The kinetics of the reactions of Pacific-porbeagle haemoglobin with CO were studied by flash-photolysis and stopped-flow methods, and the equilibrium binding curves for CO were measured in spectrophotometric titrations. Measurements were made in the pH range 6-8 and in the temperature range 0-40 degrees C. The results are discussed in terms of the allosteric model proposed by Monod, Wyman & Changeux [(1965) J. Mol. Biol. 12, 88-118]. Within this framework the results indicate that in the R-state the haem groups fall into two classes of different reactivity with different spectral characteristics, but that in the T-state the groups may be essentially equivalent. The physiological importance of the temperature-insensitivity of the equilibrium ligand-binding curves for porbeagle haemoglobin is discussed.
