It is of particular importance to note that channel proteins are not indiscriminate; each channel protein contains a selectivity filter.14 The selectivity filter is a collection of amino acid residues concentrated in the interior of the channel protein. As particles, often ions, pass into the channel protein, an electrostatic interaction occurs between the amino acid residues and the ion.15 The interaction would, for example, involve negatively charged amino acid residues in the case of ions like calcium (Na+) or potassium (K+), and positively charged amino acid residues in the case of chlorine (Cl-).2 The electrostatic interaction between amino acid residues and ions allows the channel protein to identify the ion in question by measuring its atomic radius with extremely finite accuracy. Potassium (K+) channels select K+ over Na+ by a factor of over one thousand despite differing in atomic radius by a mere 0.38 Å.While all channel proteins have an inherent selectivity filter, others have additional gating.2 Gating is a response to a predetermined trigger that allows the channel protein to undergo a conformational change. This action subsequently causes another conformational change that either opens or closes the channel, allowing or disallowing its specific particle to pass. Channel proteins can be physically or chemically modulated through a number of different mechanisms.
Voltage gating
Voltage gated protein channels play a particularly important role in excitable neuronal and muscle tissues.2
Ligand gating
Ligand gated channel proteins are activated in response to the binding of a ligand.17 Typically, ligand binding occurs at an allosteric binding site independent of the channel protein’s pore. The binding of a ligand at the allosteric binding site causes a conformational change in the structure of the channel protein, subsequently causing an influx or efflux of ions. Release of the ligand allows the channel protein to return to its original shape. Structurally, ligand gated channel proteins generally differ from other channels due to the presence of an additional protein domain that serves as the allosteric binding site.2
The prototypical example of ligand gating is the nicotinic acetylcholine receptor located on the postsynaptic side of the neuromuscular junction.18
Other gating
Channel proteins may be gated in less common instances by methods such as light activation, mechanical activation, or secondary messanger activation.2Light activated protein channels contain a photoswitch through which a photon causes a conformational change in the channel protein causing it to open or close. Only one such protein channel exists naturally.19 Mechanically activated protein channels open or close in response to a mechanical stimulus and are vital to the touch, hearing, and balance sensations in human.20 Ligand-gated protein channels are typically linked to second messanger gating.2 Second messenger gating functions stepwise in that a neurotransmitter binds to a channel protein receptor which, in turn, reveals an active site to which the conformation-changing ligand binds.
Active Transport
Active transport, simply put, is the movement of particles through a transport protein from low concentration to high concentration at the expense of metabolic energy.21 The most common energy source used by cells is adenosine triphosphate or ATP, though other sources such as light energy or the energy stored in an electrochemical gradient are also utilized.2 In the case of ATP, energy is chemically harvested through hydrolysis.22 ATP hydrolysis in turn causes a conformational change in the transport protein which allows mechanical movement of the particle in question.2 Active transport systems are, therefore, energy-coupling devices as chemical and mechanical processes are linked to achieve particle movement. Active transport is classified as either Primary Active Transport or Secondary Active Transport. Figure 4 (displayed above) displays a ribbon structure of a commonly depicted ABC Vitamin B12importer active transport protein.Primary Active Transport
Primary active transport uses the energy found in ATP, photons, and electrochemical gradients directly in the transport of molecules from low concentration to high concentration across the cellular membrane.23
Using ATP
The enzyme-catalyzed hydrolysis reaction removing a phosphate from ATP, thereby forming ADP, causes a conformational change in the transport protein allowing particles to influx or efflux.23 Enzymes catalyzing ATP-driven primary active transport are called ATPases.2
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Figure 5. Primary active transport, with the use of ATP, is depicted above progressing left to right and top to bottom.
The most universal example of ATP hydrolysis driving primary active transport in cells is the sodium-potassium pump.2 The sodium-potassium pump is responsible for controlling both sodium and potassium concentrations inside the cell. The sodium-potassium pump is extremely important in maintaining the cell’s resting potential.
Figure 6. Ribbon structure of a sodium-potassium ATPase pump.
Using Electrochemical Gradient Energy
An electrochemical gradient has two components: 1) an electrical component caused by charge difference on either side of the cellular membrane and 2) a chemical component resulting from differing concentrations of ions across the cellular membrane.24 The electrochemical gradient is generated by the presence of a proton (H+) gradient. A proton gradient is an interconvertible form of energy that can ultimately be used by the transport protein to move particles across the cellular membrane.2
A quintessential example of electrochemical gradient energy in primary active transport is the mitochondrial electron transport chain (ETC).25 The ETC uses the energy produced from the reduction of NADH to NAD+ to create a proton gradient by pumping protons into the inner mitochondrial space.
Using Photon Energy
The energy stored in a photon, the basic unit of light, is used to generate a proton gradient through a process similar to that found in electrochemical gradients.24 The stepwise passing of electrons in an electron transport chain reduces a molecule like NADH and ultimately generates a proton gradient.
Plant photosynthesis is an example of primary active transport using photon energy.2 Chlorophyll absorbs a photon of light and consequently loses an electron which it passes pheophytin causing a subsequent electron transport chain.26 This ETC ultimately ends in the reduction of NADH to NAD+ creating a proton gradient across the chloroplast membrane.
Secondary Active Transport
Secondary active transport achieves an identical result as primary active transport in that particles are moved from low concentration to high concentration at the expense of energy.2 Secondary active transport, however, functions independent of direct ATP coupling. Rather, the electrochemical energy generated from pumping ions out of the cell is used. Secondary active transport is classified as either symporter of antiporter.
Symports
Symport secondary active transport uses a downhill movement of one particle to transport another particle against its concentration gradient.27 Symports move both particles in the same direction through a transmembrane transport protein.
A common symport example is SGLT1, a glucose symport. SGLT1 tranports one glucose molecule into the cell for every two sodium ions transported into the cell.28 The SGLT1 symport is located throughout the body, particularly in the nephron of the kidney.
Antiports
Antiport secondary active transport moves two or more different particles across the cellular membrane in opposite directions.27 Antiport secondary active transport moves one particle down its concentration gradient and uses the energy generated from that process to move another particle up its concentration gradient.
The sodium-calcium exchanger found throughout humans in excitable cells is a simple and common example of an antiport. Three sodium ions travel down their concentration gradient in exchange for one calcium ion.29
References
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Contributors:
- Garret Powell (Truman State University)
- Jordan Kaminski (Truman State University)