Mechanisms and Dynamics of Ion Channel Selectivity and Gating

 

DIFFUSION VIA PROTEIN PORES AND CHANNELS: CHANNEL "GATING" AND SELECTIVE PERMEABILITY

Tubular pathways from the extracellular to the intracellular fluid have been shown by computerized three-dimensional reconstructions of protein pores and channels. As a result, materials can pass through these pores and channels and simply diffuse from one side of the membrane to the other. Integral proteins included in cell membranes create porous structures, which are always open tubes that pass through the membrane. But a pore's electrical charges and diameter enable selectivity, allowing only specific molecules to flow through. For instance, aquaporins allow water to pass through cell membranes quickly but block the passage of other molecules. Due to their small pores, aquaporins allow water molecules to diffuse across membranes in a single file. There is not enough room for any hydrated ions to flow through the pore. Certain aquaporins' (like aquaporin-2) density in cell membranes varies depending on the physiological setting.

Two key features set the protein channels apart:

1. They are frequently selectively permeable to particular chemicals
2. Chemicals that attach to the channel proteins (ligand-gated channels) or electrical impulses (voltage-gated channels) can control the gates that open or close many of the channels. 

Ion channels are therefore dynamic, flexible structures, and gating and ion selectivity are influenced by minute conformational changes.

Specificity in Protein Channel Permeability

When it comes to transporting one or more particular ions or molecules, many protein channels are quite selective. Certain features of the channel, including its diameter, shape, and the types of electrical charges and chemical interactions along its interior surfaces, are responsible for this selectivity. About 1000 times more potassium ions can pass via potassium channels than sodium ions can through the cell membrane. Since potassium ions have a slightly greater molecular diameter than sodium ions, the high degree of selectivity cannot be fully explained by the ion sizes. Potassium channels were discovered to have a tetrameric structure made up of four identical protein subunits around a central pore using x-ray crystallography. Pore loops near the top of the channel pore create a narrow selectivity filter. Carbonyl oxygens lining the selectivity filter. The dehydrated potassium ions are able to flow through the channel when the hydrated potassium ions engage with the carbonyl oxygens and release most of their attached water molecules when they enter the selectivity filter. The smaller sodium ions, on the other hand, are successfully prevented from passing through the pore by the selectivity filter because the carbonyl oxygens are too far apart to allow them to interact closely with one another.

Sodium Channel Ion Selectivity Mechanism

It is thought that the specificity of different ion channels for cations or anions or for specific ions, such calcium (Ca²⁺), potassium (K⁺), and sodium (Na⁺),  that obtain access to the channels, is mostly determined by different selectivity filters for the various ion channels. Although the sodium channel, one of the most significant protein channels, has a diameter of only 0.3 to 0.5 nanometers, its capacity to distinguish sodium ions from other competing ions in the surrounding fluids is essential for healthy cellular function. As seen in the top image, extremely negatively charged amino acid residues line the selectivity filter, the narrowest portion of the sodium channel's open pore. Although the sodium ions do not need to be completely dehydrated in order to flow through the channels, these potent negative charges have the ability to draw little dehydrated sodium ions away from their hydrating water molecules and into them. Once within the channel, the standard rules of diffusion allow the sodium ions to move in any direction. As a result, sodium ions can only pass via the sodium channel with great selectivity.

Protein Channel Gating

It is possible to regulate the ion permeability of protein channels by gating them. This method allows for the selective gating of potassium and sodium ions in both panels. Certain gates are believed to be gate-like extensions of the transport protein molecule, which have the ability to lift away from the channel opening or close it through conformational changes in the protein molecule's structure. There are two main methods for controlling when gates open and close:

Gating of voltage

When voltage gating is used, the electrical potential across the cell membrane is sensed by the gate's chemical bonds or molecular conformation. For instance, in the top panel, the outside sodium gates may stay tightly closed due to a large negative charge on the interior of the cell membrane. On the other hand, when the membrane's interior loses its negative charge, these gates abruptly open, letting sodium enter the membrane through the sodium holes. The fundamental mechanism for producing action potentials in the nerves that produce nerve signals is this procedure. When the inside of the cell membrane becomes positively charged, the potassium gates in the bottom pane, which are on the intracellular ends of the potassium channels, open. The action potential is partially terminated by the opening of these gates.

Gating Chemical (ligand)

Certain protein channel gates are activated by the binding of a ligand to the protein, which modifies the protein molecule's shape or chemical bonding to open or close the gate. The action of the neurotransmitter acetylcholine on the acetylcholine receptor, a ligand-gated ion channel, is one of the most significant examples of chemical gating. Acetylcholine opens the channel's gate, creating a negatively charged pore with a diameter of approximately 0.65 nanometers that lets uncharged molecules or positive ions with a smaller diameter pass through. The passage of nerve signals from one nerve cell to another and from nerve cells to muscle cells, which results in muscular contraction, depends critically on this gate.

Gated Channels: Open State versus Closed State

Two recordings showing a single sodium channel conducting electricity when the membrane's potential gradient was roughly 25 millivolts. Keep in mind that the channel only allows one current to flow at a time. In other words, the channel's gate opens and closes quickly, with each open state lasting anywhere from a few hundred to several thousand milliseconds. This illustrates how quickly changes can happen when protein gates open and close. The channel may be closed almost entirely at one voltage potential, whereas it may stay open almost entirely or mostly at another voltage potential. The gates have a tendency to intermittently snap open and closed at in-between voltages, resulting in an average current flow that falls between the lowest and maximum.

Method of Patch Clamping Current Flow Through Single Channels for Recording

An example of the patch clamp technique for capturing ion current flow through individual protein channels is provided. A micropipette abutted against the outside of a cell membrane has a tip diameter of only 1 or 2 micrometers. Next, a seal is formed where the pipette's edges come into contact with the cell membrane by applying suction inside the pipette to push the membrane up against the tip. The end result is a tiny membrane "patch" that allows electrical current flow to be monitored at the pipette's tip. makes it possible to change the ion concentrations inside the micropipette and outside the solution as needed. Moreover, a certain voltage can be "clamped," or established, between the two sides of the membrane. Such patches have been created tiny enough that the membrane patch under study contains just one channel protein. The transport characteristics and gating qualities of a single channel can be ascertained by manipulating the concentrations of distinct ions and the voltage across the membrane. As an alternative, you can rip the tiny portion of cell membrane from the pipette's end. The pipette is then submerged in a free solution while still wearing its sealed patch.