We have previously reported that bacterial flavohemoglobin (HMP) catalyzes both a rapid reaction of heme-bound O2 with nitric oxide (NO) to form nitrate [HMP-Fe(II)O2 + NO → HMP-Fe(III) + NO3 -] and, under anaerobic conditions, a slower reduction of heme-bound NO to an NO- equivalent (followed by the formation of N2O), thereby protecting against nitrosative stress under both aerobic and anaerobic conditions, and rationalizing our finding that NO is rapidly consumed across a wide range of O2 concentrations. It has been alternatively suggested that HMP activity is inhibited at low pO2 because the enzyme is then in the relatively inactive nitrosyl form [koff/kon for NO (0.000008 μM) ≪ koff/kon for O2 (0.012 μM) and KM for O2 = 30-100 μM]. To resolve this discrepancy, we have directly measured heme-ligand turnover and NADH consumption under various O2/NO concentrations. We find that, at biologically relevant O2 concentrations, HMP preferentially binds NO (not O2), which it then reacts with oxygen to form nitrate (in essence NO- + O2 → NO3 -). During steady-state turnover, the enzyme can be found in the ferric (FeIII) state. The formation of a heme-bound nitroxyl equivalent and its subsequent oxidation is a novel enzymatic function, and one that dominates the oxygenase activity under biologically relevant conditions. These data unify the mechanism of HMP/NO interaction with those recently described for the nematode Ascaris and mammalian hemoglobins, and more generally suggest that the peroxidase (FeIII)-like properties of globins have evolved for handling of NO.
|Number of pages||5|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Publication status||Published - 28 Aug 2001|
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