Insights into how protein dynamics affects arylamine N-acetyltransferase catalysis

Backbone resonance assignment of an insect arylalkylamine N-acetyltransferase from Bombyx mori reveals conformational heterogeneity

Arylalkylamine N-acetyltransferases (AANATs) catalyze the transfer of an acetyl group from the acetyl-group donor, acetyl-CoA, to an arylalkylamine acceptor. Although a single AANAT has been identified in mammals, insects utilize multiple AANATs in a diverse array of biological processes. AANATs belong to the GCN5-related acetyltransferase (GNAT) superfamily of enzymes, which despite their overall very low sequence homology, are characterized by a well conserved catalytic core domain. The structural properties of many GNATs have been extensively studied by X-ray crystallography that revealed common features during the catalytic cycle. Here we report the ¹H, ¹³C and ¹⁵N backbone NMR resonance assignment of the 24 kDa AANAT3 from Bombyx mori (bmAANAT3) as a first step towards understanding the role of protein dynamics in the catalytic properties of AANATs. Our preliminary solution NMR studies reveal that bmAANAT3 is well-folded in solution. The P-loop, which is responsible for cofactor binding, is flexible in the free-state, while a large region of the enzyme interconverts between two distinct conformations in the slow exchange regime.

Probing the catalytic potential of the hamster arylamine N-acetyltransferase 2 catalytic triad by site-directed mutagenesis of the proximal conserved residue, Tyr190

Arylamine N-acetyltransferases (NATs) play an important role in both the detoxification of arylamine and hydrazine drugs and the activation of arylamine carcinogens. Because the catalytic triad, Cys-His-Asp, of mammalian NATs has been shown to be essential for maintaining protein stability, rendering it impossible to assess alterations of the triad on catalysis, we explored the impact of the highly conserved proximal residue, Tyr190, which forms a direct hydrogen bond interaction with one of the triad residues, Asp122, as well as a potential pi-pi stacking interaction with the active site His107. The replacement of hamster NAT2 Tyr190 by either Phe, Ile or Ala was well tolerated and did not result in significant alterations in the overall fold of the protein. Nevertheless, stopped-flow and steady-state kinetic analysis revealed that Tyr190 was critical for maximizing the acetylation rate of NAT2 and the transacetylation rate of p-aminobenzoic acid when compared with the wild-type. Tyr190 was also shown to play an important role in determining the pK(a) of the active site Cys during acetylation, as well as the pH versus the rate profile for transacetylation. We hypothesized that the pH dependence was associated with global changes in the active site structure, which was revealed by the superposition of [(1)H, (15)N] heteronuclear single quantum coherence spectra for the wild-type and Y190A. These results suggest that NAT2 catalytic efficiency is partially governed by the ability of Tyr190 to mediate the collective impact of multiple side chains on the electrostatic potential and local conformation of the active site.