Generation of NO by Molybdoenzymes:
New catalytic performances for old enzymes

Nitric oxide radical (NO) is a signalling molecule that plays important regulatory roles in several physiological processes, in all forms of life. In addition, NO has become the focus of a huge interest by the medical scientific community, due to its suggested participation in many pathophysiological conditions. In mammals, NO is predominantly catalysed by NO synthases (NOS) and its biological effects are accomplished by posttranslational modification of transition metal centres and cysteine residues. To control the specificity of NO signalling and to limit NO toxicity, NOS activity is tightly regulated and the NO life time is controlled through its rapid oxidation to nitrate/nitrite. In fact, nitrate/nitrite have long been regarded as end-products of NO metabolism. However, recent studies have shown that nitrite can be reduced back to NO through several pathways and that nitrite administration can be cytoprotective during in vivo ischemia. Accordingly, the presence of nitrite in blood and tissues may represent a "NO storage form" that could be made available under conditions of hypoxia (when NOS activity is impaired) to ensure cell survival.
The fact that other enzymes (other than NOS) can catalyse the NO formation greatly amplifies the biological capabilities to synthesise this important molecule. Xanthine oxidoreductase (XOR) is a molybdenum-containing enzyme, belonging to the xanthine oxidase family, that catalyses the oxidation of xanthine to urate (the terminal metabolite of purine catabolism), with the simultaneous reduction of molecular oxygen, NAD⁺, and several N-oxides. The XOR ability to catalyse the nitrate reduction has been known for more than 40 years, but only recently these reactions began to be re-studied, when the (patho)physiological importance of the generated NO was realised. Yetr, the molecular mechanism of XOR nitrate/nitrite reduction remains to be elucidated.
To contribute to a better characterisation of this new NO production pathway, we are investigating if the nitrate/nitrite reductase activities are a common feature of other molybdenoenzymes (prokaryotic and eukaryotic molybdoenzymes) and we are studying the molecular mechanism of these reductions and its physiological implications.

People Involved
José J.G. Moura and Luisa Maia (PI)
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Publications
Lessons from denitrification for the human metabolism of signalling nitric oxide
Maia L, Moura JJG
in "Metalloenzymes in Denitrification: Applications and Environmental Impacts", RSC Metallobiology Series No. 9. Eds Moura, I., Moura, J.J.G., Pauleta, S.R., Maia, L., The Royal Society of Chemistry, Cambridge (ISBN: 978-1-78262-376-2), 419-443, 2017.
http://dx.doi.org/10.1039/9781782623762-00419

Nitrite reduction by molybdoenzymes: a new class of nitric oxide-forming nitrite reductases
Maia L, Moura JJG
J Biol Inorg Chem (2015) 20:403-433
http://dx.doi.org/10.1007/s00775-014-1234-2

Nitrite reductase activity of rat and human xanthine oxidase, xanthine dehydrogenase, and aldehyde oxidase: evaluation of their contribution to NO formation in vivo
Maia L, Pereira V, Mira ., Moura JJG
Biochemistry (2015) 54:685-710
http://dx.doi.org/10.1021/bi500987w

How Biology handles nitrite
Maia L, Moura JJG
Chem Rev (2014) 114:5273-5357
http://dx.doi.org/10.1021/cr400518y

Nitrite reduction by xanthine oxidase family enzymes: a new class of nitrite reductases
Maia LB, Moura JJG
J Biol Inorg Chem (2011) 16:443-460
http://dx.doi.org/10.1007/s00775-010-0741-z

Funding
Project PTDC/QUI/100366/2008 (FCT-MCTES, Portugal)
PI - José J. G. Moura

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