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Glycosylasparaginase (EC3.5.1.26) is an important catabolic enzyme in glyeobiology. This amidase consists of a and (3 subunits that are derived from a single gene product by posttranslational cleavage (Tollersrud and Aronson, 1989; Fisher et al, 1990; Fisher and Aronson, 1991; Ikonen et al, 1993; Guan et al, 1996). The enzyme can hydrolyze a number of [J-aspartyl-amides, including asparagine itself (Tanaka et al, 1973; Kaartinen et al, 1992). Mammalian glycosylasparaginase is located in lysosomes where its only physiological substrate is the Asn-GlcNAc protein-to-carbohydrate linkage that occurs in the major class of asparagine-linked glycoproteins (Mononen et al, 1993) (Figure 1). This mini-review discusses glycosylasparaginase as a member of a newly classified set of amidases that have a novel processed N-terminal threonine, serine, or cysteine involved as a nucleophile in their reaction (Duggleby et al., 1995; Lowe et al., 1995; Smith, 1995). Discovery of the unique structural biology of this family of 'Ntnhydrolases' occurred during the past year and a half based initially on the characterization of the three-dimensional structures of two members that utilize threonine and serine, respectively. Thus, Lowe et al. (1995) reported the crystal structure of the 20S (a7p7p7a7) proteasome from the archaebacterium Thermoplasma acidophilum, and this work, along with two other biochemical studies (Fenteany et al, 1995; Seemiiller et al, 1995) revealed these multifunctional proteolytic organelles use N-terminal threonines at the active sites of their mature (3 subunits as nucleophiles to catalyze peptide-bond hydrolysis. An earlier crystallographic study of E.coli penicillin amidohydrolase by Duggleby et al. (1995) showed an active-site Nterminal serine is the nucleophile in the hydrolysis of its substrate, penicillin amide. No histidine or other basic amino acid is present in the active sites of these two amidases, suggesting a unique class of amide hydrolyzing enzymes exists that is different from four previously described classical proteinase families (serine or cysteine proteinases, metallo-proteinases, and aspartic proteinases) (Goldberg, 1995). A distinctive mechanism was proposed for the catalytic reaction of these two enzymes: the free a-amino group on the N-terminal threonine or serine of their (3 subunits acts as the base required to enhance the nucleophilicity of its own side-chain hydroxyl moiety (Duggleby et al, 1995; Lowe et al., 1995). This intraresidue base on the Thr or Ser replaces the well-characterized histidine base that becomes folded into a hydrogen-bonded triad at the active-site of many typical serine proteases (Barrett and Rawlings, 1995). It now has become evident that this new amidase mechanism along with a special protein fold at the active site occurs in a number of different enzymes (Brannigan et al, 1995), and it is likely others will be discovered.

glycobiology

Lysosomal glycosylasparaginase: a member of a family of amidases that employ a processed N-terminal threonine, serine or cysteine as a combined base-nucleophile catalyst