Characterization of the Biosynthesis, Processing and Kinetic Mechanism of Action of the Enzyme Deficient in Mucopolysaccharidosis IIIC

Heparin acetyl-CoA:alpha-glucosaminide N-acetyltransferase (N-acetyltransferase, EC 2.3.1.78) is an integral lysosomal membrane protein containing 11 transmembrane domains, encoded by the HGSNAT gene. Deficiencies of N-acetyltransferase lead to mucopolysaccharidosis IIIC. We demonstrate that contrary to a previous report, the N-acetyltransferase signal peptide is co-translationally cleaved and that this event is required for its intracellular transport to the lysosome. While we confirm that the N-acetyltransferase precursor polypeptide is processed in the lysosome into a small amino-terminal alpha- and a larger ß- chain, we further characterize this event by identifying the mature amino-terminus of each chain. We also demonstrate this processing step(s) is not, as previously reported, needed to produce a functional transferase, i.e., the precursor is active. We next optimize the biochemical assay procedure so that it remains linear as N-acetyltransferase is purified or protein-extracts containing N-acetyltransferase are diluted, by the inclusion of negatively charged lipids. We then use this assay to demonstrate that the purified single N-acetyltransferase protein is both necessary and sufficient to express transferase activity, and that N-acetyltransferase functions as a monomer. Finally, the kinetic mechanism of action of purified N-acetyltransferase was evaluated and found to be a random sequential mechanism involving the formation of a ternary complex with its two substrates; i.e., N-acetyltransferase does not operate through a ping-pong mechanism as previously reported. We confirm this conclusion by demonstrating experimentally that no acetylated enzyme intermediate is formed during the reaction

A sensitive fluorescence-based assay for monitoring GM2 ganglioside hydrolysis in live patient cells and their lysates

Enzyme enhancement therapy, utilizing small molecules as pharmacological chaperones, is an attractive approach for the treatment of lysosomal storage diseases that are associated with protein misfolding. However, pharmacological chaperones are also inhibitors of their target enzyme. Thus, a major concern with this approach is that, despite enhancing protein folding within, and intracellular transport of the functional mutant enzyme out of the endoplasmic reticulum, the chaperone will continue to inhibit the enzyme in the lysosome, preventing substrate clearance. Here we demonstrate that the in vitro hydrolysis of a fluorescent derivative of lyso-GM2 ganglioside, like natural GM2 ganglioside, is specifically carried out by the ?-hexosaminidase A isozyme, requires the GM2 activator protein as a co-factor, increases when the derivative is incorporated into anionic liposomes and follows similar Michaelis\u2013Menten kinetics. This substrate can also be used to differentiate between lysates from normal and GM2 activator-deficient cells. When added to the growth medium of cells, the substrate is internalized and primarily incorporated into lysosomes. Utilizing adult Tay\u2013Sachs fibroblasts that have been pre-treated with the pharmacological chaperone Pyrimethamine and subsequently loaded with this substrate, we demonstrate an increase in both the levels of mutant ?-hexosaminidase A and substrate-hydrolysis as compared to mock-treated cells.Peer reviewed: YesNRC publication: Ye

Pyrimethamine Derivatives: Insight into Binding Mechanism and Improved Enhancement of Mutant β‑<i>N</i>-acetylhexosaminidase Activity

In order to identify structural features of pyri­meth­amine (5-(4-chloro­phenyl)-6-ethyl­pyrimidine-2,4-diamine) that contribute to its inhibitory activity (IC₅₀ value) and chaperoning efficacy toward β-<i>N</i>-acetyl­hexos­aminidase, derivatives of the compound were synthesized that differ at the positions bearing the amino, ethyl, and chloro groups. Whereas the amino groups proved to be critical to its inhibitory activity, a variety of substitutions at the chloro position only increased its IC₅₀ by 2–3-fold. Replacing the ethyl group at the 6-position with butyl or methyl groups increased IC₅₀ more than 10-fold. Surprisingly, despite its higher IC₅₀, a derivative lacking the chlorine atom in the <i>para</i>-position was found to enhance enzyme activity in live patient cells a further 25% at concentrations >100 μM, while showing less toxicity. These findings demonstrate the importance of the phenyl group in modulating the chaperoning efficacy and toxicity profile of the derivatives

Synthesis of lipophilic 1-deoxygalactonojirimycin derivatives as D-galactosidase inhibitors

N-Alkylation at the ring nitrogen of the D-galactosidase inhibitor 1-deoxygalactonojirimycin with a functionalised C6 alkyl chain followed by modification with different aromatic substituents provided lipophilic 1-deoxygalactonojirimycin derivatives which exhibit inhibitory properties against β-glycosidases from E. coli and Agrobacterium sp. as well as green coffee bean α-galactosidase. In preliminary studies, these compounds also showed potential as chemical chaperones for GM1-gangliosidosis related β-galactosidase mutants