View source for Johnson's Monday Lab Sandbox for Insulin Receptor

==Insulin Receptor==
<StructureSection load='6sof' size='350' side='right' caption='Insulin Receptor with Four Insulin Bound - 6sof' scene='83/839263/Intro_scene/1'>

==Function of the Receptor==
The insulin receptor binds the insulin hormone and initiates a cascade of events within the cell. The receptor resides within the [http://en.wikipedia.org/wiki/Cell_membrane plasma membrane] of insulin targeted cells. These cells are found in various organs, such as the liver, and tissues, including skeletal muscle and adipose <ref name="Boucher" />. The insulin receptor is activated by multiple insulin molecules binding to various sites on the receptor <ref name="Uchikawa" />. Once activated, the receptor serves as the gateway for the regulation of various cellular processes including glucose transport, glycogen storage, [http://en.wikipedia.org/wiki/Autophagy autophagy], [http://en.wikipedia.org/wiki/Apoptosis apoptosis], and gene expression. Additionally, problems with the insulin receptor are associated with the development of diseases such as Alzheimer's, type II diabetes, and cancer <ref name="Scapin" />. Recent structures of the insulin receptor have illustrated the large scale [http://en.wikipedia.org/wiki/Conformational_change conformational changes], initiated by insulin binding. Evaluation of the structural composition and the biochemical properties of the insulin receptor reveals details about the role of the receptor in crucial cellular processes.
==Insulin==
The <scene name='83/839263/Insulin_molecule/3'>insulin molecule</scene> is a [http://en.wikipedia.org/wiki/Hormone hormone] made of two separate amino acid chains that are bound by multiple disulfide bonds. Insulin is synthesized and secreted from the [http://en.wikipedia.org/wiki/Pancreatic_islets islets of Langerhans] of the pancreas in response to high concentrations of glucose in the blood. Once it is secreted, insulin moves through the bloodstream and binds to unactivated insulin receptors residing in the plasma membrane. Binding of insulin to the insulin receptor is a complex process, which involves negative cooperativity among insulin molecules <ref name="Uchikawa" /> <ref name="Schäffer" /> <ref name="Meyts" />. Current hypotheses propose that the receptor is fully activated only after multiple insulin molecules are bound <ref name="Uchikawa" />.
==Structure==
The insulin receptor is a [http://en.wikipedia.org/wiki/Receptor_tyrosine_kinase receptor tyrosine kinase]. It is a [http://en.wikipedia.org/wiki/Heterotetramer heterotetramer] that is constructed from two [http://en.wiktionary.org/wiki/homodimer homodimers]. Each homodimer maintains an extracellular domain, transmembrane helix, and an intracellular domain. The insulin receptor is divided into <scene name='83/839263/Alpha_and_beta_subunit/3'>alpha and beta</scene> [http://en.wikipedia.org/wiki/Protein_subunit subunits]. The alpha subunit is characterized by two leucine-rich regions and one cysteine-rich region. The beta subunit contains three fibronectin type III domains along with the transmembrane domain and intracellular tyrosine kinase domain that could not be shown in one continous PDB structure. The alpha and beta subunits of the extracellular domains fold over one another and form a <scene name='83/839263/V_shape/3'>"V" shape</scene> when the insulin receptor is inactivated. Upon activation, the extracellular domain undergoes a conformational change and forms a <scene name='83/839263/T-shape/4'>"T" shape</scene>. 
[[Image:SurfaceIR.png|thumb|right|250px|Figure 2: Surface representation of the insulin receptor in the active "T" shape conformation with four insulins bound (green). PDB: 6sof]]
An additional component to the [http://en.wikipedia.org/wiki/Ectodomain ectodomain] is the <scene name='83/839263/Alpha-ct/2'> ''alpha'' chain C-terminal helix</scene>, which is also referred to as the "''alpha''-CT" <ref name= "Uchikawa" />. Each of the dimers has an "alpha"-CT. The ''alpha''-CT is a single alpha-helix and it plays an important role in insulin binding and stabilization of the "T" shape activated conformation. The ''alpha''-CT interacts with a leucine-rich region of the alpha subunit and a fibronectin type III region of the beta subunit to form the insulin binding sites known as <scene name='83/839263/Insulin_molecules_at_site_1/1'>site 1 and site 1'</scene> <ref name="Uchikawa" />.

The structure of the extracellular domain is stabilized through multiple [http://en.wikipedia.org/wiki/Disulfide disulfide bonds]. The alpha subunits are linked through two disulfide bonds, with the main one being between <scene name='83/839263/Cys_holding_alphas_together/4'>Cys524</scene> of two adjacent alpha subuntis <ref name="Schäffer" />. <scene name='83/839263/Cys_683_holding_alphas_togethe/3'>Cys683</scene> of both alpha subunits  are also held together with a disulfide bond <ref name="Sparrow" />. The alpha subunit is also attached to the beta subunit by a disulfide bond between the <scene name='83/839263/Alpha_beta_link_by_disulfide/5'>Cys647 of the alpha subunit and Cys872 of the beta subunit</scene><ref name="Sparrow" />.

===Insulin Binding===
The insulin receptor unit has four separate sites for the insulin binding. There are two pairs of two identical binding sites referred to as <scene name='83/839263/Insulin_molecules_at_site_1/1'>sites 1 and 1'</scene> and <scene name='83/839263/Insulin_molecules_at_site_2/1'>sites 2 and 2'</scene>. The insulin molecules bind to these sites mostly through [http://en.wikipedia.org/wiki/Hydrophobic_effect hydrophobic interactions], with some of the most crucial residues at sites 1 and 1' being between <scene name='83/839263/Residues_of_site_1_binding/8'>Cys A7, Cys B7, and His B5 of insulin and Pro495, Phe497, and Arg498</scene> of the insulin receptor FnIII-1 domain <ref name="Uchikawa" />. Despite some of the residues included being charged they can still interact hydrophobically in this binding site. For example, due to arginine carrying its positive charge at the end of the side chain, <scene name='83/839263/Arginine_bending/1'> the side chain is bent</scene> to allow the hydrophobic part of the side chain to interact with the other hydrophobic residues. At sites 2 and 2', the major residues contributing to these hydrophobic interactions are the <scene name='83/839263/Site_2_residues_hydrophobic/4'>Leu 486, Leu 552, and Pro537 of the insulin receptor and Leu A13, Try A14, Leu A16, Leu B6, Ala B14, Leu B17 and Val B18 of the insulin molecule</scene><ref name="Uchikawa" />. While the majority of the binding interactions appear similar, sites 1 and 1' have a higher binding affinity than sites 2 and 2' due to site 1 having a larger surface area (706 Å<sup>2</sup>) exposed for insulin to bind to compared to site 2 (394 Å<sup>2</sup>)<ref name="Uchikawa" />. The binding interactions of the insulin molecules in sites 1 and 1' are facilitated by hydrophobic residues of an <scene name='83/839263/Insulin_bound_to_site_1/4'>alpha-helix</scene> of the insulin receptor. The insulin molecules in sites 2 and 2' primarily interact with the residues that comprise some of the <scene name='83/839263/Insulin_in_site_2_with_beta_sh/7'>beta-sheets</scene> of the insulin receptor. The secondary structures themselves are not what directly causes the differences in binding affinities, but the surface area that the insulin molecule can interact with. 

Recent studies have demonstrated that at least three insulin molecules have to bind to the insulin receptor to induce the active <scene name='83/839263/T-shape/4'>"T" shape</scene> conformation, as binding of two insulin molecules is insufficient to induce a full conformational change <ref name="Uchikawa" />. However, this conclusion has not yet been widely confirmed <ref name="Uchikawa" />. It has been speculated that activation of the insulin receptor can change based on the concentration of insulin. In low concentrations of insulin, the insulin receptor may not require binding of three insulin molecules in order to exhibit activation. Rather, the level of activity will change in accordance to the availability of insulin <ref name="Uchikawa" />. When higher concentrations of insulin are present, the conformational difference between the two-insulin-bound state and the three-insulin-bound state is drastic as the insulin receptor transitions from the inactive <scene name='83/839263/V_shape/3'>"V" shape</scene> to the active <scene name='83/839263/T-shape/4'>"T" shape</scene> <ref name="Uchikawa" />. However, in conditions of low insulin availability, the two-insulin-bound state may be enough to induce partial activation of the receptor <ref name="Uchikawa" />.

===Conformational Changes===
[[Image:image 6.png|thumb|left|250px|Figure 3: Conformational change of insulin receptor protomer from inactive (blue) to active (orange) form upon insulin binding. Inactive state PDB: 4zxb. Active state PDB: 6sof]]
The conformational change between the inverted, inactive <scene name='83/839263/V_shape/3'>"V" shape</scene> and the active <scene name='83/839263/T-shape/4'>"T" shape</scene> of the insulin receptor is induced by insulin binding. When an insulin molecule binds to site 1 of the alpha subunit, the respective protomer is recruited and a slight inward movement of the <scene name='83/839263/Fniii_domains/1'>Fibronectin type III domains</scene> of the beta subunit is initiated. This is accomplished by the formation of several [http://en.wikipedia.org/wiki/Salt_bridge_(protein_and_supramolecular) salt bridges], specifically between <scene name='83/839263/Salt_bridges/1'>Arg498 and Asp499 of the FnIII-1 and Lys703, Glu706, and Asp707 of the alpha-CT</scene> <ref name="Uchikawa" />. Binding of insulin to both protomers establishes a full activation of the insulin receptor. This activation is demonstrated through the inward movement of both protomers. This motion has been referred to as a "hinge" motion <ref name="Uchikawa" /> as both protomers "swing" in towards one another. Figure 3 depicts the conformational change and "hinge motion" between the inactive and active forms of an insulin receptor protomer. Upon insulin binding, the beta subunits of the inactive form, shown in blue, are "swung" inward to the active form, shown in orange. 

As the fibronectin type III domains of the beta subunit swing inward, the alpha subunits also undergo a conformational change upon insulin binding. As insulin binds to site 1, the leucine-rich region of one protomer interacts with the ''alpha''-CT and the FNIII-1 domains of the other protomer to form a binding site. These interactions are referred to as the <scene name='83/839263/Tripartite_interface/2'>tripartite interface</scene> <ref name="Uchikawa" />. In order for the tripartite interface to form, the alpha subunits of each protomer must undergo a "folding" motion. 

The proper conformational change of the ectodomain of the insulin receptor is crucial for transmitting the signal into the cell. The movements extracellularly cause the two receptor tyrosine kinase domains intracellularly to become close enough to each other to [http://en.wikipedia.org/wiki/Autophosphorylation autophosphorylate] <ref name="Boucher" />. This autophosphorylation leads enzymes to become activated in the cell that carries out processes related to insulin signaling such as metabolism and growth <ref name="Boucher" />.

While there is an explanation for which conformational changes of the insulin receptor take place, there is no full explanation for the exact mechanism by which the conformational changes are executed in the receptor <ref name="Uchikawa" />. It is known where the various domains move, but not the specifics for how this is achieved on the atomic level due to the complexity of analyzing moving structures. 

==Type II Diabetes==
[http://en.wikipedia.org/wiki/Type_2_diabetes Type II diabetes] (T2D) is a chronic condition that affects 10 percent of the world's population <ref name="Boucher" />. T2D is characterized by insulin resistance and leads to high concentrations of glucose in the bloodstream. A type II diabetic produces insulin, but when the insulin molecule binds to the insulin receptor, the signal is not properly transmitted intracellularly <ref name="Boucher" />. Insulin resistance in routine type II diabetics is not associated with mutations of the insulin receptor gene, but instead, the signal being disrupted later in the pathway <ref name="Boucher" />. Mutations of the receptor gene are associated with more severe cases of insulin resistance, as seen in [http://en.wikipedia.org/wiki/Donohue_syndrome leprechaunism]. Additionally, mutations of the insulin receptor can be fatal, as it is crucial for many cellular processes including gene expression, glucose homeostasis, and apoptosis <ref name="Boucher" />. The basis for insulin resistance in typical type II diabetics is complex and cannot yet be explained by one particular factor <ref name="Franks" /> <ref name="Boucher" />.

There are a multitude of hypotheses which discuss the reasons for the establishment of type II diabetes <ref name="Boucher" /> <ref name="Franks" />. While the specifics of the development of T2D are beyond the scope of this article, it is important to note that the molecular causes for insulin resistance and T2D have been primarily attributed to the inhibition of key proteins involved in the insulin signaling and glucose transport pathway <ref name="Boucher" />. Alterations to the phosphorylation cascade of insulin signaling can be the result of changes within the cellular environment including [http://en.wikipedia.org/wiki/Lipotoxicity lipotoxicity], inflammation, [http://en.wikipedia.org/wiki/Hyperglycemia hyperglycemia], and the presence of [http://en.wikipedia.org/wiki/Reactive_oxygen_species reactive oxygen species] (ROS) <ref name="Boucher" />. On a macroscopic level, a variety of factors influence the cellular environment, and thus the risk for T2D. These factors include gestational environment, [http://en.wikipedia.org/wiki/Human_microbiome microbiome], genetics, diet, and energy expenditure <ref name="Franks" />. Recent studies, which have evaluated the relationships between genetics and environmental factors in the progress of T2D, have shown that T2D is not uniform among the population and the biochemistry behind the development of risk factors varies for each patient <ref name="Franks" />.


== References ==
<ref name="Boucher"> PMID: 24384568</ref>
<ref name="Meyts"> DOI:10.1007/BF00400837</ref>
<ref name="Scapin"> DOI:10.1038/nature26153</ref>
<ref name="Schäffer"> PMID: 1472036</ref>
<ref name="Uchikawa"> DOI:10.7554/eLife.48630</ref>
<ref name="Franks"> DOI:10.1126/science.aaf5094</ref>
<ref name="Sparrow"> PMID: 9368005</ref>

<references/>

==Student Contributors==
*Maxwell Todd
*Abby Hillan
*Andrew Scheel