Crystal structure of SANOS, a bacterial nitric oxide synthase oxygenase protein from Staphylococcus aureus

Prokaryotic genes related to the oxygenase domain of mammalian nitric oxide synthases (NOSs) have recently been identified. Although they catalyze the same reaction as the eukaryotic NOS oxygenase domain, their biological function(s) are unknown. In order to explore rationally the biochemistry and evolution of the prokaryotic NOS family, we have determined the crystal structure of SANDS, from methicillin-resistant Staphylococcus aureus (MRSA), to 2.4 Angstrom. Haem and S-ethylisothiourea (SEITU) are bound at the SANDS active site, while the intersubunit site, occupied by the redox cofactor tetrahydrobiopterin (H4B) in mammalian NOSs, has NAD+ bound in SANDS. In common with all bacterial NOSs, SANDS lacks the N-terminal extension responsible for stable dimerization in mammalian isoforms, but has alternative interactions to promote dimer formation.