Communication between the zinc and tetrahydrobiopterin binding sites in nitric oxide synthase

Georges Chreifi, Huiying Li, Craig McInnes, Colin Gibson, Colin Suckling, Thomas L Poulos

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Abstract

The nitric oxide synthase (NOS) dimer is stabilized by a Zn2+ ion coordinated to four symmetry-related Cys residues exactly along the dimer 2-fold axis. Each of the two essential tetrahydrobiopterin (H4B) molecules in the dimer interacts directly with the heme, and each H4B molecule is ∼15 Å from the Zn2+. We have determined the crystal structures of the bovine endothelial NOS dimer oxygenase domain bound to three different pterin analogues, which reveal an intimate structural communication between the H4B and Zn2+ sites. The binding of one of these compounds, 6-acetyl-2-amino-7,7-dimethyl-7,8-dihydro-4(3H)-pteridinone (1), to the pterin site and Zn2+ binding are mutually exclusive. Compound 1both directly and indirectly disrupts hydrogen bonding between key residues in the Zn2+ binding motif, resulting in destabilization of the dimer and a complete disruption of the Zn2+ site. Addition of excess Zn2+ stabilizes the Zn2+ site at the expense of weakened binding of 1. The unique structural features of 1 that disrupt the dimer interface are extra methyl groups that extend into the dimer interface and force a slight opening of the dimer, thus resulting in disruption of the Zn2+ site. These results illustrate a very delicate balance of forces and structure at the dimer interface that must be maintained to properly form the Zn2+, pterin, and substrate binding sites.
Original languageEnglish
JournalBiochemistry
Early online date12 May 2014
DOIs
Publication statusPublished - 2014

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Keywords

  • crystal structures
  • nitric oxide synthase (NOS)
  • tetrahydrobiopterin binding sites
  • H4B

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