Nitric oxide dioxygenase function and mechanism of flavohemoglobin, hemoglobin, myoglobin and their associated reductases

Microbial flavohemoglobins (flavoHbs) and hemoglobins (Hbs) show large (NO)-N-. dioxygenation rate constants ranging from 745 to 2900 muM(-1) s(-1) suggesting a primal (NO)-N-. dioxygenase (NOD) (EC 1.14.12.17) function for the ancient Hb superfamily. Indeed, modern O-2-transporting and storing mammalian red blood cell Hb and related muscle myoglobin (Mb) show vestigial (NO)-N-. dioxygenation activity with rate constants of 34-89 muM(-1) s(-1). In support of a NOD function, microbial flavoHbs and Hbs catalyze O-2-dependent cellular (NO)-N-. metabolism, protect cells from (NO)-N-. poisoning, and are induced by (NO)-N-. exposures. Red blood cell Hb, myocyte Mb, and flavoHb-like activities metabolize (NO)-N-. in the vascular lumen, muscle, and other mammalian cells, respectively, decreasing (NO)-N-. signalling and toxicity. HbFe(III)-OO., HbFe(III)-OONO and protein-caged [HbFe(III)-O-. (NO2)-N-.] are proposed intermediates in a reaction mechanism that combines both O-atoms of O-2 with (NO)-N-. to form nitrate and HbFe(III). A conserved Hb heme pocket structure facilitates the dioxygenation reaction and efficient turnover is achieved through the univalent reduction of HbFe(III) by associated reductases. High affinity flavoHb and Hb heme ligands, and other inhibitors, may find application as antibiotics and antitumor agents that enhance the toxicity of immune cell-derived (NO)-N-. or as vasorelaxants that increase (NO)-N-. signalling. (C) 2004 Elsevier Inc. All rights reserved.