3-DIMENSIONAL STRUCTURE OF 6-PYRUVOYL TETRAHYDROPTERIN SYNTHASE, AN ENZYME INVOLVED IN TETRAHYDROBIOPTERIN BIOSYNTHESIS

The crystal structure of rat liver 6-pyruvoyl tetrahydropterin synthas e has been solved by multiple isomorphous replacement and refined to a crystallographic R-factor of 20.4% at 2.3 angstrom resolution. 6-Pyru voyl tetrahydrobiopterin synthase catalyses the conversion of dihydron eopterin triphosphate to 6-pyruvoyl tetrahydropterin, the second of th ree enzymatic steps in the synthesis of tetrahydrobiopterin from GTP. The functional enzyme is a hexamer of identical subunits. The 6-pyruvo yl tetrahydropterin synthase monomer folds into a sequential, four-str anded, antiparallel beta-sheet with a 25 residue, helix-containing ins ertion between strands 1 and 2 at the bottom of the molecule, and a se gment between strands 2 and 3 forming a pair of antiparallel helices, layered on one side of the beta-sheet. Three 6-pyruvoyl tetrahydropter in synthase monomers form an unusual 12-stranded antiparallel beta-bar rel by tight association between the N- and C-terminal beta-strands of two adjacent subunits. The barrel encloses a highly basic pore of 6-1 2 diameter. Two trimers associate in a head-to-head fashion to form th e active enzyme complex. The substrate-binding site is located close t o the trimer-trimer interface and comprises residues from three monome rs: A, A' and B. A metal-binding site in the substrate-binding pocket is formed by the three histidine residues 23, 48 and 50 from one 6-pyr uvoyl tetrahydropterin synthase subunit. Close to the metal, but appar ently not liganding it, are residues Cys42, Glu133 (both from A) and H is89 (from B), which might serve as proton donors and acceptors during catalysis.