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G-protein Coupled Receptors- Lijiana Ma 

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G-protein Coupled Receptors- Lijiana Ma 

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G-protein Coupled Receptors
The largest family of cell surface receptors are the GPRCS.
A diverse set of ligands binds to this type of receptor, including peptide hormones, neurotransmitters, and odor molecules. These receptors all have a similar structure with seven transmembrane domains. Proteins that were identified by sequence homology, but whose ligands are not known, are termed orphan receptors. GPCRs associated with heterotrimeric G-proteins (green), that is, G-proteins composed of three different subunits:  alpha, beta, and gamma.  The subunits are tethered at the membrane surface by covalently attached lipid molecules.
When a ligand binds, the receptor activates the attached G-protein by causing the exchange of GTP (yellow) for GDP (red).  The activated G-protein then dissociates into an alpha (G-alpha) and a beta-gamma complex.   G-alpha bound to GTP is active and can diffuse along the membrane surface to activate (and sometimes inhibit) target proteins, often enzymes that generate second messengers.  Likewise, the beta-gamma complex is also able to diffuse along the inner membrane surface and affect protein activity.
Inactivation occurs because G-alpha has intrinsic GTPase activity. After GTP hydrolysis, G-alpha bound to GDP will reassociate with a beta-gamma complex to form an inactive G-protein that can again associate with a receptor.
The GTPase activity of the G-alpha can be made faster by other proteins--sometimes the target protein, sometimes a separate regulatory protein. Cholera toxin causes a chemical modification that prevents GTP hydrolysis and leads to unregulated signaling.

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Direct contact between cells occur when two or more cells send signals through the physical exchange of molecules that call for a specific action within the receiving cell. Membrane signaling involves proteins shaped into receptors embedded in the cell’s membrane that connect the triggers in the external environment to the ongoing dynamic chemistry and make up inside the cell. This allows the direct passage of molecules between external and internal compartments of the cell.

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chemically gated ion channels ~ Shaun Mayekar

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chemically gated ion channels ~ Shaun Mayekar

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Ion channel, protein expressed by virtually all living cells that creates a pathway for charged ions from dissolved salts, including sodium, potassium, calcium, and chloride ions, to pass through the otherwise impermeant lipid cell membrane. Operation of cells in the nervous system, contraction of the heart and of skeletal muscle, and secretion in the pancreas are examples of physiological processes that require ion channels. In addition, ion channels in the membranes of intracellular organelles are important for regulating cytoplasmic calcium concentration and acidification of specific subcellular compartments (e.g., lysosomes).

The nicotinic acetylcholine receptor is an example of a ligand-gated ion channel. It is composed of five subunits arranged symmetrically around a central conducting pore. Upon binding acetylcholine, the channel opens and allows diffusion of sodium (Na+) and potassium (K+) ions through the conducting pore.
The nicotinic acetylcholine receptor is an example of a ligand-gated ion channel.

Ions flow passively through channels toward equilibrium.

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G proteins are binders of guanine nucleotides GDP and GTP. The inner surface of the plasma membrane and transmembrane receptors of hormones. These are called G-protein-coupled receptors. G protein-coupled receptors are found only in eukaryotes, including yeast, choanoflagellates, and animals. The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. The same ligands can activate many different receptor family members; at least 9 distinct G-protein-linked receptors are activated by adrenaline, for example, another 5 or more by acetylcholine, and at least 15 by the neurotransmitter serotonin. Despite the chemical and functional diversity of the signal molecules that bind to them, all G protein-linked receptors have a similar structure. They consist of a single polypeptide chain that threads back and forth across the lipid bilayer seven times.

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Voltage-Gated Ion Channel By Brynne Davies

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Voltage-Gated Ion Channel By Brynne Davies

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Voltage-gated ion channels are a class of transmembrane proteins that are activated by changes in electrical potential difference across the cell membrane. Voltage-gated ion channels are generally composed of several subunits arranged in such a way that there is a central pore through which ions can travel down their electrochemical gradients. The channels tend to be ion-specific, although similarly sized and charged ions may sometimes travel through them. Voltage-gated ion channels that are selectively permeable to each of the major physiological ions—Na+, K+, Ca2+, and Cl-The functionality of voltage-gated ion channels is attributed to its three main discrete units: closed-activatable, activated (open and conducting), and closed-inactivatable. Na channels are made up of 1800 to 4000 amino acids with four transmembrane repeat domains. The molecules of the protein interact with each other and surrounding molecules to form a structure that defines its function.
Each of the four transmembrane domains contains a voltage-sensitive alpha helix that is displaced in the open or conduction state. The linker between the III and IV repeat domains act as a ball and chain to fold up into the channel opening to block sodium ions (Na+) from moving through the channel in the inactivatable state. When a channel opens Na+ moves from outside the membrane, through the channel, to the inside following both a concentration gradient and a voltage gradient. Shortly after the channel opens it becomes energetically favorable for the linker between the III and IV repeat to move into the opening and block further Na+ movement. Voltage-gated sodium channels and calcium channels are made up of a single polypeptide with four homologous domains. Each domain contains 6 membrane-spanning alpha helices. One of these helices, S4, is the voltage sensing helix. The S4 segment contains many positive charges such that a high positive charge outside the cell repels the helix, keeping the channel in its closed state. In general, the voltage-sensing port.
The picture shows a voltage-gated ion channel. At resting, the voltage-gated ion channel is closed and ions cannot pass. neuron stimulation -> the membrane potential will increase -> conformational change in the channel and allows ions to pass. The channel remains open briefly -> further conformational change -> inactivated by the so-called "ball and chain" portion of the protein. The channel will remain inactive and refractory for a brief period of time before returning to the active, but closed, resting position.

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Another picture of ion channels.

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Ions have a significant effect on many cell processes. Cells use ions for transmitting signals across the cell membrane or along the surface of the cell. An ion is a small particle that requires a specific portal protein to facilitate its transport through the membrane. Such a protein molecule is called an ion channel.

Ion channel is used to restore the balance of ions across a membrane. It serves as a counterbalance to active transport, a process where a cell uses energy to actively pump ions and other charged molecules across a membrane in order to establish ion gradients or alter the pH of an organelle to activate enzymes. Ion channels are regionally located in the neuron.
An ion channel is usually equipped with four basic parts: a central conduction pathway (opening) for ions to pass through, an ion recognition site to allow passage of specific ions (selectivity filter), one or more gates that may open or close, and a sensor that senses the triggering signal and transmits it to the gate.

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Autocrine signaling is a form of cell signaling in which a cell secretes an autocrine agent, such as a hormone or chemical messenger, that binds to autocrine receptors on the same cell, leading to changes in the cell. These signals are produced by signaling cells that can also bind to the ligand that is released, which means the signaling cell and the target cell can be the same or a similar cell; hence the name autocrine, auto- for self.

Autocrine signaling is important because it is the way cells regulate pain sensation and inflammatory responses. If a cell is infected with a virus, the cell can signal itself to undergo programmed cell death, killing the virus in the process. In some cases, neighboring cells of the same type are also influenced by the released ligand. In embryological development, this process of stimulating a group of neighboring cells may help to direct the differentiation of identical cells into the same cell type, thus ensuring the proper developmental outcome.
I chose this photo above as it demonstrates how autocrine signalling works; in this scenario, the cell releases signals to itself that targets receptor sites on itself.

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Chemically gated ion channels - Rachelle Atallah

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Chemically gated ion channels - Rachelle Atallah

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Ligand-gated ion channels are proteins, also known as ionotropic receptors. They are a group of transmembrane that open to allow ions such as Na, K, Ca and/or Cl to pass through the cell membrane in response to a ligand (chemical messenger) such as a neurotransmitter. Ligand-gated channels, located at sites of synaptic contact are found predominantly on dendritic spines, dendrites and somata of nerve cells, or neurons. Ligand gated ion channels are classified into three superfamilies which lack evolutionary relationship: Cys-loop receptors, Ionotropic glutamate receptors and ATP-gated channels. The cys-loop family includes the glycine receptor, the nicotinic acetylcholine receptor, serotonin, the zinc activated ion channel and the GABA receptor, which can regulate the cell cycle.

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The largest family of cell surface receptors are the G-protein coupled receptors (GPCRs).  There are hundreds of different GPCR proteins, and nearly a third drugs target this type of receptor.  A diverse set of ligands bind to this type of receptor, including peptide hormones, neurotransmitters, and odor molecules.  These receptors all have a similar structure with seven transmembrane domains.  On the basis of their seven-transmembrane domain structure, many GPCRs have been identified in the human genome.  Proteins that were identified by sequence homology, but whose ligands are not known, are termed orphan receptors.

GPCRs associate with heterotrimeric G-proteins (green), that is, G-proteins composed of three different subunits:  alpha, beta, andgamma.  The subunits are tethered at the membrane surface by covalently attached lipid molecules.

When a ligand binds, the receptor activates the attached G-protein by causing the exchange of GTP (yellow) for GDP (red).  The activated G-protein then dissociates into an alpha (G-alpha) and a beta-gamma complex.   G-alpha bound to GTP is active, and can diffuse along the membrane surface to activate (and sometimes inhibit) target proteins, often enzymes that generate second messengers.  Likewise, the beta-gamma complex is also able to diffuse along the inner membrane surface and affect protein activity.

Inactivation occurs because G-alpha has intrinsic GTPase activity. After GTP hydrolysis, G-alpha bound to GDP will reassociate with a beta-gamma complex to form an inactive G-protein that can again associate with a receptor.

The GTPase activity of the G-alpha can be made faster by other proteins--sometimes the target protein, sometimes a separate regulatory protein. Cholera toxin causes a chemical modification that prevents GTP hydrolysis and leads to unregulated signaling.

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Cells usually communicate by sending chemical signals (which are called ligands), and once they’re produced by a cell, they’re released into extracellular space. After that, specific cells receive these chemical signals (and they must have a specific receptor in order to be able to receive them). In autocrine signaling, a cell releases a ligand (called an autocrine agent in this case) that binds to receptors in its own surface. So basically, the cell signals itself. This is really important during embryological development because once a cell has been directed along a particular way of differentiation, it will send signals to itself that reinforce this developmental decision. It is also likely to encourage other groups of identical cells to make the same developmental decisions. Scientists believe that autocrine signaling plays a major role in cancer. This is because cancerous cells use autocrine signaling to receive signals of growth and survival, which causes metastasis and the formation of tumors.

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Ion gated channels are important for cells to communicate. This type of communication allows for muscles to relax and contract. Chemically gated ions are proteins located on the membrane. They have small passages which open and close allowing chemicals to move down the concentration gradient, this movement allows for the muscles to receive to act accordingly. The main difference from other transporters is that the rate of ion transport is very quick. Chemically gated ions work better if the body is satisfied in its potassium requirements its lack can lead to frequent muscle cramps which are failures of the chanel to send a signal for the muscle to relax.  Attached is an illustration of such channel where

1 - channel domains (typically four per channel), 2 - outer vestibule, 3 - selectivity filter, 4 - diameter of selectivity filter, 5 - phosphorylation site, 6 - cell membrane.

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nerve cells are able to send signals to other nerve cells at high velocities. Receptors on dendrites detect signals that travel through axons to the presynaptic terminal. once the signals reach the terminals, neurotransmitters make a fusion with the membrane releasing other hormones and signals into the synaptic space. the target cell detects the contents with receptors.

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G-protein-linked receptors bind a ligand and initiate a membrane protein called a G-protein. The activated G-protein then cooperates with either an ion channel or an enzyme in the membrane. All G-protein-linked receptors have seven transmembrane domains. However, each receptor has its own specific extracellular domain and G-protein-binding site.

Cell signaling through the use of G-protein-linked receptors occurs in a cyclic series of events. Before the ligand binds, the inactive G-protein can bind to a newly-revealed site on the receptor specific for its binding. Once the G-protein binds to the receptor, the resultant shape change activates the G-protein, which releases GDP and picks up GTP. The subunits of the G-protein then split into the α subunit and the β subunit. One or both of these G-protein fragments may be able to activate other proteins as a result. Later, the GTP on the active α subunit of the G-protein is hydrolyzed to GDP and the β subunit is deactivated. The subunits re-associate to form the inactive G-protein, and the cycle starts over.

The picture is just a representation of the process and part of the reason why I chose it.

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G-protein linked receptors - Linh Nguyen

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G-protein linked receptors - Linh Nguyen

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G-protein-linked receptors form the largest family of cell-surface receptors and are found in all eukaryotes. G-protein-linked receptors mediate responses to signal molecules. These signal molecules are varied in structure like function: proteins and small peptides.

All G-protein-linked receptors have a similar structure. They consist of a single polypeptide chain that threads back and forth across the lipid bilayer. Receptors that bind protein ligands have a large extracellular domain formed by a polypeptide.

The first step in this complex signalling system involves the binding of specific ligands. This then activates the receptor. The signal is transmitted into the cell via a change in the receptor, which activates the bound G protein. This allows regulation of the intracellular concentrations of secondary messengers. Finally, the subunits bind to the receptor, which terminates the signal.

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Autocrine Signalling is a form of cell signalling in which a cell secretes a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on that same cell, leading to changes in the cell.This means the signaling cell and the target cell can be the same or a similar cell.  Autocrine signaling plays an important role in many processes.For instance, autocrine signaling is important during development, helping cells take on and reinforce their correct identities. Autocrine signaling also regulates pain sensation and inflammatory responses. Further, if a cell is infected with a virus, the cell can signal itself to undergo programmed cell death, killing the virus in the process. In some cases, neighboring cells of the same type are also influenced by the released ligand. In embryological development, this process of stimulating a group of neighboring cells may help to direct the differentiation of identical cells into the same cell type, thus ensuring the proper developmental outcome.

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Enzyme-linked receptors, aka catalytic receptors, are cell-surface receptors with intracellular domains that are associated with an enzyme. Simply, they are transmembrane proteins with the binding of extracellular ligand domain on the outer surface of the plasma membrane that causes enzymatic activity on the intracellular side . If they are connected to a signalling molecule, these receptors form dimers, receptor pairs perform tyrosine phosphorylation of each other. Thus, the binding of receptor signalling molecule leads to transmit information into the cell. In some cases, the intracellular domain of the receptor itself is an enzyme, that has an intracellular domain that interacts directly with an enzyme. The enzyme-linked receptors normally have large extracellular and intracellular domains, but the membrane-spanning region consists of a single alpha-helical region of the peptide strand. When a ligand binds to the extracellular domain, a signal is transferred through the membrane and activates the enzyme, which sets off a chain of events within the cell that eventually leads to a response.
There are several types of enzymatic receptors, such as receptor tyrosine kinase, serine/threonine-specific protein kinase, guanylyl cyclases, and tyrosine-kinase-associated receptors. Of one of the enzyme-linked receptors, tyrosine kinase receptor, such an example would be: The tyrosine kinase receptor transfers phosphate groups to tyrosine molecules. Signalling molecules bind to the extracellular domain of two nearby tyrosine kinase receptors, which then dimerize. Phosphates are then added to tyrosine residues on the intracellular domain of the receptors and can then transmit the signal to the next messenger within the cytoplasm. There are two steps to the T-K Receptor reaction (as shown below). One being that the ligand binding cases 2 receptor polypeptides to aggregate (dimer). The second activates the T-K parts of polypeptides, which adds phosphates to the tyrosine on the tail of the second polypeptide.
The effects of enzyme-linked receptors typically are slow requiring the expression of new genes. Mutations, in receptor tyrosine kinases, are responsible for a wide array of diseases, including cancers, or neurodegeneration.

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chemically gated ion channels. Ion gated channels are very important in the way cells communicate. This communication from cells allows heart to beat muscles to contract. Chemically gated ion channels are proteins on the membrane. They have tunnels, the tunnels open and close to allow chemicals to move down their concentration gradient. The movement of the chemicals allows for different things to happen.

Basically, a signaling protein attaches to the gate, this causes the gate to open, the opening of the gate causes a response.

A great example of this would be a neuron cell. They have chemically gated ion channels that work in quick section to achieve an effect.

My picture displays that neuron cell.

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Enzymatic receptors bind to an extracellular ligand causing enzymatic movement on the intracellular side. They send the extracellular signals to the nucleus by a course of interactions. Then, specific transcription elements turn on, affecting expression. The basic structure has three parts: a ligand-binding domain, a transmembrane domain, and finally an "active" domain. The ligand-binding domain makes easy accesses for ligands. The transmembrane domain has multiple hydrophobic amino acids. It is also the bind used to keep the receptor near the cell membrane. Finally the "active" domain is naturally in the receptor or is closely tied through cytoplasm.
There are many types of enzymatic receptors, but the main three are: the receptor serine-threonine kinases,
the receptor tyrosine kinases (RTKs), and finally the tyrosine-kinase-associated receptors. These different types are looked upon as path ways a cell can signal. The picture below depicts RTK, due to the "active" and "inactive" parts.
Over all, enzymatic receptors are cell-surface, associated with many key domains. Due to its cell-surface label, it is known to communicate between multicellular organisms.  Just like what it states in its name, it requires an enzyme to be activated through ligand binds transferring through membranes. Although this is one of the more simpler receptors, it is equally as significant.

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To add your post, select the red + symbol in the bottom R hand corner of the screen. Don't forget to upload a picture too!