Effect of Phosphate on the Kinetics of Assembly of Oxygenated Hemoglobin from Isolated α and β Chains

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Abstract
The kinetics of assembly of oxygenated hemoglobin from isolated α and β chains was investigated under various buffer conditions by use of a circular dichroism (CD) stopped-flow apparatus. The difference CD spectra of hemoglobin against its constituent chains were independent of the buffer conditions, while the time courses of the Soret CD after mixing equimolar amounts of the α and β chains changed with the buffer conditions. The time courses were analyzed on the basis of a scheme which included a monomertetramer equilibrium of the β chain (β4⇌4β), dissociation of the β4(β4→4β), and a second-order combination of α and β monomers (α+β→αβ). The analysis showed that buffer conditions affected the dissociation of the β4, rather than the monomer combination: The rate of the dissociation of the β4, accelerated with decreasing phosphate concentration, while the rate of the monomer combination was less sensitive to the phosphate concentration. This result indicates that the stability of the β4 depends on the phosphate concentration. It was furthermore suggested that the inorganic phosphate was bound to the β4 with an association constant of 133 M−1 and a Hill coefficient of 1.2.