Glycosylasparaginase inhibition studies: competitive inhibitors, transition state mimics, noncompetitive inhibitors

Identification of Small Molecule Compounds for Pharmacological Chaperone Therapy of Aspartylglucosaminuria

Aspartylglucosaminuria (AGU) is a lysosomal storage disorder that is caused by genetic deficiency of the enzyme aspartylglucosaminidase (AGA) which is involved in glycoprotein degradation. AGU is a progressive disorder that results in severe mental retardation in early adulthood. No curative therapy is currently available for AGU. We have here characterized the consequences of a novel AGU mutation that results in Thr122Lys exchange in AGA, and compared this mutant form to one carrying the worldwide most common AGU mutation, AGU-Fin. We show that T122K mutated AGA is expressed in normal amounts and localized in lysosomes, but exhibits low AGA activity due to impaired processing of the precursor molecule into subunits. Coexpression of T122K with wildtype AGA results in processing of the precursor into subunits, implicating that the mutation causes a local misfolding that prevents the precursor from becoming processed. Similar data were obtained for the AGU-Fin mutant polypeptide. We have here also identified small chemical compounds that function as chemical or pharmacological chaperones for the mutant AGA. Treatment of patient fibroblasts with these compounds results in increased AGA activity and processing, implicating that these substances may be suitable for chaperone mediated therapy for AGU.

Carbohydrate-Processing Enzymes of the Lysosome: Diseases Caused by Misfolded Mutants and Sugar Mimetics as Correcting Pharmacological Chaperones

Lysosomal storage diseases are hereditary disorders caused by mutations on genes encoding for one of the more than fifty lysosomal enzymes involved in the highly ordered degradation cascades of glycans, glycoconjugates, and other complex biomolecules in the lysosome. Several of these metabolic disorders are associated with the absence or the lack of activity of carbohydrate-processing enzymes in this cell compartment. In a recently introduced therapy concept, for susceptible mutants, small substrate-related molecules (so-called pharmacological chaperones), such as reversible inhibitors of these enzymes, may serve as templates for the correct folding and transport of the respective protein mutant, thus improving its concentration and, consequently, its enzymatic activity in the lysosome. Carbohydrate-processing enzymes in the lysosome, related lysosomal diseases, and the scope and limitations of reported reversible inhibitors as pharmacological chaperones are discussed with a view to possibly extending and improving research efforts in this area of orphan diseases.

Identification of an aspartylglucosaminidase-like protein in the venom of the parasitic wasp Asobara tabida (Hymenoptera: Braconidae)

This study was designed to identify one of the main components of venomous secretions of the endoparasitic wasp Asobara tabida. By using electrophoretic methods, partial amino acid sequencing and immunostaining, we demonstrated the presence of an aspartylglucosaminidase (AGA)-like protein in the venom of this insect. The enzyme had a polymeric conformation and was formed of 30 and 18 kDa subunits. The relative positions of several amino acids involved in substrate binding and catalytic activity of known AGA-proteins, which are usually lysosomal enzymes, were conserved in the NH(2)-terminal ends of these subunits. Antibodies raised against human AGA recognized the two subunits of the protein and a 44 kDa protein, suggesting the presence of a precursor molecule of the enzyme in the venom. However, no reliable measurement of the AGA activity could be performed on the venom extracts, which could be explained by the fact the enzyme would be stored in the reservoir of the venom apparatus under an inactive form. These results constitute the first description of an AGA-like protein in an insect venom and are discussed with respect to the knowledge acquired on lysosomal and venom enzymes.

Acylation is rate-limiting in glycosylasparaginase-catalyzed hydrolysis of N4-(4'-substituted phenyl)-L-asparagines

Glycosylasparaginase catalyzes the hydrolysis of the N-glycosylic bond between N-acetyl-D-glucosamine and L-asparagine in the catabolism of glycoproteins. The mechanism has been proposed to resemble that of serine proteases involving an acylation step where a nucleophilic attack by a catalytic Thr residue on the carbonyl carbon of the N-glycosylic bond gives rise to a covalent beta-aspartyl-enzyme intermediate, and a deacylation step to give the final products. The question posed in this study was: Is the acylation step the rate-limiting step in the hydrolysis reaction as in serine proteases? To answer this question a series of mostly new substituted anilides was synthesized and characterized, and their hydrolysis reactions catalyzed by glycosylasparaginase from human amniotic fluid were studied. Five N4-(4'-substituted phenyl)-L-asparagine compounds were synthesized and characterized: 4'-hydrogen, 4'-ethyl, 4'-bromo, 4'-nitro, and 4'-methoxy. Each of these anilides was a substrate for the enzyme. Hammett plots of the kinetic parameters showed that acylation is the rate-limiting step in the reaction and that upon binding the electron distribution of the substrate is perturbed toward the transition state. This is the first direct evidence that acylation is the rate-limiting step in the enzyme-catalyzed reaction. A Brønsted plot indicates a small, negative charge (-0.25) on the nitrogen atom of the leaving group anilines containing electron-withdrawing groups, and a small, positive charge (0.43) on the nitrogen atom of the leaving group anilines containing electron-donating groups. The free energy (incremental) change of binding (delta deltaGb) in the enzyme-substrate transition state complexes shows that substitution of a substituted phenyl group for the pyranosyl group in the natural substrate results in an overall loss of binding energy equivalent to a weak hydrogen bond, the magnitude of which is dependent on the substituent group. The data are consistent with a mechanism for glycosylasparaginase involving rapid formation of a tetrahedral structure upon substrate binding, and a rate-limiting breakdown of the tetrahedral structure to a covalent beta-aspartyl-enzyme intermediate that is dependent on the electronic properties of the substituent group and on the degree of protonation of the leaving group in the transition state by a general acid.

Glycosylasparaginase activity requires the alpha-carboxyl group, but not the alpha-amino group, on N(4)-(2-Acetamido-2-deoxy-beta-D-glucopyranosyl)-L-asparagine

Glycosylasparaginase catalyzes the hydrolysis of the N-glycosylic bond in N(4)-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-L-asparagine in the catabolism of N-linked oligosaccharides. A deficiency, or absence, of enzyme activity gives rise to aspartylglycosaminuria, the most common disorder of glycoprotein metabolism. The enzyme catalyzes the hydrolysis of a variety of asparagine and aspartyl compounds containing a free alpha-carboxyl group and a free alpha-amino group; computational studies suggest that the alpha-amino group actively participates in the catalytic mechanism. In order to study the importance of the alpha-carboxyl group and the alpha-amino group on the natural substrate to the reaction catalyzed by the enzyme, 14 analogues of the natural substrate were studied where the structure of the aspartyl group of the substrate was changed. The incremental binding energy (DeltaDeltaGb) for those analogues that were substrates was calculated. The results show that the alpha-amino group may be substituted with a group of comparable size, for the alpha-amino group contributes little, if any, to the transition state binding energy of the natural substrate. The alpha-amino group position acts as an "anchor" in the binding site for the substrate. On the other hand, the alpha-carboxyl group is necessary for enzyme activity; removal of the alpha-carboxyl group or changing it to an alpha-carboxamide group results in no hydrolysis reaction. Also, N-acetyl-D-glucosamine is not sufficient for binding to the active site for efficient hydrolysis by the enzyme. These results provide supporting evidence for a proposed intramolecular autoproteolytic activation reaction for the enzyme. However, the results raise a question as to an important role for the alpha-amino group in the catalytic mechanism as indicated in computational studies.