Fig. 9.

Plausible catalytic mechanism for cyanuric acid hydrolysis by AtzD. Ser85 is activated by Lys42 and acts as nucleophile with bound CA (1), allowing nucleophilic attack at one of the three ketone positions of CA. The tetrahedral intermediate formed undergoes ring opening to form an acyl intermediate. The acylated serine is subsequently hydrolysed by a solvent water molecule, again activated by Lys42, via a tetrahedral intermediate to form carboxybiuret (2) which spontaneously decarboxylates to form biuret (3) and the active site is regenerated by intramolecular proton transfer.