Binding of carbon monoxide to hemoglobin Zürich. Proposal for a kinetic model.

When the spectroscopic properties and kinetics of combination with carbon monoxide of hemoglobin Zurich (HbZh) and its isolated abnormal chains with two sulphydryl groups blocked by mercuribenzoate (βZhSHgR) were studied by flash photolysis and rapid-mixing techniques, large differences compared to human hemoglobin (HbA) and normal isolated chains were observed as follows. 1 The position of the absorption maximum and its coefficient in the Soret region is very different for the CO derivatives of βZhSHgR and tetrameric abnormal HbZh from that of normal chains and hemoglobin. 2 Flash photolysis on βZhSHgR chains show they react with a single rate constant of 2.7 × 107 M−1 s−1 compared to 4.5 × 106 M−1 s−1 for isolated α chains; photochemical efficiency of CO dissociation is also less. 3 Kinetic analysis of CO binding to HbZh by the stopped-flow technique gave a second-order rate constant for βZh chains within the tetramer in the deoxy (T) quaternary conformation of 13.4 × 106 M−1 s−1 compared to 4.7 × 106 M−1 s−1 for α chain in the tetramer in its liganded (R) state. 4 Flash photolysis on solutions of fully saturated HbZh reveal a very fast initial phase during recombination with CO with a second-order rate constant comparable to that of βZhSHgR and attributed to βZh in the dimer. At very low levels of saturation there is a single phase only with a rate constant similar to that of βZh chains in the tetramer. Reduction of the intensity of the photodissociating light causes a decrease in the relative amplitude of the intial fast phase. The proposed kinetic scheme, based on a modified two-state (R and T) allosteric model, implies that βZh chains in the tetramer with a deoxy T configuration have a higher affinity for CO than α chains and a very high combination rate constant. However, when the tetramer flops to the liganded R configuration these chains increase their combination velocity to that of βZh chains in the dimer (R state).