Glycosylasparaginase-catalyzed synthesis and hydrolysis of beta-aspartyl peptides

beta-Aspartyl di- and tripeptides are common constituents of mammalian metabolism, but their formation and catabolism are not fully understood. In this study we provide evidence that glycosylasparaginase (aspartylglucosaminidase), an N-terminal nucleophile hydrolase involved in the hydrolysis of the N-glycosidic bond in glycoproteins, catalyzes the hydrolysis of beta-aspartyl peptides to form L-aspartic acid and amino acids or peptides. The enzyme also effectively catalyzes the synthesis of beta-aspartyl peptides by transferring the beta-aspartyl moiety from other beta-aspartyl peptides or beta-aspartylglycosylamine to a variety of amino acids and peptides. Furthermore, the enzyme can use L-asparagine as the beta-aspartyl donor in the formation of beta-aspartyl peptides. The data show that synthesis and degradation of beta-aspartyl peptides are new, significant functions of glycosylasparaginase and suggest that the enzyme could have an important role in the metabolism of beta-aspartyl peptides.

Recombinant human glycosylasparaginase catalyzes hydrolysis of L-asparagine

Glycosylasparaginase is a lysosomal amidase involved in the degradation of glycoproteins. Recombinant human glycosylasparaginase is capable of catalyzing the hydrolysis of the amino acid L-asparagine to L-aspartic acid and ammonia. For the hydrolysis of L-asparagine the Km is 3-4-fold higher and Vmax 1/5 of that for glycoasparagines suggesting that the full catalytic potential of glycosylasparaginase is not used in the hydrolysis of the free amino acid. L-Asparagine competitively inhibits the hydrolysis of aspartylglucosamine indicating that both the amino acid and glycoasparagine are interacting with the same active site of the enzyme. The hydrolytic mechanism of L-asparagine and glycoasparagines will be discussed.

Enzymatic synthesis of the N-glycosidic bond by beta-aspartylation of glycosylamines

Glycosylasparaginase (EC 3.5.1.26) is an amidase, which cleaves the N-glycosidic linkage during glycoprotein degradation leading to the liberation of L-aspartic acid from various glycoasparagines. In this work we demonstrate that glycosylasparaginase is also capable of catalyzing the synthesis of the N-glycosidic bond by N-beta-aspartylation of beta-glycosylamine using 1-amino-N-acetylglucosamine as the nucleophile and L-aspartic acid beta-methyl ester as the beta-aspartyl donor. Kinetic studies indicated that beta-glycosylamine has 1390-fold higher reactivity than water in the de-beta-aspartylation of the beta-aspartylenzyme, indicative of the presence of a beta-glycosylamine binding sub-site at the substrate binding site of glycosylasparaginase. The reaction can be applied to glycosylaparaginase-catalyzed biosynthesis of novel glycoasparagines.