  3.5.1.122 | N-terminal L-glutaminyl-[protein] + H2O = N-terminal L-glutamyl-[protein] + NH3 |
catalytic mechanism determination of hNtaq1 based on the crystal structure of hNtaq1 and docking study. In the first step, nucleophilic sulfhydryl group of Cys28 approaches Cd of the amide group of the N-terminal glutamine and becomes deprotonated by His81. The sulfhydryl group of Cys28 plays a crucial role in the nucleophilic attack on acyl group in the N-terminal glutamine side chain of substrates, which results in formation of a tetrahedral intermediate. Asp97 facilitates the process by forming a hydrogen bond and electrostatic interactions with His81. The ammonia is released upon productive collapse of the tetrahedral intermediate and a water molecule enters the active site cavity and attacks S-acyl intermediate to convert glutamine to a glutamate. As the final step, the glutamate side chain is cleaved from S-acyl of Cys28. His81 first acts as a general base activation water for a nucleophilic attack on the S-acyl intermediate, and then upon collapse of the tetrahedral intermediate acts a general acid to protonate the leaving group, i.e. the thiolate of Cys28. The substrate peptide with newly formed N-terminal glutamate is released from the binding cleft at this stage and the enzyme is ready for another round of catalysis |
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