Channel Proteins

As the name suggests, it is a barrier that allows the movement or passage of anything across it specifically. Channel proteins are membrane proteins usually with two or more ⍶-helices crossing the lipid bilayer. These proteins are classified under the transport protein of membrane proteins. They are responsible for the selective transport of molecules. These proteins are more specific, selective and accept only one type of molecule for transport. Channel proteins are arranged in a special form across the cell membrane that provides a hydrophilic way of transportation for water molecules, small ions, and polar molecules. Channel proteins carry very rapid movement of molecules as there are no conformational changes due to interaction with specific ions or molecules to be transported. 

Channel proteins transport solutes down their concentration gradient. They behave like a pore in the membrane and allow the inward and outward movement of molecules. Channel proteins are classified into two types based on the passage of molecules and ions. They are channel gated proteins and channel non-gated proteins.

Gated Channel proteins

Gated channel proteins are more selective and open only in response to specific signals. The main types of signals that cause ion channels to open are voltage changes across the membrane and are referred to as voltage-gated channels. Mechanical stress also causes some specific channels to open. They are known as mechanical-gated channels. Some channels proteins open in response to ligand binding. They are known as ligand-gated channels. The ligand can be either an extracellular or intracellular mediator, such as ions, and are referred to as ion-gated channels.

• Voltage-Gated Ion channels

Gates of ion channels have voltage sensing loops which lead to the opening or closing of gates according to the changes caused by voltage. For example, Voltage-gated Sodium ions (Na+) channels, which are found in Axons of Neurons, are responsible for generating an action potential. Voltage-gated Calcium ion channels (Ca++) are present on the sarcoplasmic reticulum (SER) membrane during muscle contraction. They are also responsible for slowing back to polyspermy. 

• Mechanical-Gated Ion channels

These channels are open by the effect of mechanical pressure that causes the passage of specific ions across them. For example, Sodium ions channels in Mechanoreceptors. These channels are also found in hair cells of the internal ear for audition and balancing. Mechanical-gated ion channels are most commonly found in Touch receptors (like Ruffini’s corpuscles, Pacinian corpuscles). 

• Ligand-Gated Ion channels

Ligand-gated ion channels are responsible for transporting molecules or ions as well as receptors for signalling. These channel proteins are of two types depending on the location where they work. They are-

• Extrinsic Ligand-gated ion channels

These channel proteins are found on the extracellular side of the membrane for generating a free passage of ions when a specific impulse works on it. For example, Acetylcholine gated Na+ ion channels. These channel proteins are found on the neuromuscular junction, a muscle endplate, and dendrites of postsynaptic cells of Neurons. Neurotransmitters such as Glycine and GABA open Cl¯ ion channels, while aspartate opens Na+ ion channels. 

• Intrinsic Ligand-gated ion channels 

Ligand is produced with the cell, such as cAMP, cGMP, etc. They bind to ion channels, which leads to their opening and allows the specific ions to get transported. cGMP opens up the Na+ ion channel, IP3 opens up the Ca2+ ion channel. The potassium ions (K+) channel in cardiac muscle also follows this pathway. For example, These channel proteins are found in rod cells.

Non-Gated Channel Proteins

Non-Gated channel proteins are leaky channel proteins that always remain in the open state. These channel proteins are not common as compared to gated channel proteins but are found in the membrane. 

For example, an Open K+ ion channel on neurons and other cells is responsible for maintaining resting potential (-60mV). 

Examples of Channel Proteins

Aquaporin is an example of a channel protein that allows the rapid movement of water across the cell membrane. It conducts the facilitated diffusion of water. Aquaporin proteins are present in the kidney, nephron, and other parts of it for absorbing the water back into the body. Aquaporin protein is made up of β-barrels (secondary structure). 

All the ion channel proteins like Na+, Cl¯, Ca++, K+, etc., are considered examples of Channel proteins. Acetylcholine-mediated nerve impulses in muscle cells are also known as the example of channel proteins. 

Difference between Channel Protein and Carrier Protein

Carrier proteins are those proteins that bind and transport the molecules across the cell membrane. These proteins are also involved in facilitated diffusion. But basically, carrier proteins are needed for the active transport of molecules that require significant energy. 

Channel proteins are the site of the passageway for small molecules, ions, polar molecules like water without the need for energy, while the carrier proteins are required for the movement of large molecules, and the energy is needed for such movement. Carrier proteins have an active site for binding specific molecules and allow the passage of large molecules into and out of the cell membrane.

Channel proteins carry the fast movement of molecules, or ions, while the carrier proteins mediate the slow transport of large molecules. Carrier proteins have conformational changes in the binding of molecules, ions to be transported, while channel proteins do not show the process of conformational changes on binding. 

Carrier proteins can be uniporters or co-transporters. Uniporters transport a single solute from one side of the membrane to the other. Co-transporters mediate coupled transport. In comparison, such types of arrangements are not seen in channel proteins. . 

Conclusion

Channel proteins are proteins that completely span the membrane and form a hydrophilic tunnel that allows the transport of target molecules across it by the process of diffusion. Channel proteins are gated and non-gated proteins. Non-gated proteins are leaky channel proteins that are not significant as compared to gated channel proteins. Gated channel proteins are voltage sensitive, stress-sensitive (mechanical), and ligand-dependent.

Channel proteins are different from carrier proteins based on their function, process, and specificity. Also, the time is taken for the passage or transport of target molecules is different in both cases. Channel protein allows the transport by diffusion, while the carrier proteins need the energy to transport molecules because it actively transports the large molecules.