6‐Pyruvoyl‐Tetrahydropterin Synthase

Abstract

The 6‐pyruvoyl‐tetrahydropterin synthase (PTPS), a zinc‐dependent, heat‐stable, homohexameric protein complex of approximately 90 kDa molecular weight catalyzes the conversion of dihydroneopterin triphosphate to 6‐pyruvoyl‐tetrahydropterin, the second of three enzymatic steps in the synthesis of tetrahydrobiopterin from GTP. The 6‐pyruvoyl‐tetrahydropterin synthetase monomer forms a compact, single‐domain α + β structure containing a sequential, four‐stranded, antiparallel β‐sheet. Oligomerization occurs through association of monomers via their β‐sheets that form a 12‐stranded β‐barrel structure that is duplicated in the functional hexameric form via a head‐to‐head association of two trimers. The PTPS active site is a 12‐Å deep cavity that is located on the interface of three monomers, two from one trimer and one subunit from the other trimer. The Zn(II) binding site is located at the center of the cavity. The transition metal is bound to the NE2 atoms of the three histidine residues. The role of the catalytic metal is to correctly position the substrate, to polarize the carbon–oxygen bonds for facile proton release from the substrate, and to prevent cleavage of the C1′‐C2′ substrate side‐chain bond. In addition, PTPS contains an intersubunit catalytic triad motif composed of the amino acid residues CysA42, HisB89, and AspB88 from two neighboring monomers (A and B) that putatively function as a relay system for proton abstraction from substrate and reaction intermediate. Residue GluA133 may represent the key residue for the stereospecificity of the catalyzed reaction.