UGGT, or-glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...

 glucosyltransferase (160kDa) is a soluble enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 resident in the lumen of the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

 (ER). The main function of UGGT is to recognize misfolded glycoproteins and transfer a glucose (Glc) monomer (monoglucosylate) to the terminal mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....

 on the glycoprotein. It uses UDP-Glc as the glycosyl donor and requires calcium ions for its activity.

UGGT is part of the ER quality control system of glycoprotein folding and its activity increases the potential for correctly folded glycoproteins. The main proteins involved in the ER quality control system are UGGT, the ER lectin
Lectins are sugar-binding proteins that are highly specific for their sugar moieties. They play a role in biological recognition phenomena involving cells and proteins. For example, some viruses use lectins to attach themselves to the cells of the host organism during infection...

s and chaperones (calnexin
Calnexin is a 90kDa integral protein of the endoplasmic reticulum . It consists of a large N-terminal calcium-binding lumenal domain, a single transmembrane helix and a short , acidic cytoplasmic tail....

 and calreticulin
Calreticulin also known as calregulin, CRP55, CaBP3, calsequestrin-like protein, and endoplasmic reticulum resident protein 60 is a protein that in humans is encoded by the CALR gene....

), and glucosidase II. UGGT first recognize the incompletely folded glycoprotein and monoglucosylate it. The lectins, calnexin and calreticulin, have high affinities for monoglucosylated proteins and these chaperones may attempt to assist in folding of the misfolded protein. Subsequently, glucosidase II will deglucosylate the glycoprotein. If the glycoprotein is still misfolded, UGGT will monoglycosylate it and allow the glycoprotein to go through the cycle again.

Currently, it is unclear how UGGT recognize misfolded glycoprotein. It has been proposed that UGGT may bind to exposed hydrophobic stretches, a characteristic feature of misfolded proteins.
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