Importance of valine 567 in substrate recognition and oxidation by neuronal nitric oxide synthase

A nitric oxide synthase-like protein from Synechococcus produces NO/NO3- from l-arginine and NADPH in a tetrahydrobiopterin- and Ca2+-dependent manner

Nitric oxide synthases (NOSs) are heme-based monooxygenases that convert l-Arg to l-citrulline and nitric oxide (NO), a key signaling molecule and cytotoxic agent in mammals. Bacteria also contain NOS proteins, but the role of NO production within these organisms, where understood, differs considerably from that of mammals. For example, a NOS protein in the marine cyanobacterium Synechococcus sp. PCC 7335 (syNOS) has recently been proposed to function in nitrogen assimilation from l-Arg. syNOS retains the oxygenase (NOSox) and reductase (NOSred) domains present in mammalian NOS enzymes (mNOSs), but also contains an N-terminal globin domain (NOSg) homologous to bacterial flavohemoglobin proteins. Herein, we show that syNOS functions as a dimer and produces NO from l-Arg and NADPH in a tetrahydrobiopterin (H4B)-dependent manner at levels similar to those produced by other NOSs but does not require Ca2+-calmodulin, which regulates NOSred-mediated NOSox reduction in mNOSs. Unlike other bacterial NOSs, syNOS cannot function with tetrahydrofolate and requires high Ca2+ levels (>200 μm) for its activation. NOSg converts NO to NO3- in the presence of O2 and NADPH; however, NOSg did not protect Escherichia coli strains against nitrosative stress, even in a mutant devoid of NO-protective flavohemoglobin. We also found that syNOS does not have NOS activity in E. coli (which lacks H4B) and that the recombinant protein does not confer growth advantages on l-Arg as a nitrogen source. Our findings indicate that syNOS has both NOS and NO oxygenase activities, requires H4B, and may play a role in Ca2+-mediated signaling.

Importance of Val567 on heme environment and substrate recognition of neuronal nitric oxide synthase

Nitric oxide (NO) produced by mammalian nitric oxide synthases (mNOSs) is an important mediator in a variety of physiological functions. Crystal structures of mNOSs have shown strong conservation of the active‐site residue Val567 (numbering for rat neuronal NOS, nNOS). NOS‐like proteins have been identified in several bacterial pathogens, and these display striking sequence identity to the oxygenase domain of mNOS (NOSoxy), with the exception of a Val to Ile mutation at the active site. Preliminary studies have highlighted the importance of this Val residue in NO‐binding, substrate recognition, and oxidation in mNOSs. To further elucidate the role of this valine in substrate and substrate analogue recognition, we generated five Val567 mutants of the oxygenase domain of the neuronal NOS (nNOSoxy) and used UV‐visible and EPR spectroscopy to investigate the effects of these mutations on the heme distal environment, the stability of the heme‐Fe^II‐CO complexes, and the binding of a series of substrate analogues. Our results are consistent with Val567 playing an important role in preserving the integrity of the active site for substrate binding, stability of heme‐bound gaseous ligands, and potential NO production.

AtNOS/AtNOA1 is a functional Arabidopsis thaliana cGTPase and not a nitric-oxide synthase

AtNOS1 was previously identified as a potential nitric-oxide synthase (NOS) in Arabidopsis thaliana, despite lack of sequence similarity to animal NOSs. Although the dwarf and yellowish leaf phenotype of Atnos1 knock-out mutant plants can be rescued by treatment with exogenous NO, doubts have recently been raised as to whether AtNOS1 is a true NOS. Moreover, depending on the type of physiological responses studied, Atnos1 is not always deficient in NO induction and/or detection, as previously reported. Here, we present experimental evidence showing that AtNOS1 is unable to bind and oxidize arginine to NO. These results support the argument that AtNOS1 is not a NOS. We also show that the renamed NO-associated protein 1 (AtNOA1) is a member of the circularly permuted GTPase family (cGTPase). AtNOA1 specifically binds GTP and hydrolyzes it. Complementation experiments of Atnoa1 mutant plants with different constructs of AtNOA1 show that GTP hydrolysis is necessary but not sufficient for the physiological function of AtNOA1. Mutant AtNOA1 lacking the C-terminal domain, although retaining GTPase activity, failed to complement Atnoa1, suggesting that this domain plays a crucial role in planta. cGTPases appear to be RNA-binding proteins, and the closest homolog of AtNOA1, the Bacillus subtilis YqeH, has been shown to participate in ribosome assembly and stability. We propose a similar function for AtNOA1 and discuss it in the light of its potential role in NO accumulation and plant development.