<p> The dimensionless
equilibrium constant for the allosteric conformation change, K<sub>ΔC</sub> =
0.02602 (Knowles & Magde, linked ms 2) following binding of O<sub>2</sub>
by α-chains in <sup>T</sup>state Hb<sub>4</sub>/BPG (whole blood under standard
conditions) is shown to be comprised of: (i) an endothermic change in
conformation, from <sup>T</sup>state to <sup>R</sup>state, of 24.3 kJ/mol; (ii)
exothermic conversion of <sup>T</sup>state <sup>T</sup>αO<sub>2</sub>-chains to
<sup>R</sup>state <sup>R</sup>αO<sub>2</sub>-chains of -13.8 kJ/mol;
(iii)exothermic binding of BPG by R-states.
Eq. (1) defines the component steps whereby the <sup>T</sup>state
conformation is converted to the <sup>R</sup>state conformation.</p>
<p>ΔG<sup>o</sup>(<sup>R</sup>(Hb<sub>4</sub>), BPG) describes
the endothermic decomposition of the binary complex, <sup>T</sup>Hb<sub>4</sub>/BPG
into <sup>R</sup>Hb<sub>4</sub> and BPG, equal to + 33.7 kJ/mol (DeBruin et al.
(1973). J. Biol. Chem. <u>248</u>, 2774-2777).
ΔG<sup>o</sup> for the
equilibrium constant for ΔG<sup>O</sup>(K<sub>ΔC</sub>)
and Σ ΔG<sup>o</sup> for binding of O<sub>2</sub> by the pair of equivalent <sup>T</sup>state
α-chains, ΔG<sup>O</sup>(<sup>T</sup>α<sup>*</sup>O<sub>2</sub>), + 9.41 kJ/mol
and – 49.6 kJ/mol, respectively, are determined by fitting of O<sub>2</sub> equilibrium
binding data to the Perutz-Adair equation.
ΔG<sup>o</sup> for reaction of a pair of equivalent <sup>R</sup>state
α-chains with O<sub>2</sub>, ΔG<sup>O</sup>(<sup>R</sup>αO<sub>2</sub>), was
estimated from the known affinity of myoglobin for O<sub>2</sub> at 37<sup>o</sup>C
(Theorell H. (1936). Biochem. Z., <u>268</u>,
73-81), -63.4 kJ/mol. The unknown
quantity, ∆G<sup>O</sup>(<sup>R</sup>(HbO<sub>2</sub>)<sub>4</sub>/BPG), was obtained
by solving Eq. (1), being -10.5 kJ/mol, K (<sup>R</sup>(HbO<sub>2</sub>)<sub>4</sub>/BPG)
= 58.4 L/mol. The value of the equilibrium
constant for binding BPG to R-state conformations represents 0.0073% of the value
of the binding constant of BPG to <sup>T</sup>state conformations: 800,000
L/mol. The value of K<sub>ΔC</sub>; (i) accounts
for the ability of O<sub>2</sub> to escape, virtually unhindered from rbcs and
(ii) provides a biophysical basis for manifestation of high resting rates of
metabolism in warm blooded species.</p>
Stabilization of the T-state of human hemoglobin by proflavine, an antiseptic drug