GGT3P

Model summary

A secreted human enzyme that is produced as an inactive precursor and autocatalytically matures into a heterodimeric catalyst that cleaves and transfers gamma-glutamyl groups. By operating in the extracellular space and within exosomes, it degrades glutathione and related gamma-glutamyl conjugates, recycling amino acids and shaping lipid mediator pathways. Its N-terminal nucleophile chemistry enables both hydrolysis and transpeptidation, thereby coupling glutathione turnover to broader proteolysis and leukotriene biosynthesis.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The sequence is dominated by IPR029055 (Nucleophile aminohydrolases, N-terminal homologous superfamily, residues 36–568), which defines an N-terminal nucleophile (Ntn) hydrolase fold that undergoes autocatalytic maturation to expose a catalytic threonine. Nested within this scaffold is IPR000101 (Gamma-glutamyltranspeptidase family, residues 12–568), specifying the enzyme class that cleaves γ-glutamyl bonds. The internal layout resolves into two cooperating modules: IPR043138 (Gamma-glutamyltranspeptidase, large subunit homologous superfamily, residues 264–374) followed by IPR043137 (Gamma-glutamyltranspeptidase, small subunit, C-terminal domain homologous superfamily, residues 375–568). This arrangement dictates autocatalytic cleavage that yields a heterodimeric enzyme composed of a large and a small subunit; the small subunit contributes the catalytic N-terminus that becomes the active-site threonine. IPR055262 (Gamma-glutamyltranspeptidase, conserved site, residues 381–405) marks the catalytic region that stabilizes the Ntn fold and positions the nucleophile for acyl transfer and hydrolysis.

2. 02

   This Ntn hydrolase architecture causes two complementary molecular functions.

   First, the exposed threonine and surrounding oxyanion-stabilizing pocket catalyze γ-glutamyl bond cleavage and transpeptidation, which corresponds to GO:0016755 aminoacyltransferase activity and GO:0008242 omega peptidase activity. Second, the extensive β-sheet/α-helix surfaces and dimer interface create multivalent interfaces that mediate GO:0005515 protein binding, both for zymogen assembly and for transient substrate/adaptor interactions.

3. 03

   From these activities, the biological processes follow directly.

   Hydrolysis and transfer of γ-glutamyl groups drive GO:0006749 glutathione metabolic process by degrading extracellular glutathione and channeling its constituent amino acids. The enzyme’s core role in peptide bond cleavage places it in GO:0006508 proteolysis and GO:0006536 glutamate metabolic process, as γ-glutamyl turnover feeds into amino acid pools. Autocatalytic maturation of the proenzyme into its active heterodimer is intrinsic to function and embodies GO:0031638 zymogen activation. By generating cysteinylglycine and processing γ-glutamyl adducts, the enzyme supplies substrates and cofactors that feed lipid mediator pathways, aligning with GO:0006631 fatty acid metabolic process and GO:0019370 leukotriene biosynthetic process.

4. 04

   The cellular context is inferred from secretion and vesicular trafficking.

   The absence of transmembrane domains and the presence of a soluble Ntn hydrolase fold support an extracellular soluble enzyme that is secreted and packaged into vesicles. This supports localization to GO:0070062 extracellular exosome and, more broadly, the extracellular milieu where glutathione and leukotriene precursors are encountered.

5. 05

   Mechanistically, the enzyme is synthesized as a single-chain precursor that autocatalytically cleaves into large and small subunits; the small subunit exposes the catalytic threonine that forms a transient acyl-enzyme intermediate.

   In the presence of acceptor amino acids or peptides, the active site performs transpeptidation (aminoacyltransferase chemistry), whereas in their absence it proceeds to hydrolysis (omega peptidase chemistry). This dual-mode catalysis explains its role in amino acid recycling and in shaping extracellular leukotriene pathways.

6. 06

   Given its extracellular and exosomal residence and its need for transient partners, I hypothesize interactions with glutathione and cysteinylglycine handling systems, exosome biogenesis factors, and leukotriene pathway enzymes.

   Likely partners include extracellular matrix and vesicle-associated proteins that scaffold the enzyme near sites of glutathione turnover and lipid mediator production, as well as adaptor proteins that stabilize the heterodimer and guide its packaging into exosomes.