Autoantibody binding and unique enzyme-substrate intermediate conformation of human transglutaminase 3

Structural and mechanistic analysis of Ca2+-dependent regulation of transglutaminase 2 activity using a Ca2+-bound intermediate state

Mammalian transglutaminases, a family of Ca2+-dependent proteins, are implicated in a variety of diseases. For example, celiac disease (CeD) is an autoimmune disorder whose pathogenesis requires transglutaminase 2 (TG2) to deamidate select glutamine residues in diet-derived gluten peptides. Deamidation involves the formation of transient γ-glutamyl thioester intermediates. Recent studies have revealed that in addition to the deamidated gluten peptides themselves, their corresponding thioester intermediates are also pathogenically relevant. A mechanistic understanding of this relevance is hindered by the absence of any structure of Ca2+-bound TG2. We report the X-ray crystallographic structure of human TG2 bound to an inhibitory gluten peptidomimetic and two Ca2+ ions in sites previously designated as S1 and S3. Together with additional structure-guided experiments, this structure provides a mechanistic explanation for how S1 regulates formation of an inhibitory disulfide bond in TG2, while also establishing that S3 is essential for γ-glutamyl thioester formation. Furthermore, our crystallographic findings and associated analyses have revealed that i) two interacting residues, H305 and E363, play a critical role in resolving the thioester intermediate into an isopeptide bond (transamidation) but not in thioester hydrolysis (deamidation); and ii) residues N333 and K176 stabilize preferred TG2 substrates and inhibitors via hydrogen bonding to nonreactive backbone atoms. Overall, the intermediate-state conformer of TG2 reported here represents a superior model to previously characterized conformers for both transition states of the TG2-catalyzed reaction.

Coeliac disease: the paradox of diagnosing a food hypersensitivity disorder with autoantibodies

Serum antibodies to the autoantigen transglutaminase 2 (TG2) are increasingly harnessed to diagnose coeliac disease. Diagnostic guidelines for children give recommendation for a no-biopsy-based diagnosis through detection of high amounts of IgA anti-TG2 antibodies in serum with confirmation of positivity in a separate blood sample by characteristic autoantibody-staining of tissue. While measurement of IgA anti-TG2 also is important in the diagnostic workup of adults, the adult guidelines still mandate examination of gut biopsies. This requirement might well change in the future, as might the necessity for confirming autoantibody positivity by tissue staining. The key role of autoantibody serology for diagnosis of coeliac disease is paradoxical. Coeliac disease was considered, and still can be considered, a food intolerance disorder where autoantibodies at face value are out of place. The immunological mechanisms underlying the formation of autoantibodies in response to gluten exposure have been dissected. This review presents the current insights demonstrating that the autoantibodies in coeliac disease are intimately integrated in the maladapted immune response to gluten.