Autocatalytic cleavage of human gamma-glutamyl transpeptidase is highly dependent on N-glycosylation at asparagine 95

J Biol Chem. 2011 Aug 19;286(33):28876-28888. doi: 10.1074/jbc.M111.248823. Epub 2011 Jun 28.

Abstract

γ-Glutamyl transpeptidase (GGT) is a heterodimeric membrane enzyme that catalyzes the cleavage of extracellular glutathione and other γ-glutamyl-containing compounds. GGT is synthesized as a single polypeptide (propeptide) that undergoes autocatalytic cleavage, which results in the formation of the large and small subunits that compose the mature enzyme. GGT is extensively N-glycosylated, yet the functional consequences of this modification are unclear. We investigated the effect of N-glycosylation on the kinetic behavior, stability, and functional maturation of GGT. Using site-directed mutagenesis, we confirmed that all seven N-glycosylation sites on human GGT are modified by N-glycans. Comparative enzyme kinetic analyses revealed that single substitutions are functionally tolerated, although the N95Q mutation resulted in a marked decrease in the cleavage efficiency of the propeptide. However, each of the single site mutants exhibited decreased thermal stability relative to wild-type GGT. Combined mutagenesis of all N-glycosylation sites resulted in the accumulation of the inactive propeptide form of the enzyme. Use of N-glycosylation inhibitors demonstrated that binding of the core N-glycans, not their subsequent processing, is the critical glycosylation event governing the autocleavage of GGT. Although N-glycosylation is necessary for maturation of the propeptide, enzymatic deglycosylation of the mature wild-type GGT does not substantially impact either the kinetic behavior or thermal stability of the fully processed human enzyme. These findings are the first to establish that co-translational N-glycosylation of human GGT is required for the proper folding and subsequent cleavage of the nascent propeptide, although retention of these N-glycans is not necessary for maintaining either the function or structural stability of the mature enzyme.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Amino Acid Substitution
  • Asparagine / genetics
  • Asparagine / metabolism
  • Catalysis
  • Enzyme Stability / physiology
  • Glycosylation
  • HEK293 Cells
  • Humans
  • Kinetics
  • Mutagenesis, Site-Directed
  • Mutation, Missense
  • Protein Folding*
  • Protein Modification, Translational / physiology*
  • Structure-Activity Relationship
  • gamma-Glutamyltransferase / genetics
  • gamma-Glutamyltransferase / metabolism*

Substances

  • Asparagine
  • gamma-Glutamyltransferase
  • gamma-glutamyltransferase, human