RESUMO
Canavan disease is an autosomal-recessive neurodegenerative disorder caused by a lack of aspartoacylase, the enzyme that degrades N-acetylaspartate (NAA) into acetate and aspartate. With a view to studying the mechanisms underlying the action of human aspartoacylase (hASP), this enzyme was expressed in a heterologous Escherichia coli system and characterized. The recombinant protein was found to have a molecular weight of 36 kDa and kinetic constants K(m) and k(cat) of 0.20 +/- 0.03 mM and 14.22 +/- 0.48 s(-1), respectively. Sequence alignment showed that this enzyme belongs to the carboxypeptidase metalloprotein family having the conserved motif H(21)xxE(24)(91aa)H(116). We further investigated the active site of hASP by performing modelling studies and site-directed mutagenesis. His21, Glu24 and His116 were identified here for the first time as the residues involved in the zinc-binding process. In addition, mutations involving the Glu178Gln and Glu178Asp residues resulted in the loss of enzyme activity. The finding that wild-type and Glu178Asp have the same K(m) but different k(cat) values confirms the idea that the carboxylate group contributes importantly to the enzymatic activity of aspartoacylase.
Assuntos
Amidoidrolases/química , Amidoidrolases/metabolismo , Doença de Canavan/enzimologia , Amidoidrolases/genética , Sequência de Aminoácidos , Substituição de Aminoácidos , Sequência de Bases , Doença de Canavan/genética , Domínio Catalítico/genética , DNA Complementar/genética , Escherichia coli/genética , Ácido Glutâmico/química , Histidina/química , Humanos , Técnicas In Vitro , Cinética , Ligantes , Modelos Moleculares , Dados de Sequência Molecular , Peso Molecular , Mutagênese Sítio-Dirigida , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Homologia de Sequência de Aminoácidos , Zinco/metabolismoRESUMO
This is the first report of a patient with aminoacylase I deficiency. High amounts of N-acetylated amino acids were detected by gas chromatography-mass spectrometry in the urine, including the derivatives of serine, glutamic acid, alanine, methionine, glycine, and smaller amounts of threonine, leucine, valine, and isoleucine. NMR spectroscopy confirmed these findings and, in addition, showed the presence of N-acetylglutamine and N-acetylasparagine. In EBV transformed lymphoblasts, aminoacylase I activity was deficient. Loss of activity was due to decreased amounts of aminoacylase I protein. The amount of mRNA for the aminoacylase I was decreased. DNA sequencing of the encoding ACY1 gene showed a homozygous c.1057 C>T transition, predicting a p.Arg353Cys substitution. Both parents were heterozygous for the mutation. The mutation was also detected in 5/161 controls. To exclude the possibility of a genetic polymorphism, protein expression studies were performed showing that the mutant protein had lost catalytic activity.