**(A) Allosteric autocatalysis**. Reaction of nitrite with deoxyhemoglobin exhibits R-state catalysis. Tetrameric T-state deoxyHb reduces nitrite to NO, generating a met-heme (3^{+}) and an iron-nitrosyl-heme (Fe^{+2}-NO), on the same or different Hb tetramers, which stabilize the tetramer(s) in the R state. Increasing R-state character is associated with a higher bimolecular rate constant for nitrite reduction. As a result, ferrous deoxyhemes on these R-state stabilized tetramers react with nitrite faster than those on T-state stabilized tetramers, thereby exponentially propagating nitrite reduction and R-state stabilization. This process therefore represents a unique allosteric autocatalytic reaction mechanism. Please note that we are showing the bimolecular rate constant, not the overall reaction rate, and that in the case of hemoglobin, the overall hemoglobin rate constant is not constant, but changes with the T-to-R allosteric transition. The overall rate constant is dependent on the intrinsic reactivity of nitrite with heme and is highest in R state. The actual rate of a second-order reaction is determined by the concentration of deoxyheme multiplied by the concentration of nitrite and multiplied by the bimolecular rate constant. Panel A is reproduced from Grubina et al^{} with permission. (B) Distribution of R and T ligand populations modulates nitrite reduction. The fractions of R and T oxygen-liganded species are plotted as a function of oxygen saturation. The quaternary state (R or T) is indicated and the number of ligands bound to each tetramer is indicated by the subscript, so that R_{3} indicates an R-state conformation with 3 oxygen ligands bound. The fraction of each species was calculated using the MWC Perutz 2 state model with the value of c set at 0.015, where c is the ratio of equilibrium-binding constants for T (taken as 1/77 mm Hg) and R states. The ratio L = T_{0}/R_{0} was taken as 10^{5}. The fractions of some intermediate species are so small that they do not appear on the graph. (C) Contribution of quaternary states to nitrite reaction rate. The reaction rates of nitrite with deoxygenated Hb are plotted as a function of oxygen saturation for cases where the product of the nitrite and Hb concentrations are 10^{−6} M^{2}. At each oxygen saturation the rate of the reaction was calculated as [nitrite] × {*k*_{t}(4[T_{0}] + 3[T_{1}] + 2[T_{2}] + [T_{3}]) + *k*_{R}(4[R_{0}] + 3[R_{1}] + 2[R_{2}] + [R_{3}]), where the square brackets refer to Hb concentrations. Here, *k*_{R}/*k*_{T} was rounded to 100 and *k*_{T} was set to 0.2 M^{−1}s^{−1}. The contribution by R-state and T-state molecules was obtained by calculating the products of *k*_{R} and *k*_{T} separately (so, for example, the R-state contribution is [nitrite] × *k*_{R}(4[R_{0}] + 3[R_{1}] + 2[R_{2}] + [R_{3}])).

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