Oxygen-Affinity of Hemoglobin is Regulated by Effector-Linked Dynamic Modulations of High-Frequency Thermal Fluctuations

Abstract

Heterotropic effectors reduce the O2-affinity of hemoglobin (Hb) as much as >10\3-folds: P50 = 0.4 → 200 mmHg, Klow = 0.3 → 0.004 mmHg-1, and Khigh = 10 → 0.004 mmHg-1. However, there are no detectable changes in either static T/R-quaternary and associated tertiary structures or the stereochemical and electronic structures of the heme coordination that are attributable to such changes in the O2-affinity of Hb, upon binding of the heterotropic effectors to either T(deoxy)- or R(oxy)-Hb1,2, indicating that the O2-affinity of Hb is not regulated by static T/R-quaternary and associated tertiary structures of Hb and that the reactivity (or the O2-affinity) of the heme Fe itself is not altered1, in sharp contrast to the mainstream idea of the heme-centric regulation of the O2-affinity in Hb3. Our 6ns-molecular dynamics simulations4,5 indicated that the amplitudes of very high-frequency (>GHz) thermal fluctuations of the globin matrix are entropically enhanced by the dissociation of O2 from oxy-Hb and/or binding of heterotropic effectors to deoxy- and oxy-Hb, in parallel to the reduction of the O2-affinity. The rate of dissociation of O2 from oxy-Hb is enhanced by the effector-linked enhanced thermal fluctuations, which simultaneously reduce the rate of geminate-recombination of O2. This results in the reduction of the apparent O2-affinity of Hb by heterotropic effectors.