Crystal structure of 7,8-dihydroneopterin triphosphate epimerase

Background: Dihydroneopterin triphosphate (H2NTP) is the central substrate in the biosynthesis of folate and tetrahydrobiopterin. Folate serves as a c ofactor in amino acid and purine biosynthesis and tetrahydrobiopterin is us ed as a cofactor in amino acid hydroxylation acid nitric oxide synthesis. I n bacteria, H2NTP enters the folate biosynthetic pathway after nonenzymatic dephosphorylation; in vertebrates, H2NTP is used to synthesize tetrahydrob iopterin. The dihydroneopterin triphosphate epimerase of Escherichia coli c atalyzes the inversion of carbon 2' of H2NTP. Results: The crystal structure of the homo-octameric protein has been solve d by a combination of multiple isomorphous replacement, Patterson search te chniques and cyclic averaging and has been refined to a crystallographic R factor of 18.8% at 2.9 Angstrom resolution. The enzyme is a torus-shaped, D -4 symmetric homo-octamer with approximate dimensions of 65 x 65 Angstrom. Four epimerase monomers form an unusual 16-stranded antiparallel beta barre l by tight association between the N- and C-terminal beta strands of two ad jacent subunits. Two tetramers associate in a head-to-head fashion to form the active enzyme complex. Conclusions: The folding topology, quaternary structure and amino acid sequ ence of epimerase is similar to that of the dihydroneopterin aldolase invol ved in the biosynthesis of the vitamin folio acid. The monomer fold of epim erase is also topologically similar to that of GTP cyclohydrolase I (GTP CH -1), 6-pyrovoyl tetrahydropterin synthase (PTPS) and uroate oxidase (UO), D espite a lack of significant sequence homology these proteins share a commo n subunit fold and oligomerize to form central beta barrel structures emplo ying different cyclic symmetry elements, D-4, D-5, D-3 and D-2, respectivel y. Moreover, these enzymes have a topologically equivalent acceptor site fo r the 2-amino-4-oxo pyrimidine (2-oxo-4-oxo pyrimidine in uroate oxidase) m oiety of their respective substrates.