A density functional theory study on the role of His-107 in arylamine N-acetyltransferase 2 acetylation.

Arylamine N-acetyltransferases (NATs, EC 2.3.1.5) catalyze an acetyl group transfer from acetyl coenzyme A (AcCoA) to primary arylamines, and are responsible for the biotransformation and metabolism of drugs, carcinogens, etc. Structure analysis revealed that His-107 was likely the residue accountable for mediating acetyl transfer. We have examined the full catalytic mechanism of this system by means of DFT method. The results indicate that if the acetyl group directly transferred from the donor, p-nitrophenyl acetate, to the acceptor, cysteine, the high activation energy will be a great hindrance. These energies have dropped a little in a range of 20-25 kJ/mol when His-107 is assisting the transfer process. However, when protonated His-107 is mediating the reaction, the activation energies have dropped about 70-85 kJ/mol. Our calculations strongly support an enzymatic acetylation mechanism that experiences a thiolate-imidazolium pair, which have verified the presumption from experiments.