Tissue transglutaminase (abbreviated as tTG or TG2) is a 78-kDa, calcium-dependent enzyme (EC 2.3.2.13) of the protein-glutamine γ-glutamyltransferases family (or simply transglutaminase family). Like other transglutaminases, it crosslinks proteins between an ε-amino group of a lysine residue and a γ-carboxamide group of glutamine residue, creating an inter- or intramolecular bond that is highly resistant to proteolysis (protein degradation). Aside from its crosslinking function, tTG catalyzes other types of reactions including deamidation, GTP-binding/hydrolyzing, and isopeptidase activities. Unlike other members of the transglutaminase family, tTG can be found both in the intracellular and the extracellular spaces of various types of tissues and is found in many different organs including the heart, the liver, and the small intestine. Intracellular tTG is abundant in the cytosol but smaller amounts can also be found in the nucleus and the mitochondria. Intracellular tTG is thought to play an important role in apoptosis. In the extracellular space, tTG binds to proteins of the extracellular matrix (ECM), binding particularly tightly to fibronectin. Extracellular tTG has been linked to cell adhesion, ECM stabilization, wound healing, receptor signaling, cellular proliferation, and cellular motility. tTG is the autoantigen in celiac disease, a lifelong illness in which the consumption of dietary gluten causes a pathological immune response resulting in the inflammation of the small intestine and subsequent villous atrophy. It has also been implicated in the pathophysiology of many other diseases, including such as many different cancers and neurogenerative diseases.
Structure
Gene The human tTG gene is located on the 20th chromosome (20q11.2-q12).
Protein TG2 is a multifunctional enzyme that belongs to transglutaminases which catalyze the crosslinking of proteins by epsilon-(gamma-glutamyl)lysine isopeptide bonds. Similarly to other transglutaminases, tTG consists of a GTP/ GDP binding site, a catalytic domain, two beta barrel and a beta-sandwich. Crystal structures of TG2 with bound GDP, GTP, or ATP have demonstrated that these forms of TG2 adopt a "closed" conformation, whereas TG2 with the active site occupied by an inhibitory gluten peptide mimic or other similar inhibitors adopts an "open" conformation. In the open conformation the four domains of TG2 are arranged in an extended configuration, allowing for catalytic activity, whereas in the closed conformation the two C-terminal domains are folded in on the catalytic core domain which includes the residue Cys-277. The N-terminal domain only shows minor structural changes between the two different conformations.
Mechanism The catalytic mechanism for crosslinking in human tTG involves the thiol group from a Cys residue in the active site of tTG. The thiol group attacks the carboxamide of a glutamine residue on the surface of a protein or peptide substrate, releasing ammonia, and producing a thioester intermediate. The thioester intermediate can then be attacked by the surface amine of a second substrate (typically from a lysine residue). The end product of the reaction is a stable isopeptide bond between the two substrates (i.e. crosslinking). Alternatively, the thioester intermediate can be hydrolyzed, resulting in the net conversion of the glutamine residue to glutamic acid (i.e. deamidation). The deamidation of glutamine residues catalyzed by tTG is thought to be linked to the pathological immune response to gluten in celiac disease. A schematic for the crosslinking and the deamidation reactions is provided in Figure 1.
Regulation The expression of tTG is regulated at the transcriptional level depending on complex signal cascades. Once synthesized, most of the protein is found in the cytoplasm, plasma membrane and ECM, but a small fraction is translocated to the nucleus, where it participates in the control of its own expression through the regulation of transcription factors. Crosslinking activity by tTG requires the binding of Ca2+ ions. Multiple Ca2+ can bind to a single tTG molecule. Specifically, tTG binds up to 6 calcium ions at 5 different binding sites. Mutations to these binding sites causing lower calcium affinity, decrease the enzyme's transglutaminase activity. In contrast, the binding of one molecule of GTP or GDP inhibits the crosslinking activity of the enzyme. Therefore, intracellular tTG is mostly inactive due to the relatively high concentration of GTP/GDP and the low levels of calcium inside the cell. Although extracellular tTG is expected to be active due to the low concentration of guanine nucleotides and the high levels of calcium in the extracellular space, evidence has shown that extracellular tTG is mostly inactive. Recent studies suggest that extracellular tTG is kept inactive by the formation of a disulfide bond between two vicinal cysteine residues, namely Cys 370 and Cys 371. When this disulfide bond forms, the enzyme remains in an open confirmation but becomes catalytically inactive. The, oxidation/reduction of the disulfide bond serves as a third allosteric regulatory mechanism (along with GTP/GDP and Ca2+) for the activation of tTG. Thioredoxin-1 has been shown to activate extracellular tTG by reducing the disulfide bond. Another disulfide bond can form in tTG, between the residues Cys-230 and Cys-370. While this bond does not exist in the enzyme's native state, it appears when the enzyme is inactivated via oxidation. The presence of calcium protects against the formation of both disulfide bonds, thus making the enzyme more resistant to oxidation.
Recent studies have suggested that interferon-γ may serve as an activator of extracellular tTG in the small intestine; these studies have a direct implication to the pathogenesis of celiac disease. Activation of tTG has been shown to be accompanied by large conformational changes, switching from a compact (inactive) to an extended (active) conformation. (see Figure 3)
In the extracellular matrix, TG2 is "turned off", due primarily to the oxidizing activity of endoplasmic reticulum protein 57 (ERp57). Thus, tTG is allosterically regulated by two separate proteins, Erp57 and TRX-1. (See Figure 4).
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