Review
Bacterial nitric oxide synthases: what are they good for?

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Nitric oxide synthases (NOSs) are heme-based monooxygenases that oxidize L-arginine to nitric oxide (NO), a signaling molecule and cytotoxic agent in higher organisms. Although NOS-like activity has been reported in many bacteria, only a few bacterial homologs of mammalian NOSs (mNOSs) have been characterized to date. In contrast to mNOSs, which possess both a catalytic and a reductase domain, the bacterial enzymes lack reductase domains and require the supply of suitable reductants to produce NO. A notable exception is a NOS from a gram-negative bacterium that contains a new type of reductase module. Remarkably, bacterial NOSs seem to have functions that differ from those of mNOSs, including nitration of different metabolites and protection against oxidative stress. Studies of bacterial NOSs will probably result in a better understanding of the mechanism of NO synthesis and unveil a variety of new functions for NO in microbes.

Section snippets

Nitric oxide synthases in bacteria?

In mammals, nitric oxide (NO) is involved in many biological processes that range from regulation of blood pressure to protection against pathogens [1]. Mammalian NO synthases (mNOSs) are highly regulated, complex enzymes that catalyze the oxidation of L-arginine (L-Arg) to NO and citrulline [2] (Figure 1). Hence, the identification of NO synthase (NOS) homologs encoded in bacterial genomes – more than ten years ago – was received with excitement. Given the importance of mNOSs in multicellular

Biochemistry and genomics

For more than a decade, attempts have been made to find and characterize sources of NOS activity from microbial samples. In several organisms, such as Nocardia [3] and Lactobacillus [4], L-Arg-dependent nitrite production has been demonstrated and shown to be attenuated by known NOS inhibitors. However, when considering these studies, one should keep in mind that bacteria can and do produce NO from a variety of pathways, many of which are not dependent on NOS. For example, nitrate reductase can

Enzymology and structure

Spectroscopic properties, structures and catalytic profiles of bacterial NOSs are for the most part very similar to mNOS, with a few interesting exceptions 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24. Mammalian NOSs are homodimers that contain an N-terminal NOSox and a C-terminal flavoprotein reductase domain (NOSred). NOSox binds L-Arg, heme and the redox-active cofactor 6R-tetrahydrobiopterin (H4B) and contains the catalytic center of the enzyme. NOSred has binding sites for flavin

Reductase partners

NO production by NOS from gram-positive bacteria has so far been characterized to be similar to that of mNOS, with the exception that the reducing equivalents cannot be supplied from a covalently attached reductase domain. A B. subtilis flavodoxin (YkuN) was demonstrated to be an efficient electron donor for bsNOS that could support NO synthesis in vitro [35]. However, when the ykuN gene is deleted, B. subtilis still presents phenotypes indicative of NOS activity. Furthermore, bacterial NOSs

Functions of bacterial NOS

NO was first identified as a biological product in 1967, when it was shown to be an intermediate in anaerobic denitrification in the marine bacterium Pseudomonas perfectomarinus [44]. It is now well known that during denitrification, NO is produced from Cu- or heme-dependent nitrite reductases [45]. NO from host and environmental sources seems to be an important signal for many bacteria; several NO-specific sensing systems have been identified 46, 47. However, only a few bacteria have been

Concluding remarks and future directions

Studies of bacterial NOSs have increased our understanding of the synthesis and function of NO in prokaryotes and raised several interesting questions regarding NO biochemistry in these systems (Box 1). Bacterial NOSs continue to serve as tractable subjects for unveiling the mechanistic details of NO synthesis. Moreover, studies of bacterial NOSs promise to reveal unanticipated biochemical pathways and new functions for NO in microbes. In fact, prokaryotes might employ NOSs as general sources

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