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The Ntn hydrolases (N-terminal nucleophile) are a diverse superfamily of of enzymes that are activated autocatalytically via an N-terminally lcated nucleophilic amino acid. N-terminal nucleophile (NTN-) hydrolase superfamily, which contains a four-layered alpha, beta, beta, alpha core structure. This family of hydrolases includes penicillin acylase, the 20S proteasome alpha and beta subunits, and glutamate synthase. The mechanism of activation of these proteins is conserved, although they differ in their substrate specificities. All known members catalyze the hydrolysis of amide bonds in either proteins or small molecules, and each one of them is synthesized as a preprotein. For each, an autocatalytic endoproteolytic process generates a new N-terminal residue. This mature N-terminal residue is central to catalysis and acts as both a polarizing base and a nucleophile during the reaction. The N-terminal amino group acts as the proton acceptor and activates either the nucleophilic hydroxyl in a Ser or Thr residue or the nucleophilic thiol in a Cys residue. The position of the N-terminal nucleophile in the active site and the mechanism of catalysis are conserved in this family, despite considerable variation in the protein sequences.
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