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Studies on the Relations between Molecular and Functional Properties of Hemoglobin

VII. KINETIC EFFECTS OF THE REVERSIBLE DISSOCIATION OF HEMOGLOBIN INTO SINGLE CHAIN MOLECULES

From the ¹ From the Institute of Biochemistry, University of Rome, The Consiglio Nazionale delle Ricerche Center for Molecular Biology, Rome, and the "Regina Elena" Institute for Cancer Research, Rome, Italy

The kinetics of the reactions of human hemoglobin with carbon monoxide and oxygen has been studied in photochemical and rapid mixing experiments over a large range of hemoglobin concentration.

When the reaction is initiated by rapid removal of the ligand from ligand-bound hemoglobin, the kinetics of the combination of hemoglobin with CO shows a marked concentration dependence in both the photochemical and the rapid mixing experiments. In dilute hemoglobin solutions (below 10^-5 m in heme), dissociation of the ligand from oxyhemoglobin or carbonmonoxyhemoglobin is followed by slow changes (half-time of the order of seconds) in the properties of the system.

These results lead to the following picture, which is also consistent with other as yet unexplained aspects of hemoglobin kinetics. (a) Ligand-bound hemoglobin dissociates reversibly into single chain molecules at concentrations below 10^-5 m. (b) Deoxygenated hemoglobin has a much lower tendency to dissociate into single chain molecules, and there is no appreciable dissociation even at concentrations of the order of 10^-6 to 10^-7 m. (c) The association of deoxygenated and ß chains is a relatively slow process. Therefore, after sudden dissociation of the ligand from dilute hemoglobin solutions, the properties of the system, for a brief time, are those of a mixture of deoxygenated hemoglobin and deoxygenated and ß chains. (d) The properties of the single chain molecules obtained by dilution of ligand-bound hemoglobin are the same as those of isolated and ß hemoglobin chains as obtained by preparative procedures.

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