Heterotropic effectors exert more significant strain on monoligated than on unligated hemoglobin

The effect of allosteric effectors, such as inositol hexakisphosphate and/or bezafibrate, has been investigated on the unliganded human adult hemoglobin both spectroscopically (employing electronic absorption, circular dichroism, resonance Raman, and x-ray absorption near-edge spectroscopies) and functionally (following the kinetics of the first CO binding step up to a final 4% ligand saturation degree). All data indicate that the unliganded T-state is not perturbed by the interaction with either one or both effectors, suggesting that their functional influence is only exerted when a ligand molecule is bound to the heme. This is confirmed by the observation that CO dissociation from partially liganded hemoglobin (