title: Atomic structure of GTP cyclohydrolase I.
abstract: BACKGROUND
  Tetrahydrobiopterin serves as the cofactor for enzymes involved in neurotransmitter biosynthesis and as regulatory factor in immune cell proliferation and the biosynthesis of melanin. The biosynthetic pathway to tetrahydrobiopterin consists of three steps starting from GTP. The initial reaction is catalyzed by GTP cyclohdrolase I (GTP-CH-I) and involves the chemically complex transformation of the purine into the pterin ring system.
  
  
  RESULTS
  The crystal structure of the Escherichia coli GTP-CH-I was solved by single isomorphous replacement and molecular averaging at 3.0 A resolution. The functional enzyme is a homodecameric complex with D5 symmetry, forming a torus with dimensions 65 A x 100 A. The pentameric subunits are constructed via an unprecedented cyclic arrangement of the four-stranded antiparallel beta-sheets of the five monomers to form a 20-stranded antiparallel beta-barrel of 35 A diameter. Two pentamers are tightly associated by intercalation of two antiparallel helix pairs positioned close to the subunit N termini. The C-terminal domain of the GTP-CH-I monomer is topologically identical to a subunit of the homohexameric 6-pyruvoyl tetrahydropterin synthase, the enzyme catalyzing the second step in tetrahydrobiopterin biosynthesis.
  
  
  CONCLUSIONS
  The active site of GTP-CH-I is located at the interface of three subunits. It represents a novel GTP-binding site, distinct from the one found in G proteins, with a catalytic apparatus that suggest involvement of histidines and, possibly, a cystine in the unusual reaction mechanism. Despite the lack of significant sequence homology between GTP-CH-I and 6-pyruvoyl tetrahydropterin synthase, the two proteins, which catalyze consecutive steps in tetrahydrobiopterin biosynthesis, share a common subunit fold and oligomerization mode. In addition, the active centres have an identical acceptor site for the 2-amino-4-oxo pyrimidine moiety of their substrates which suggests an evolutionarily conserved protein fold designed for pterin biosynthesis.
authors.affiliation: Max Planck Institute für Biochemie, Abteilung Strukturforschung, Martinsried, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Nar
authors.forename: R
authors.initials: R
authors.lastname: Huber
authors.forename: W
authors.initials: W
authors.lastname: Meining
authors.forename: C
authors.initials: C
authors.lastname: Schmid
authors.forename: S
authors.initials: S
authors.lastname: Weinkauf
authors.forename: A
authors.initials: A
authors.lastname: Bacher
mesh_terms: D000429:Alcohol Oxidoreductases / Q000737:chemistry; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D001708:Biopterins / Q000031:analogs & derivatives / Q000096:biosynthesis; D002384:Catalysis; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology; D006136:GTP Cyclohydrolase / Q000737:chemistry* / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D008958:Models, Molecular; D019798:Neopterin / Q000031:analogs & derivatives; D019761:Phosphorus-Oxygen Lyases; D011487:Protein Conformation; D011621:Pteridines / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D011621:Pteridines; C013490:dihydroneopterin triphosphate; D001708:Biopterins; D019798:Neopterin; D006160:Guanosine Triphosphate; D000429:Alcohol Oxidoreductases; D006136:GTP Cyclohydrolase; D019761:Phosphorus-Oxygen Lyases; C043497:6-pyruvoyltetrahydropterin synthase; C003402:sapropterin