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The Adiponectin receptor 1 is one of the two receptors for the hormone called adiponectin.

Function

The function of adipoR1 is directly linked with the adiponectin. It is an hormone, and more precisely an adipokine ^{[1] [2]}, present in the blood at high concentration, approximatively 0,01 % of the total amount of proteins in plasma^[3].The human adiponectin monomer has a molecular weight of about 28 kDa and is composed of 244 amino acids. However, the molecular weight of the hormone depends on the multimerization of this one ^[3]. The hormone is mainly created by adipocytes present in brown and white adipose tissues but according to researches it could also be produced in some non-adipose tissues as in skeletal muscle ^[3][2].Two forms of adiponectin exist: the full-length adiponectin, presents in the liver and the globular adiponectin presents in skeletal muscles and in the liver.The adiponectin receptor 1 is a receptor sensitive in particular to the globular form^[2]. This hormone is known to be anti-diabetic, antiatherogenic and a regulator of tissue inflammation and insulin sensitivity^[4]. These adiponectin's properties are linked to the fatty oxidation trigger by the hormone and the adipoR1 receptor. Different fatty acid oxidation pathway exists. The major pathway regulated by adipoR1 is the AMP kinase channel, but this pathway is not completely known. However, several studies show that adipoR1 decreases the hepatic glucose production by activating this channel. AdipoR1 is also able to limit the expression of enzymes, like glucose-6-phosphatase, phosphoenolpyruvate carboxykinase and carboxykinase1, involved in gluconeogenesis ^[4]

Structure

The Adiponectin receptor 1 is an integral membrane protein composed of 375 amino acids and its molecular weight is 42,4 kDa. This protein can be decomposed into different parts: an internal (residues 89 to 120), a short intracellular domain called (residues 121 to 129), (residues 134 to 364) and an external (residues 365 to 375). ^[1]

The organisation of the structure of the Adiponectin receptor 1 is the opposite to G protein-coupled receptor family. Indeed, the Adiponectin receptor has an internal and an external while the G-protein family has an internal N-terminus domain and an external C-terminus domain. ^{[5] [1] [6]}

The Adiponectin receptor 1 contains linked thanks to three and . The is formed by the residues 135 to 157, by the residues 169 to 192, the by the residues 198 to 227, the by the residues 232 to 252, the by the residues 264 to 288 by the residues 305 to 319 and the by the residues 336 to 364 .Besides, the and are longer than the other helices. These have a clockwise circular specific organisation (from helix I to helix VII) and form a bundle. Concerning the extracellular faces, the three which connect the transmembrane helices are exposed and it is the same for the . Besides, and the are longer than the other helices and as a result the two turns of the are exposed too. ^[1] In the middle of the seven transmembrane helices there is a large internal cavity extended from the cytoplasmic surface to the outer lipid layer where a can be found. This cavity has small openings between the and , and between the and . It has been assumed that these openings are involved in the entrance and exit of both substrate and product. In this cavity, there is a zinc ion which is coordinated thanks to three histidine residues. These three histidine residues are in the , and in the . As a result, the zinc ion is in the intracellular layer of the membrane, in the neighbourhood of 4° deep from the inner surface of the plasma membrane. Thanks to its tetrahedral coordination, this zinc ion binds the , and together. The adiponectin-stimulated AMPK phosphorylation doesn’t directly require the zinc binding site, nevertheless it has been supposed that the zinc ion allows a stabilizing effect. ^[1]

AdipoR1 has the capacity to form oligomers. ^[5] Indeed in living cell both monomers and oligomers are present. A specific motif was identified to contribute to the AdipoR1 dimerization: it is the motif GxxxG in the transmembrane . Besides, the dimerization of AdipoR1 is also regulated. This dimerization is inhibited by the fixation of the full-length adiponectin while the globular adiponectin has any impact on the dimerization level of the AdipoR1 receptor. Thanks to mutant experiment, it can be supposed that the collagen-like domain of the full-length adiponectin is responsible to the dimer dissociation. There are strong evidences that dimerization of the AdipoR1 receptor has a role during the biosynthesis, the trafficking and the signalling of the seven transmembrane receptors. ^[7]

Diseases

AdipoR1 is involved in different diseases and symptoms. One major diseases linked with this proteine is the Type II diabete which is caracterized by a chronic hyperglycemia. This disease is mainly present in obese or overweight people. In case of type II diabete, adiponectin level in plasma is reduced signficantly and the expression of the adiponectin receptors,AdipoR1, decreases, especially in skeletal muscle and adipose tissues .^{[5] [3]}

Another symptom linked to adipoR1 is the insulin resistance. An insulin resistant organism is an organism which will require more insulin to obtain the biological effects compare to a normal organism^[2].Insulin resistance can be caused by any defects in the insulin signaling cascade, including problems with AdipoR1^[2].Hyperglycemia and hyperinsulinemia are probably the cause of insulin resistance^[5]. In fact, The adiponectin levels and the expression of AdipoR1 drop, reducing adiponectin sensitivity and so lead to insulin resistance^[2]. Then, insulin resistance provoque a drop of adiponectin secretion and circulation level, increasing hyperinsulinemia.This is the start of a vicious cycle^{[3] [5]}.

Obesity and insulin resistance also lead to metabolic syndrome.Indeed,adipoR1 can display metabolic dysfunction, especially in case of the drop of its expression^[6].However agonist of adiponectin receptor 1 should be a new treatment strategie for metabolic syndrome or obesity-linked diseases^[5].Moreover,a treatment against type II diabete and insulin resistance could be AdipoR1 overexpression. It could,in the one hand, improves insulin resistance and promotes glucose uptake in skeletal muscle,and, in the other hand, attenuates weight gain and improves glucose metabolism models of metabolic dysfunction in macrophages^[6].

3D visualisation

Use of Jsmol and Jmol ^[8][9]