Recebido em 16/10/11; aceito em 26/3/11; publicado na web em 26/6/12 UDP-N-ACETYLGLUCOSAMINE-ENOLPYRUVYL TRANSFERASE: DETERMINATION OF PROTONATION STATE OF ACTIVE SITE AMINOACID RESIDUES BY PM6 METHOD. UDP-N-acetylglucosamine-enolpyruvyl transferase (MurA) catalyzes the reaction between phosphoenol pyruvate and UDP-N-acetylglucosamine. We present a theoretical approach using the semiempirical PM6 method for defining protonation state of three active site residues, K22, H125, and K160. Prior comparison with neutron diffraction data showed that PM6 accurately predicted protonation states of active site residues of b-trypsin and D-xylose isomerase. Using the same methodology with MurA crystallographic data, we conclude that when reaction intermediate is located at the active site, H125 and K22 are in protonated form and K160 in neutral form.