CO binding and valency exchange in asymmetric Hb hybrids

Confirmation of a unique intra-dimer cooperativity in the human hemoglobin alpha(1)beta(1)half-oxygenated intermediate supports the symmetry rule model of allosteric regulation

The contribution of the alpha(1)beta(1)half-oxygenated tetramer [alphabeta:alphaO(2)betaO(2)] (species 21) to human hemoglobin cooperativity was evaluated using cryogenic isoelectric focusing. The cooperative free energy of binding, reflecting O(2)-driven protein structure changes, was measured as (21)DeltaG(c) = 5.1 +/- 0. 3 kcal for the Zn/FeO(2) analog. For the Fe/FeCN analog, (21)DeltaG(c) was estimated as 4.0 kcal after correction for a CN ligand rearrangement artifact, demonstrating that ligand rearrangement does not invalidate previous conclusions regarding this species. In the context of the entire Hb cooperativity cascade, which includes eight intermediate species, the 21 tetramer is highly abundant relative to the other doubly-ligated species, providing strong support for the previously determined consensus partition function of O(2) binding and for the Symmetry Rule model of hemoglobin cooperativity (Ackers et al., Science 1992;255:54-63). Cooperativity of normal human hemoglobin is shown to depend on site-configuration, and not solely the number of O(2) bound, nor the occupancy of alpha vs. beta subunits. Verification of a unique contribution from the alpha(1)beta(1)doubly-oxygenated species to the equilibrium O(2) binding curve strongly reinforces the Symmetry Rule interpretation that the alpha(1)beta(1)dimer acts both as a structural and functional element in cooperative O(2) binding.

Asymmetric (deoxy dimer/azido-met dimer) hemoglobin hybrids dissociate within seconds

Double mixing stopped-flow experiments have been performed to study the stability of asymmetric hemoglobin (Hb) hybrids, consisting of a deoxy and a liganded dimer. The doubly liganded [deoxy/cyano-met] hybrid (species 21) was reported to have an enhanced stability, with tetramer to dimer dissociation requiring over 100 seconds, based on a method that required an incubation of over two days. However, kinetic experiments revealed rapid ligand binding to species 21, as for triply liganded tetramers, which dissociate within a few seconds. For the present study, [deoxy dimer/azido-met dimer] hybrids are formed within 200 ms by stopped-flow mixing of dithionite with a solution containing oxyHb and azido-metHb. The dithionite scavenges oxygen, thus transforming oxyHb to deoxyHb, and the [oxy dimer/azido-met dimer] hybrid to the asymmetric [deoxy/azido-met] hybrid (species 21). After a variable aging time of the asymmetric hybrids, their allosteric state is probed by CO binding in a second mixing. As previously observed the freshly produced asymmetric hybrids bind CO rapidly as for R-state Hb. As the hybrids are aged from 0.1 to 10 seconds, the fraction of slow CO binding increases, consistent with a dissociation of the asymmetric hybrid to form the more stable deoxy Hb tetramer which reacts slowly with CO. Control experiments showed a predominantly slow phase for deoxy Hb, and fast rebinding for the symmetric hybrids. The kinetic data can be simulated with a tetramer to dimer dissociation rate for species 21 of 1.5/second at 100 mM NaCl (pH 7.2) and 1.9/second at 180 mM NaCl (pH 7.4). These values are similar to those reported for liganded Hb, as opposed to deoxy (T-state) tetramers which dissociate over four orders of magnitude more slowly. As expected from simulations of dimer exchange, the observed transition rate depends on the initial fractions of oxy- and metHb; this effect is not consistent with a slow R to T transition. These results, showing a lifetime of about one second for species 21, do not support the symmetry rule which is based on an enhanced stability of the asymmetric hybrid.

The stereochemical mechanism of the cooperative effects in hemoglobin revisited

In 1970, Perutz tried to put the allosteric mechanism of hemoglobin, proposed by Monod, Wyman and Changeux in 1965, on a stereochemical basis. He interpreted their two-state model in terms of an equilibrium between two alternative structures, a tense one (T) with low oxygen affinity, constrained by salt-bridges between the C-termini of the four subunits, and a relaxed one (R) lacking these bridges. The equilibrium was thought to be governed primarily by the positions of the iron atoms relative to the porphyrin: out-of-plane in five-coordinated, high-spin deoxyhemoglobin, and in-plane in six-coordinated, low-spin oxyhemoglobin. The tension exercised by the salt-bridges in the T-structure was to be transmitted to the heme-linked histidines and to restrain the movement of the iron atoms into the porphyrin plane that is necessary for oxygen binding. At the beta-hemes, the distal valine and histidine block the oxygen-combining site in the T-structure; its tension was thought to strengthen that blockage. Finally, Perutz attributed the linearity of proton release with early oxygen uptake to the sequential rupture of salt-bridges in the T-structure and to the accompanying drop in pKa of the weak bases that form part of them. Almost every feature of this mechanism has been disputed, but evidence that has come to light more than 25 years later now shows it to have been substantially correct. That new evidence is reviewed below.