Mechanism and Catalytic Site Atlas

Introduction

MTG is involved in catalysing an acyl transfer reaction between protein glutamine and alkylamine, producing protein N(5)-alkylglutamine and ammonia. The residues Cys 64, Asp 255 and His 274 superimpose well on the 'Cys, His, Asp' catalytic triad of other transglutaminases, such as Factor XIII. However, the relative positions of MTG His and Asp seem to be reversed relative to the Cys residue and therefore the catalytic triad is not conserved in the active site of MTG, Hence, a cysteine-like protease mechanism is proposed, in which Asp 255 plays the role of the His residue in factor XIII-like TGases. Cys 64 is sufficiently exposed to the solvent and is deprotonated by water. The thiolate ion of Cys 64 nucleophilically attacks an acyl donor, the carbonyl C of the substrate Gln residue side chain. Electrostatic interaction of Asp 255 with water facilitates donation of a proton from Asp 255 to the resulting oxyanion from nucleophilic attack, and ammonia is released. The Asp 255 gamma-carboxyl group and the His 274 imidazole ring form a hydrogen bond to allow a favourable conformation. The Asp 255 oxygen is negatively charged and nucleophilically attacks a proton of a Lys residue acyl acceptor. Lys 269 side chain acts as an acyl acceptor as the NH attacks the transition state acyl group. Cys 64 N and Trp 272 Ne1 contribute to the stabilising oxyanion hole by forming hydrogen bonds. The product is released from the oxyanion intermediate.