Hemoglobin A (HbA) is an allosterically regulated
nitrite reductase that reduces
nitrite to NO under physiological
hypoxia. The efficiency of this reaction is modulated by two intrinsic and opposing properties: availability of unliganded ferrous hemes and R-state character of the
hemoglobin tetramer.
Nitrite is reduced by deoxygenated ferrous hemes, such that
heme deoxygenation increases the rate of NO generation. However,
heme reactivity with
nitrite, represented by its bimolecular rate constant, is greatest when the tetramer is in the R quaternary state. The mechanism underlying the higher reactivity of R-state hemes remains elusive. It can be due to the lower
heme redox potential of R-state ferrous hemes or could reflect the high
ligand affinity geometry of R-state tetramers that facilitates
nitrite binding. We evaluated the
nitrite reductase activity of unpolymerized
sickle hemoglobin (HbS), whose
oxygen affinity and cooperativity profile are equal to those of HbA, but whose
heme iron has a lower redox potential. We now report that HbS exhibits allosteric
nitrite reductase activity with competing
proton and redox Bohr effects. In addition, we found that
solution phase HbS reduces
nitrite to NO significantly faster than HbA, supporting the thesis that
heme electronics (i.e. redox potential) contributes to the high reactivity of R-state deoxy-hemes with
nitrite. From a pathophysiological standpoint, under conditions where HbS
polymers form, the rate of
nitrite reduction is reduced compared with HbA and
solution-phase HbS, indicating that HbS
polymers reduce
nitrite more slowly.