G Protein-Linked Receptors an Organism Term Paper

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[. . .] Messenger molecules may be amino acids, peptides, proteins, fatty acids, lipids, nucleosides or nucleotides. Hydrophobic messengers bind to intracellular receptors that regulate or influence expression of specific genes.

A ligand binds its receptor through specific weak non-covalent bonds by suiting into a specific binding site or "pocket." Binding of most of cognate receptors where a ligand has low concentrations means a high receptor affinity and low receptor affinity happens when a high concentration of the ligand is needed (Department of Biology). Prolonged exposure to a ligant, and the resulting occupancy of the receptor, often results in desensitization. Desensitization depends on receptor down-regulation through the removal of the receptor from the surface or by changing to the receptor that will lower the affinity to the ligand or disable it in initiating changes in cellular function (Department of Biology). Desensitization may likewise develop tolerance, which reduces or eliminates the effectiveness of some medicines when over prescribed.

A proteins are guanine-nucletide-binding that change the target protein's activity (Department of Biology 2003). Their receptors possess an extra-cellular N-terminus and a cytosolic C-terminus, which are separated by seven trans-membrane alpha helices, in turn, connected by peptide loops. The cytosolic loop between the 5th and 6th alpha helices binds a specific G. protein (Department of Biology).

A proteins that are bound to GTP are active, while those bound to GDP are not (Department of Biology 2003). These proteins are classified into large heterotrimeric and small monomeric G. proteins. When a messenger binds the heterotrimeric G. protein-linked receptor, it changes shape to allow linking with the trimeric G. protein (Department of Biology).

Many G. proteins make signal transduction events a diversified affair. Some bind potassium or calcium ion channels in neurotransmitters, while some either release or form major second messengers, such as cyclic AMP and calcium ions (Department of Biology 2003). Still some activate kinases. cAMP os produced by adenylyl cyclase, embedded in the plasma membrane and activated by binding an activated alpha subunit of the Gs G. protein. Phosphodiesterase progressively decreases cAMP in the absence of the ligand and active G. protein, thus reducing cAMP levels. Protein kinase A is a cAMP-dependent kinase, which is cAMP's main intracellular target (Department of Biology).

A protein signaling is quite important because its disturbance or disruption leads to several human diseases (Department of Biology 2003). For example, the micro-organism vibrio cholerae secretes the cholera toxin, which modifies the salt and fluid balance in the intestines. Hormones that activate Gs G. protein to increase cAMP normally maintain that balance. The disease occurs when the cholera toxin enzymatically changes Gs to disable it from converting GTP to GDP. Under this condition, Gs cannot be inactivated, keeping cAMP levels high and leading intestinal cells to secrete salt and water (Department of Biology). Extreme dehydration can lead to death.

The release of calcium ions is another key element in many signaling processes. Calcium ions help regulate many cellular functions (Department of Biology). Calcium ionophore releases calcium from the intracellular stores that imitate the effect of Insp3 activation. These stores can be released by the InsP3 channel and the ryanodine receptor channel, which opens when calcium is present. And although other proteins bind calcium to regulate its activity, binding to the protein calmodulin to produce a calcium-calmodulin complex is most frequently resorted to as an intermediate measure (Department of Biology).

The fertilization of animal ova illustrates calcium-mediated signal transduction, following a receptor-ligand interaction (Department of Biology 2003). The sperm, at first, binds the egg's surface at the membrane. Within 30 seconds, a wave of calcium releases spreads from the location of the sperm contact. This release of calcium determines two events. The first is the stimulation of the fusion of the cortical granules with the egg's plasma membrane to change or modify the surrounding coat of the egg and help prevent the binding of another sperm to it (Department of Biology). This is the natural and slow block to the phenomenon called polyspermy. The second event consists of the egg activation and the resumption of metabolic processes.

Nitric oxide is also a signal. It is a toxic and short-living gas molecule that plays a part as a signaling molecule in the cardiovascular system (Department of Biology 2003). It couples G. protein-linked receptor stimulation in endothelial cells to the relaxation of smooth muscle cells in blood vessels. This relaxation of the vascular smooth muscles or vasodilator involves six events or stages. First is the binding of acetylcholine to G. protein receptors that produces InsP3. Second is the release of calcium ions by InsP3 from endoplasmic reticulum. Third is the formation of complex calcium ions and calmodulin, which stimulates nitric oxide synthase in order to produce nitric oxide. Fourth is the diffusion of nitric oxide from the endothelial cell into adjacent smooth muscle cells. Fifth the activation of guanylyl cyclase in smooth muscle cells in order to produce cyclic GMP or cGMP. The sixth and last is the activation of the protein kinase G. To produce several muscle proteins that induce muscle relaxation (Department of Biology).

A ligand that binds a protein kinase-associated receptor stimulates a kinase activity and the blend of phosphorylation transmits the signal (Department of Biology 2003). Receptor tyrosine kinases assemble and go through autophosphorylation and begin a chain reaction, leading to cell growth, proliferation or differentiation. Most receptor tyrosine kinases have one trans membrane or TM domain, an extra-cellular ligand-binding domain and a cytosolic tail with tyrosine residue targets of the tyrosine activity.

Ligand binding begins the activation of the receptor tyrosine kinases, conducing to receptor aggregation, often as dimers. As soon as the epidermal growth factor receptor is autophosphorylated in response to the EGF ligand, a variety of GRB2 SH2-domain- containing and Sos guanine-nucleotide release protein binds the receptor. This change activates the Sos and causes the release of GDP, which in turn, allows Ras to bind a new GTP and get activated (Department of Biology). When activated, Ras creates a pathway, which includes the mitoen-activated protein kinases or MAPKs. These receptor tyrosine kinases stimulate a divergence of signaling pathways. Receptor tyrosine kinases can activate a form of phophlipase C. And phosphatidylinositor-3-kinase or PI3K (Department of Biology).

Growth factors act as messengers (Department of Biology 2003). Besides nutrients, growth factors are needed by the cell for growth. These include platelet-derived growth factors, insulin, insulin-like growth factor 1 or IGF-1, fibroblast growth factor or EGF, and nerve growth factor or NGF. This ligand function has a more significant value than growth and cell division. Impairment of the growth factor by affecting receptor tyrosine kinases can have telling consequences on embryonic development (Department of Biology). Fibroblast growth factors and fibrolast growth factor receptors have strong roles in both embryonic and adult signaling. These receptors play an important role in the development of the mesoderm, the embryonic tissue that eventually develops into muscle, cartilage, bone and blood cells (Department of Biology). When dimerized with normal versions of the receptor or becomes a mutant receptor, it will have a dominant inhibitory effect on normal activity as a dominant negative mutation. Such a mutant version of the receptor, when injected into frog eggs, can cause the failure of mesodermal tissue to develop. In the case of the frog experiment, tadpoles developed with heads but without bodies. Defects of this receptor in humans lead to achondroplasia or dwarfism and thanatophoric dysplasia or severe bone abnormalities, which are fatal in infancy (Department of Biology).

Hormones are chemical signals secreted by a tissue to regulate another, which is located at a distance (Department of Biology 2003), often transmitted through the circulatory system. They may be proteins, peptides, steroids or other molecules and their signals can be classified according to the distance they negotiate in reaching target cells.

Hormones can be proteins, peptides, steroids and other molecules. An endocrine hormone is transmitted by the circulatory system, while a paracrine hormone functions on proximate cells. Endocrine tissues directly secrete into the bloodstream while exocrine tissues do so into ducts, which bring the secretions to target parts of the body (Department of Biology). The pancreas secretes both endocrine and paracrine hormones. Insulin and glucagons are the endocrine hormones it secretes, while its paracrine hormones are digestive enzymes. Endocrine hormones eventually reach target tissues, such as the heart and liver, in the case of epinephrine, or liver and skeletal muscles, in the case of insulin (Department of Biology).

Endocrine hormones are categorized into four. These are amino acid derivatives or epinephrine; peptides, such as anti-diuretic hormone or vasopressin; proteins, such as insulin; and lipid-like hormones, including steroids such as testosterone.

Paracrine hormones, on the other hand, include a histamine, which is a histine derivative, and the prostaglandins, which arachidonic acid derivatives (Department of Biology).

Cells also regulate what is called programmed cell death or apoptosis, a well-ordered way of eliminating cells (Department of Biology 2003). Apoptosis is, however, quite different from… [END OF PREVIEW]

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