Dihydropyrimidinase, a dimetalloenzyme containing a carboxylated lysine within the active site, is a member of the cyclic\namidohydrolase family, which also includes allantoinase, dihydroorotase, hydantoinase, and imidase. Unlike all known dihydropyrimidinases,\nwhich are tetrameric, pseudomonal dihydropyrimidinase forms a dimer at neutral pH. In this paper, we report\nthe crystal structure of P. aeruginosa dihydropyrimidinase at pH 5.9 (PDB entry 5YKD). The crystals of P. aeruginosa dihydropyrimidinase\nbelonged to space group C2221 with cell dimensions of a 108.9, b 155.7, and c 235.6 Ã?Å¡ A. The structure of\nP. aeruginosa dihydropyrimidinase was solved at 2.17Ã?Å¡A resolution. An asymmetric unit of the crystal contained four crystallographically\nindependent P. aeruginosa dihydropyrimidinase monomers. Gel filtration chromatographic analysis of purified\nP. aeruginosa dihydropyrimidinase revealed a mixture of dimers and tetramers at pH 5.9. Thus, P. aeruginosa dihydropyrimidinase\ncan form a stable tetramer both in the crystalline state and in the solution. Based on sequence analysis and structural\ncomparison of the dimer-dimer interface between P. aeruginosa dihydropyrimidinase and Thermus sp. dihydropyrimidinase,\ndifferent oligomerization mechanisms are proposed
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