Recently, Guenter Schwarz and colleagues published an elegant study in the Biochemical Journal (2019) 476, 1805–1815 which combines kinetic and spectroscopic studies with protein engineering to provide a mechanism for sulfite oxidase (SO)-catalyzed nitrite reduction that yields nitric oxide (NO). This work is noteworthy as it demonstrates that (i) for NO generation, both sulfite and nitrite must bind to the same molybdenum (Mo) center; (ii) upon sulfite reduction, Mo is reduced from +6 (MoVI) to +4 (MoIV) and MoIV reduces nitrite to NO yielding MoV; (iii) the heme moiety, linked to the Mo-center by an 11 amino acid residue tether, gets reduced by intramolecular electron transfer (IET) resulting in MoV being oxidized to MoVI; (iv) the reduced heme transfers its electron to a second nitrite molecule converting it to NO; (v) the authors demonstrate steady-state NO production in the presence of the natural electron acceptor cytochrome c; (vi) Finally, the authors use protein engineering to shorten the heme tether to reduce the heme-Mo-center distance with the aim of increasing NO production. Consequently, the rate of IET to cytochrome c is decreased but the enzymatic turnover rate for NO production is increased by ∼10-fold. This paper is unique as it provides strong evidence for a novel mechanism for steady-state NO production for human mitochondrial SO and serves as a potential template for studying NO production mechanisms in other enzymes by integrating the information gained from enzyme kinetics with EPR and UV/vis spectroscopy and protein engineering.

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