Mode of Action:

The mechanism through which aspartylglucosaminidase reacts is thought to be autocatalytic. The catalytic mechanism of AGA is like that of a Ser proteinase. The reaction catalyzed by AGA is one of the last steps in glycoprotein degradation. AGA breaks the amide bond between the asparagine and the oligosaccharide chain. The asparagine requires both a free alpha amino group and a free carboxy group for catalysis. The active site is buried deep within a funnel-shaped cleft that is formed by both subunits. The Thr206 is the key conserved amino acid during catalysis. The Thr 206 functions as a base (the alpha amino group) which increases the nucleophilicity of the alcohol of Thr206 . The alcohol oxygen forms a covalent bond to the carbonyl carbon of the glycoasparagine substrate. The transition state consists of the negative carbonyl oxygen which is stabalized by the oxyanion hole. The oxyanion hole is formed by the OH group of Thr257, which is hydrogen bonded to the hydroxyl group of Thr257 and Gly258 . During the catalytic mechanism, Thr 206, Thr224 , Arg234 , Asp237 and Thr257 are completely conserved. Ser72 is not conserved but is hydrogen bonded to Thr206 at the alpha amino group. Trp11 and Phe288 (shown in grey on the far right of the picture-be patient) are involved in the binding of the carbohydrate. The aspartic acid and carbohydrate are released when water comes in and breaks the complex.(2)

Fig. 1

The picture above discribes the stated autocatalytic mechanism in organic terms. It was obtained from an EMBO J., article titled Functional analyses of active site residues of human lysosomal aspartylglucosaminidase: implications for catalytic mechanism and autocatalytic activation, written by Tikkanen, Riikonen, Oinonen, Rouvinen and Peltonen. (2)

Reaction Catalyzed by AGA:

Fig.2

The reaction catalyzed by AGA. The amide linkage between the L-asparagine residue and glycan. Picture obtained from Nat. of struct. biology, article entitiled Three-dimensional structure of human lysosomal aspartylglucosaminidase.(6)