Biochemistry of

HNO/NO and Hemes
 

     In the past, possible biological activity of nitrite oxide, NO, was discounted because of its high reactivity and toxicity. But in the last twenty years, NO has  been demonstrated to play many different roles in our bodies: controlling blood pressure, long-term memory and the immune response.  The physiological importance of nitric oxide has generated tremendous interest in the chemistry of heme-nitrosyls,  as both the formation and activity of NO is directly attributable to heme cofactors in nitric oxide synthase and soluble guanyl cyclase.

    There is also considerable interest in the biological activity of the one-electron reduced form of nitric oxide, termed a nitroxyl or nitrosyl hydride (NO- or HNO).  Nitroxyl intermediates have been proposed in the catalytic cycles of the heme enzymes nitric oxide synthase and the nitrite and nitric oxide reductases.  The pharmacological activity of nitroxyl-releasing drugs has also been suggested to be due to their reactivity with the various heme targets.

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          Metal complexes of HNO are rare and typically short-lived in solution. A notable exception is the HNO adduct of myoglobin, Mb-HNO, which we first observed during an electrochemical study (Bayachou JACS 1998). Subsequently, we synthesized it directly by reduction of Mb-NO (Lin JACS 2000); the nitroxyl was protonated at nitrogen, as demonstrated by the splitting of its  1H NMR at ca. 15 ppm.

      The unique ¹H NMR signal of the HNO adduct allowed us to determine its solution structure by NOE and COSY NMR methods in collaboration with Gerd LaMar at UC Davis (Sulc, JBIC 2003).
 

        HNO itself is very short-lived in aqueous solution due to a nearly diffusion-controlled dimerization, but we have shown that deoxymyoglobin efficiently traps HNO to give Mb-HNO directly (Sulc JACS 2004). The binding of HNO to Mb is rapid and essentially irreversible; this implies that the beneficial effects of nitroxyl for heart disease may be mediated by its reactivity with myoglobin and hemoglobin in the heart and blood. We have also recently collaborated on a re-evaluation of the oxidation potential and pKa of HNO, which overturned some common misconceptions on its possible biological generation (Bartberger PNAS 2002).

    Selected recent publications on N-oxide chemistry:

      “Chemistry of the HNO Ligand with Hemes and Synthetic Coordination Complexes” Farmer, P. J.; Sulc, F. invited review in press, J. Inorg. Biochem.

“Trapping of Nitroxyl by Deoxy Myoglobin” Sulc, F.; Immoos, C.; Pervitsky, D. Farmer, P. J.  J. Amer. Chem. Soc. 2004, 125, 1096-1101.

"¹H NMR Structure of the Heme Pocket of HNO-Myoglobin" Sulc, F.; Fleischer, E.; Farmer, P. J.; Ma, D.; La Mar, G. J. Biol. Inorg. Chem. 2003, 8, 348-352.

"The reduction potential of nitric oxide (NO) and its importance to NO biochemistry" Bartberger, M.D.; Liu, W.; Ford, E.; Miranda, K. M.;  Switzer, C.; Fukuto, J. M.; Farmer, P. J.; Wink, D.A.; Houk K. N. Proc. Nat. Acad. Sci. 2002, 99, 10958-10963.

"O-Atom Transfer from Nitric Oxide Catalyzed by Fe(TPP)" Lin, R.; Farmer, P. J.  J. Am. Chem. Soc. 2001, 123, 1143 -1150.

“The HNO Adduct of Myoglobin: Synthesis and Characterization” Lin, R.; Farmer, P. J. J. Am. Chem. Soc 2000, 122, 2393 –2394.