Probing the specificity of gamma-glutamylamine cyclotransferase: an enzyme involved in the metabolism of transglutaminase-catalyzed protein crosslinks

Abstract

γ-Glutamylamine cyclotransferase (gGACT) catalyzes the intramolecular cyclization of a variety of L-γ-glutamylamines producing 5-oxo-L-proline and free amines. Its substrate specificity implicates it in the downstream metabolism of transglutaminase products, and is distinct from that of γ-glutamyl cyclotransferase which acts on L-γ-glutamyl amino acids. To elucidate the mechanism by which gGACT distinguishes between L-γ-glutamylamine and amino acid substrates, the specificity of the rabbit kidney enzyme for the amide region of substrates was probed through the kinetic analysis of a series of L-γ-glutamylamines. The isodipeptide N(ε)-(L-γ-glutamyl)-L-lysine 1 was used as a reference. The kinetic constants of the L-γ-glutamyl derivative of n-butylamine 7, were nearly identical to those of 1. Introduction of a methyl or carboxylate group on the carbon adjacent to the side-chain amide nitrogen in L-γ-glutamylamine substrates resulted in a dramatic decrease in substrate properties for gGACT thus providing an explanation of why gGACT does not act on L-γ-glutamyl amino acids except for L-γ-glutamylglycine. Placement of substituents on carbons further removed from the side-chain amide nitrogen in L-γ-glutamylamines restored activity for gGACT, and L-γ-glutamylneohexylamine 19 had a higher specificity constant (k(cat) /K(m)) than 1. gGACT did not exhibit any stereospecificity in the amide region of L-γ-glutamylamine substrates. In addition, analogues (26-30) with heteroatom substitutions for the γ methylene position of the L-γ-glutamyl moiety were examined. Several thiocarbamoyl derivatives of L-cysteine (28-30) were excellent substrates for gGACT.