N-Acetyl-D-glucosamine-6-phosphate deacetylase: substrate activation via a single divalent metal ion.
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abstract
NagA is a member of the amidohydrolase superfamily and catalyzes the deacetylation of N-acetyl-d-glucosamine-6-phosphate. The catalytic mechanism of this enzyme was addressed by the characterization of the catalytic properties of metal-substituted derivatives of NagA from Escherichia coli with a variety of substrate analogues. The reaction mechanism is of interest since NagA from bacterial sources is found with either one or two divalent metal ions in the active site. This observation indicates that there has been a divergence in the evolution of NagA and suggests that there are fundamental differences in the mechanistic details for substrate activation and hydrolysis. NagA from E. coli was inactivated by the removal of the zinc bound to the active site and the apoenzyme reactivated upon incubation with 1 equiv of Zn2+, Cd2+, Co2+, Mn2+, Ni2+, or Fe2+. In the proposed catalytic mechanism the reaction is initiated by the polarization of the carbonyl group of the substrate via a direct interaction with the divalent metal ion and His-143. The invariant aspartate (Asp-273) found at the end of beta-strand 8 in all members of the amidohydrolase superfamily abstracts a proton from the metal-bound water molecule (or hydroxide) to promote the hydrolytic attack on the carbonyl group of the substrate. A tetrahedral intermediate is formed and then collapses with cleavage of the C-N bond after proton transfer to the leaving group amine by Asp-273. The lack of a solvent isotope effect by D2O and the absence of any changes to the kinetic constants with increases in solvent viscosity indicate that net product formation is not limited to any significant extent by proton-transfer steps or the release of products. N-Trifluoroacetyl-d-glucosamine-6-phosphate is hydrolyzed by NagA 26-fold faster than the corresponding N-acetyl derivative. This result is consistent with the formation or collapse of the tetrahedral intermediate as the rate limiting step in the catalytic mechanism of NagA.
