On the Mechanism of Action of Urocanase: Observation of the Enzyme-Bound NAD~+-Inhibitor Adduct by ~(13)C NMR Spectroscopy

摘要

Urocanase (EC 4.2.1,49) catalyzes the second step in the degradation of histidine in most cells (Scheme 1) and contains a tightly bound NAD~+ which is essential for catalytic activity. NAD~+ is supposed to function as an electrophilic catalyst and forms a transient covalent adduct with the imidazole ring of the substrate, urocanic acid (1). According to the mechanism postulated by us, the unusual addition of water leading to 5'-hydroxyimidazolepropionic acid 2 occurs in a chemically plausible manner.Intermediate 2 is in a spontaneous equilibrium with imidazolonepropionic acid 3.