Catalytic domain of bacterial PuuE allantoinases, Schizosaccharomyces pombe chitin deacetylase 1 (SpCDA1), and similar proteins
Allantoinase (EC 3.5.2.5) can hydrolyze allantoin((2,5-dioxoimidazolidin-4-yl)urea), one of the most important nitrogen carrier for some plants, soil animals, and microorganisms, to allantoate. DAL1 gene from Saccharomyces cerevisiae encodes an allantoinase. However, some organisms possess allantoinase activity but lack DAL1 allantoinase. In those organisms, a defective allantoinase gene, named puuE (purine utilization E), encodes an allantoinase that specifically catalyzes the hydrolysis of (S)-allantoin into allantoic acid. PuuE allantoinase is related to polysaccharide deacetylase (DCA), one member of the carbohydrate esterase 4 (CE4) superfamily, that removes N-linked or O-linked acetyl groups of cell wall polysaccharides, and lacks sequence similarity with the known DAL1 allantoinase that belongs to the amidohydrolase superfamily. PuuE allantoinase functions as a homotetramer. Its monomer is composed of a 7-stranded barrel with detectable sequence similarity to the 6-stranded barrel NodB homology domain of DCAs. It appears to be metal-independent and acts on a small substrate molecule, which is distinct from the common features of DCAs that are normally metal ion dependent and recognize multimeric substrates. This family also includes a chitin deacetylase 1 (SpCDA1) encoded by the Schizosaccharomyces pombe cda1 gene. Although the general function of chitin deacetylase (CDA) is the synthesis of chitosan from chitin, a polymer of N-acetyl glucosamine, to build up the proper ascospore wall, the actual function of SpCDA1 might involve allantoin hydrolysis. It is likely orthologous to PuuE allantoinase, whereas it is more distantly related to the CDAs found in other fungi, such as Saccharomyces cerevisiae and Mucor rouxii. Those CDAs are similar with rizobial NodB protein and are not included in this family.
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