains the K+ channel signature "GYG". Figure 3-4 shows the arrangement of subunits in a K+ channel.
Figure 3-2. The amino acid sequence of the Shaker protein.
Figure 3-3. The domain structure of the Shaker protein.
The open state of the Shaker K+ channel has been determined by x-ray crystallography (Long et al., 2005). The selectivity filter is formed by the backbone oxygen atoms which facilitate the passage of K+ ions (Figure 3-5).
Figure 3-5. The x-ray structure of the K+ channel selectivity filter. [Source: PLOS One]
Na+ and Ca2+ Channels
A Na+ or Ca2+ channel consists of a major pore-forming subunit and optionally other small auxiliary subunits. The major pore-forming subunit is called the α subunit which can be divided into four similar domains. Each domain is analogous to a Shaker protein with six transmembrane segments and a P-region (Figure 3-6). Thus, an α subunit is sufficient to form an ion channel.
Figure 3-6. The domain structure of the α subunit of a Na+ or Ca2+ channel.
Ligand-Activated Ion Channels
An AMPA receptor or NMDA receptor is made up of four subunits. However, the nicotinic acetylcholine receptor (nAChR) is composed of five subunits (Figure 3-7).