Tissue transglutaminase

Tissue transglutaminase (abbreviated as tTG or TG2) is a 78-kDa, calcium dependent is_associated_with::enzyme of the protein-glutamine γ-glutamyltransferases family (or simply is_associated_with::transglutaminase family). Like other transglutaminases, it crosslinks is_associated_with::proteins between an ε-is_associated_with::amino group of a is_associated_with::lysine residue and a γ-is_associated_with::carboxamide group of is_associated_with::glutamine residue, creating an inter- or intramolecular bond that is highly resistant to is_associated_with::proteolysis (protein degradation). Aside from its crosslinking function, tTG catalyzes other types of reactions including is_associated_with::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 is_associated_with::cytosol but smaller amounts can also be found in the nucleus and the is_associated_with::mitochondria. Intracellular tTG is thought to play an important role in is_associated_with::apoptosis. In the extracellular space, tTG binds to proteins of the extracellular matrix (ECM), binding particularly tightly to is_associated_with::fibronectin. Extracellular tTG has been linked to cell adhesion, ECM stabilization, wound healing, receptor signaling, cellular proliferation, and cellular motility.

tTG is particularly notable for being the autoantigen in is_associated_with::coeliac disease, a lifelong illness in which the consumption of dietary is_associated_with::gluten causes a pathological immune response resulting in the inflammation of the small intestine and subsequent villous atrophy.

Mechanism
The catalytic mechanism for crosslinking in human tTG involves the is_associated_with::thiol group from a Cys residue in the active site of tTG. The thiol group attacks the carboxamide of a is_associated_with::glutamine residue on the surface of a protein or peptide substrate, releasing is_associated_with::ammonia, and producing a is_associated_with::thioester intermediate. The thioester intermediate can then be attacked by the surface amine of a second substrate (typically from a is_associated_with::lysine residue). The end product of the reaction is a stable is_associated_with::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 is_associated_with::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
Crosslinking activity by tTG requires the binding of Ca2+ ions. Multiple Ca2+ can bind to a single tTG molecule. 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 is_associated_with::guanine is_associated_with::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 is_associated_with::disulfide bond between two vicinal Cys residues. Therefore, 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 has been shown to activate extracellular tTG by reducing the disulfide bond. 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 2)



Genetics
The human tTG gene is located on the 20th chromosome (20q11.2-q12).

Physiology
tTG is expressed ubiquitously. It requires calcium as a cofactor for transamidation activity. Transcription is increased by is_associated_with::retinoic acid. Among its many supposed functions, it appears to play a role in is_associated_with::wound healing, is_associated_with::apoptosis, and is_associated_with::extracellular matrix development tTG is thought to be involved in the regulation of the cytoskeleton by crosslinking various cytoskeletal proteins including myosin,  actin, and is_associated_with::spectrin. Evidence shows that intracellular tTG crosslinks itself to myosin. It is also believed that tTG may stabilize the structure of the dying cells during apoptosis by polymerizing the components of the cytoskeleton, therefore preventing the leakage of the cellular contents into the extracellular space.

tTG also has is_associated_with::GTPase activity: In the presence of GTP, it suggested to function as a G protein participating in signaling processes. Besides its transglutaminase activity, tTG is proposed to also act as kinase, and protein disulfide isomerase, and deamidase. This latter activity is important in the deamidation of gliadin peptides, thus playing important role in the pathology of is_associated_with::coeliac disease.

Clinical significance
tTG is best known for its link with is_associated_with::celiac disease. is_associated_with::Anti-transglutaminase antibodies (ATA) result in a form of is_associated_with::gluten sensitivity in which a cellular response to Triticeae glutens that are crosslinked to tTG are able to stimulate transglutaminase specific is_associated_with::B-cell responses that eventually result in the production of ATA IgA and IgG.

tTG is believed to be involved in several neurodegenerative disorders including is_associated_with::Alzheimer, Parkinson and Huntington diseases. Such neurological diseases are characterized in part by the abnormal aggregation of proteins due to the increased activity of protein crosslinking in the affected brain. Additionally, specific proteins associated with these disorders have been found to be in vivo and in vitro substrates of tTG. Although tTG is up regulated in the areas of the brain affected by Huntington's disease, a recent study showed that increasing levels of tTG do not affect the onset and/or progression of the disease in mice.

Recent studies suggest that tTG also plays a role in is_associated_with::inflammation, and tumor biology. tTG expression is elevated in multiple cancer cell types and is implicated in drug resistance and metastasis due to its ability to promote mesenchymal transition and stem cell like properties.

Diagnostic use
is_associated_with::Serology for anti-tTG antibodies has superseded older serological tests (anti-endomysium, anti-gliadin, and anti-reticulin) and has a strong sensitivity (99%) and specificity (>90%) for identifying coeliac disease. Modern anti-tTG assays rely on a human recombinant protein as an antigen.

Therapeutic use
Use of tTG as a form of surgical glue is still experimental. It is also being studied as an attenuator of is_associated_with::metastasis in certain tumors.