Ion transporter

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Diffusion vs. Transport Figure 2.jimmyjohnslaser2.pdf
Diffusion vs. Transport

In biology, an ion transporter is a transmembrane protein that moves ions (or other small molecules) across a biological membrane to accomplish many different biological functions, including cellular communication, maintaining homeostasis, energy production, etc. [1] There are different types of transporters including pumps, uniporters, antiporters, and symporters. Active transporters or ion pumps are transporters that convert energy from various sources—including adenosine triphosphate (ATP), sunlight, and other redox reactions—to potential energy by pumping an ion up its concentration gradient. [2] This potential energy could then be used by secondary transporters, including ion carriers and ion channels, to drive vital cellular processes, such as ATP synthesis. [3]

Contents

This article is focused mainly on ion transporters acting as pumps, but transporters can also function to move molecules through facilitated diffusion. Facilitated diffusion does not require ATP and allows molecules that are unable to quickly diffuse across the membrane (passive diffusion), to diffuse down their concentration gradient through these protein transporters. [4]

Ion transporters are essential for proper cell function and thus they are highly regulated by the cell and studied by researchers using a variety of methods. Some examples of cell regulations and research methods will be given.

Classification and disambiguation

Ion transporters are classified as a super family of transporters that contain 12 families of transporters. [5] These families are part of the Transport Classification (TC) system that is used by the International Union of Biochemistry and Molecular Biology (IUBMB) and are grouped according to characteristics such as the substrates being transported, the transport mechanism, the energy source used, and also by comparing the DNA sequences making up each protein. The most important unifying factor being the charged nature of the substrate which indicates the transport of an ion and not a neutral species. [5] Ion transporters differ significantly from ion channels. Channels are pores that run through the membrane, whereas transports are proteins that must change shape to switch which side of the membrane it is open to. Because of this, transporters are much slower at moving molecules than channels.

An electrochemical gradient or concentration gradient is a difference in concentration of a chemical molecule or ion in two separate areas. [6] At equilibrium the concentrations of the ion in both areas will be equal, so if there is a difference in concentration the ions will seek to flow "down" the concentration gradient or from a high concentration to low concentration. Ion channels allows the specific ions that will fit into the channel to flow down their concentration gradient, equalizing the concentrations on either side of the cell membrane. Ion channels and ion transporters accomplish this via facilitated diffusion which is a type of passive transport. However, only ion transporters can also perform active transport, which involves moving ions against their concentration gradient. [7] Using energy sources such as ATP, ion transporters are able to move ions against their concentration gradient which can then be used by secondary transporters or other proteins as a source of energy. [6]

Energy source

Primary transporter

ATPsynthase uses a chemical (proton) gradient to generate ATP ATP-Synthase.svg
ATPsynthase uses a chemical (proton) gradient to generate ATP

Primary transporters use energy to transport ions such as Na +, K+, and Ca2+ across a cells membrane and can create concentration gradients. [6] This transport can use ATP as an energy source or it can be used to generate ATP through methods such as the electron transport chain in plants. [7] [6]

Active transporter

Active transporters use ATP to convert the energy in ATP into potential energy in the form of a concentration gradient. They use the ATP to transport an ion from a low concentration to a higher concentration. Examples of proteins that use ATP are P-type ATPases that transfer Na +, K+, and Ca2+ ions by phosphorylation, A-type ATPases that transfer anions, and ABC transporters (ATP binding cassette transporters) that transport a broad set of molecules. [6] Examples of the P-type ATPase include Na+/K+-ATPase [7] [8] [9] that is regulated by Janus Kinase-2 [10] as well as Ca2+ ATPase which exhibits sensitivity to ADP and ATP concentrations [3] P-glycoprotein is an example of an ABC transport binding protein in the human body.

ATP producing transporter

ATP producing transporters run in the opposite direction of ATP Utilizing transporters. These proteins transport ions from high to low concentration with the gradient but in the process ATP is formed. Potential energy in the form of the concentration gradient is used to generate ATP. [6] In animals, this ATP synthesis takes place in the mitochondria using F- type ATPase otherwise known as ATP synthase. This process utilizes the electron transport chain in a process called oxidative phosphorylation. [11] [2] V-type ATPase serves the opposite function as F-type ATPase and is used in plants to hydrolyze ATP to create a proton gradient. Examples of this are lysosomes that use V-type ATPase to acidify vesicles or plant vacuoles during the process of photosynthesis in the chloroplasts. [7] This process can be regulated through various methods such as pH. [12]

Secondary transporter

Na+ Glu Symporter Sodium Glucose symporter all in one.pdf
Na+ Glu Symporter
Porters.PNG

Secondary transporters also transport ions (or small molecules) against the concentration gradient – from low concentration to high concentration - but unlike primary transporters which use ATP to create a concentration gradient, secondary transporters use the potential energy from the concentration gradient created by the primary transporters to transport ions. [6] For example, the sodium-dependent glucose transporter found in the small intestine and kidney use the sodium gradient created in the cell by the sodium potassium pump (as mentioned above) to help carry glucose into the cell. [13] This happens as sodium flows down its concentration gradient which provides enough energy to push glucose up its concentration gradient back into the cell. This is important in the small intestine and the kidney to prevent them from losing glucose. Symporters such as the sodium-glucose symporter transport an ion with its concentration gradient, and they couple the transport of a second molecule in the same direction. Antiporters also use the concentration gradient of one molecule to move another up its concentration gradient but the coupled molecule is transported in the opposite direction. [6]

Regulation

Ion transporters can be regulated in a variety of different ways such as phosphorylation, allosteric inhibition or activation, and sensitivity to ion concentration. Using protein kinases to add a phosphate group or phosphatases to dephosphorylate the protein can change the activity of the transporter. [14] Whether the protein is activated or inhibited with the addition of the phosphate group depends on the specific protein. With allosteric inhibition, the regulatory ligand can bind into the regulatory site and either inhibit or activate the transporter. Ion transporters can also be regulated by the concentration of an ion (not necessarily the ion it transfers) in solution. For example, the electron transport chain is regulated by the presence of H+ ions (pH) in solution. [6]

Techniques for studying ion transporters

Patch clamp

A patch clamp is an electrophysiology technique used to study channels and transporters in cells by tracking the current that run through them. This technique was perfected by Hodgkin and Huxley before the existence of channels and transporters was known. [11] [15] Besides its groundbreaking work early on patch clamping legacy continues on and is commonly used by researchers still to study ion transporters and how environments and ligands effects the function of the transporter. [1] [16]

X-ray crystallography

X-ray crystallography is an incredible tool that allows the structure of proteins to be visualized, however, it is only a snapshot of one protein conformation. The structure of transport proteins allows researchers to further understand how and what the transporter does to move molecules across the membrane. [17] [18]

Fluorescence recovery after photobleaching

Fluorescence recovery after photobleaching (FRAP) is a technique used to track diffusion of lipids or proteins in a membrane. This technique is used to better understand transporters mobility in the cell and its interactions with lipid domains and lipid rafts in the cell membrane.

Förster resonance energy transfer

Förster resonance energy transfer (FRET) is a technique that uses fluorescence to track how close two proteins are to each other. This has been used in studying transporters to see how they interact with other cellular proteins. [1]

Table of ion transporters

Ion Transporters
Neurotransmitter transporter
Glutamate transporter
Monoamine transporter
GABA transporters
Glycine transporters
Adenosine transporters
Plasma membrane Ca2+ ATPase
Sodium-calcium exchanger
Sodium-chloride symporter

See also

Related Research Articles

<span class="mw-page-title-main">Facilitated diffusion</span> Biological process

Facilitated diffusion is the process of spontaneous passive transport of molecules or ions across a biological membrane via specific transmembrane integral proteins. Being passive, facilitated transport does not directly require chemical energy from ATP hydrolysis in the transport step itself; rather, molecules and ions move down their concentration gradient according to the principles of diffusion.

In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellular energy to achieve this movement. There are two types of active transport: primary active transport that uses adenosine triphosphate (ATP), and secondary active transport that uses an electrochemical gradient. This process is in contrast to passive transport, which allows molecules or ions to move down their concentration gradient, from an area of high concentration to an area of low concentration, without energy.

<span class="mw-page-title-main">ATPase</span> Dephosphorylation enzyme

ATPases (EC 3.6.1.3, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or the inverse reaction. This dephosphorylation reaction releases energy, which the enzyme (in most cases) harnesses to drive other chemical reactions that would not otherwise occur. This process is widely used in all known forms of life.

<span class="mw-page-title-main">Sodium–potassium pump</span> Enzyme found in the membrane of all animal cells

The sodium–potassium pump is an enzyme found in the membrane of all animal cells. It performs several functions in cell physiology.

<span class="mw-page-title-main">Passive transport</span> Transport that does not require energy

Passive transport is a type of membrane transport that does not require energy to move substances across cell membranes. Instead of using cellular energy, like active transport, passive transport relies on the second law of thermodynamics to drive the movement of substances across cell membranes. Fundamentally, substances follow Fick's first law, and move from an area of high concentration to an area of low concentration because this movement increases the entropy of the overall system. The rate of passive transport depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins. The four main kinds of passive transport are simple diffusion, facilitated diffusion, filtration, and/or osmosis.

<span class="mw-page-title-main">Mediated transport</span> Transportation of substances via membrane

Mediated transport refers to transport mediated by a membrane transport protein. Substances in the human body may be hydrophobic, electrophilic, contain a positively or negatively charge, or have another property. As such there are times when those substances may not be able to pass over the cell membrane using protein-independent movement. The cell membrane is imbedded with many membrane transport proteins that allow such molecules to travel in and out of the cell. There are three types of mediated transporters: uniport, symport, and antiport. Things that can be transported are nutrients, ions, glucose, etc, all depending on the needs of the cell. One example of a uniport mediated transport protein is GLUT1. GLUT1 is a transmembrane protein, which means it spans the entire width of the cell membrane, connecting the extracellular and intracellular region. It is a uniport system because it specifically transports glucose in only one direction, down its concentration gradient across the cell membrane.

A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane proteins; that is they exist permanently within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion, active transport, osmosis, or reverse diffusion. The two main types of proteins involved in such transport are broadly categorized as either channels or carriers. Examples of channel/carrier proteins include the GLUT 1 uniporter, sodium channels, and potassium channels. The solute carriers and atypical SLCs are secondary active or facilitative transporters in humans. Collectively membrane transporters and channels are known as the transportome. Transportomes govern cellular influx and efflux of not only ions and nutrients but drugs as well.

<span class="mw-page-title-main">Membrane potential</span> Electric potential difference between interior and exterior of a biological cell

Membrane potential is the difference in electric potential between the interior and the exterior of a biological cell. It equals the interior potential minus the exterior potential. This is the energy per charge which is required to move a positive charge at constant velocity across the cell membrane from the exterior to the interior.

In cellular biology, membrane transport refers to the collection of mechanisms that regulate the passage of solutes such as ions and small molecules through biological membranes, which are lipid bilayers that contain proteins embedded in them. The regulation of passage through the membrane is due to selective membrane permeability – a characteristic of biological membranes which allows them to separate substances of distinct chemical nature. In other words, they can be permeable to certain substances but not to others.

<span class="mw-page-title-main">Resting potential</span> Static membrane potential in biology

The relatively static membrane potential of quiescent cells is called the resting membrane potential, as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential. The resting membrane potential has a value of approximately -70mV or -0.07V.

<span class="mw-page-title-main">Uniporter</span>

Uniporters, also known as solute carriers or facilitated transporters, are a type of membrane transport protein that passively transports solutes across a cell membrane. It uses facilitated diffusion for the movement of solutes down their concentration gradient from an area of high concentration to an area of low concentration. Unlike active transport, it does not require energy in the form of ATP to function. Uniporters are specialized to carry one specific ion or molecule and can be categorized as either channels or carriers. Facilitated diffusion may occur through three mechanisms: uniport, symport, or antiport. The difference between each mechanism depends on the direction of transport, in which uniport is the only transport not coupled to the transport of another solute.

<span class="mw-page-title-main">Antiporter</span> Class of transmembrane transporter protein

An antiporter is an integral membrane protein that uses secondary active transport to move two or more molecules in opposite directions across a phospholipid membrane. It is a type of cotransporter, which means that uses the energetically favorable movement of one molecule down its electrochemical gradient to power the energetically unfavorable movement of another molecule up its electrochemical gradient. This is in contrast to symporters, which are another type of cotransporter that moves two or more ions in the same direction, and primary active transport, which is directly powered by ATP.

<span class="mw-page-title-main">Cotransporter</span> Type of membrane transport proteins

Cotransporters are a subcategory of membrane transport proteins (transporters) that couple the favorable movement of one molecule with its concentration gradient and unfavorable movement of another molecule against its concentration gradient. They enable coupled or cotransport and include antiporters and symporters. In general, cotransporters consist of two out of the three classes of integral membrane proteins known as transporters that move molecules and ions across biomembranes. Uniporters are also transporters but move only one type of molecule down its concentration gradient and are not classified as cotransporters.

<span class="mw-page-title-main">Electrochemical gradient</span> Gradient of electrochemical potential, usually for an ion that can move across a membrane

An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts:

Method of glucose uptake differs throughout tissues depending on two factors; the metabolic needs of the tissue and availability of glucose. The two ways in which glucose uptake can take place are facilitated diffusion and secondary active transport. Active transport is the movement of ions or molecules going against the concentration gradient.

Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa (enterocytes) of the small intestine (SGLT1) and the proximal tubule of the nephron. They contribute to renal glucose reabsorption. In the kidneys, 100% of the filtered glucose in the glomerulus has to be reabsorbed along the nephron. If the plasma glucose concentration is too high (hyperglycemia), glucose passes into the urine (glucosuria) because SGLT are saturated with the filtered glucose.

The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger, exchange protein, or NCX) is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium (Na+) by allowing Na+ to flow down its gradient across the plasma membrane in exchange for the countertransport of calcium ions (Ca2+). A single calcium ion is exported for the import of three sodium ions. The exchanger exists in many different cell types and animal species. The NCX is considered one of the most important cellular mechanisms for removing Ca2+.

<span class="mw-page-title-main">Symporter</span> Class of membrane transport proteins

A symporter is an integral membrane protein that is involved in the transport of two different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the cell membrane at the same time, and is, therefore, a type of cotransporter. The transporter is called a symporter, because the molecules will travel in the same direction in relation to each other. This is in contrast to the antiport transporter. Typically, the ion(s) will move down the electrochemical gradient, allowing the other molecule(s) to move against the concentration gradient. The movement of the ion(s) across the membrane is facilitated diffusion, and is coupled with the active transport of the molecule(s). In symport, two molecule move in a 'similar direction' at the 'same time'. For example, the movement of glucose along with sodium ions. It exploits the uphill movement of other molecules from low to high concentration, which is against the electrochemical gradient for the transport of solute molecules downhill from higher to lower concentration.

Transcellular transport involves the transportation of solutes by a cell through a cell. Transcellular transport can occur in three different ways active transport, passive transport, and transcytosis.

The P-type plasma membrane H+
-ATPase is found in plants and fungi. For the gastric H+
/K+
 ATPase, see Hydrogen potassium ATPase.

References

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  12. ↑ Tikhonov AN (October 2013). "pH-dependent regulation of electron transport and ATP synthesis in chloroplasts". Photosynthesis Research. 116 (2–3): 511–34. Bibcode:2013PhoRe.116..511T. doi:10.1007/s11120-013-9845-y. PMID   23695653. S2CID   12903551.
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