Homology modelling and structural analysis of human arylamine N-acetyltransferase NAT1: evidence for the conservation of a cysteine protease catalytic domain and an active-site loop

Arylamine N-acetyltransferases (EC 2.3.1.5) (NATs) catalyse the biotransfor mation of many primary arylamines, hydrazines and their N-hydroxylated meta bolites, thereby playing an important role in both the detoxification and m etabolic activation of numerous xenobiotics. The recently published crystal structure of the Salmonella typhimurium NAT (StNAT) revealed the existence of a cysteine protease-like (Cys-His-Asp) catalytic triad. In the present study, a three-dimensional homology model of human NAT1, based upon the cry stal structure of StNAT [Sinclair, Sandy, Delgoda, Sim and Noble (2000) Nat . Struct. Biol. 7, 560-564], is demonstrated. Alignment of StNAT and NAT1, together with secondary structure predictions, have defined a consensus reg ion (residues 29-131) in which 37 % of the residues are conserved. Homology modelling provided a good quality model of the corresponding region in hum an NAT1. The location of the catalytic triad was found to be identical in S tNAT and NAT1. Comparison of active-site structural elements revealed that a similar length loop is conserved in both species (residues 122-131 in NAT 1 model and residues 122-133 in StNAT). This observation may explain the in volvement of residues 125, 127 and 129 in human NAT substrate selectivity. Our model, and the fact that cysteine protease inhibitors do not affect the activity of NAT1, suggests that human NATs may have adapted a common catal ytic mechanism from cysteine proteases to accommodate it for acetyl-transfe r reactions.