Members of this family are ring-opening amidohydrolases, including cyanuric acid amidohydrolase (CAH) (EC:3.5.2.15) (AtzD and TrzD) and barbituric acid hydrolase (BAH). Note that barbiturase does not act as defined for EC:3.5.2.1 (barbiturate + water = malonate + urea) but rather catalyses the ring opening of barbiturase acid to ureidomalonic acid. Cyanuric acid hydrolases (AtzD) and barbiturases are homologous and found almost exclusively in bacteria [1]. CAH is a homotetramer, the monomer of which possesses a fold that is dubbed the Toblerone fold. Although not obvious from the protein sequence, each CAH monomer possesses 3-fold rotational symmetry, for which the repeated structural motif is structurally similar to that of the YjgF superfamily of proteins (two alpha helices and a four-strand antiparallel beta-sheet), despite having low sequence similarity. This suggests that the Toblerone fold evolved from concatenated YjgF family proteins. The 3-fold symmetry of each CAH monomer extends to the active site, which contains three Arg-Lys-Ser triads, which bind the substrate via an extensive hydrogen bonding network [2].

1: Defining sequence space and reaction products within the cyanuric acid hydrolase (AtzD)/barbiturase protein family. Seffernick JL, Erickson JS, Cameron SM, Cho S, Dodge AG, Richman JE, Sadowsky MJ, Wackett LP; J Bacteriol. 2012;194:4579-4588. PMID:22730121

2: High-Resolution X-Ray Structures of Two Functionally Distinct Members of the Cyclic Amide Hydrolase Family of Toblerone Fold Enzymes. Peat TS, Balotra S, Wilding M, Hartley CJ, Newman J, Scott C; Appl Environ Microbiol. 2017; [Epub ahead of print] PMID:28235873