mammalian-like beta-alanine synthase (beta-AS) and similar proteins (class 5 nitrilases)
This subgroup includes mammalian-like beta-AS (EC 126.96.36.199, also known as beta-ureidopropionase or N-carbamoyl-beta-alanine amidohydrolase). This enzyme catalyzes the third and final step in the catabolic pyrimidine catabolic pathway responsible for the degradation of uracil and thymine, the hydrolysis of N-carbamyl-beta-alanine and N-carbamyl-beta-aminoisobutyrate to the beta-amino acids, beta-alanine and beta-aminoisobutyrate respectively. This subgroup belongs to a larger nitrilase superfamily comprised of nitrile- or amide-hydrolyzing enzymes and amide-condensing enzymes, which depend on a Glu-Lys-Cys catalytic triad. This superfamily has been classified in the literature based on global and structure based sequence analysis into thirteen different enzyme classes (referred to as 1-13), this subgroup corresponds to class 5. Members of this superfamily generally form homomeric complexes, the basic building block of which is a homodimer. Beta-ASs from this subgroup are found in various oligomeric states, dimer (human), hexamer (calf liver), decamer (Arabidopsis and Zea mays), and in the case of Drosophila melanogaster beta-AS, as a homooctamer assembled as a left-handed helical turn, with the possibility of higher order oligomers formed by adding dimers at either end. Rat beta-AS changes its oligomeric state (hexamer, trimer, dodecamer) in response to allosteric effectors. Eukaryotic Saccharomyces kluyveri beta-AS belongs to a different superfamily.