The reaction kinetics of four sheep haemoglobins with identical alpha-chains

Sheep haemoglobins A, B, C and F are known to have identical alpha-chains but differing beta-chains. Rate constants were determined for the combination of CO with these four deoxy haemoglobins (l') and for the dissociation of O2 from the fully saturated tetramers (k4) from 15 to 38 degrees C at physiological pH in the presence of CO2, and at pH 9.1 in the absence of CO2. The constant for the replacement of O2 from the tetramer by CO (m' infinity) and the ratio of the combination constants of CO and O2 with the three parts saturated tetramer (l'4/k'4), were also determined over this range of temperature at physiological pH in the presence of CO2. In no respect did Hb A differ from Hb C. Apart from this the values for l' differed significantly between each pair of haemoglobins. The constant k4 showed significant differences only when Hb B was compared with Hb A or Hb C. Values of m' infinity were similar for the four haemoglobins; and the values of the ratio l'4/k'4 were similar for Hbs A, C and F, and about half as great for Hb B. The results do not support the hypothesis of predominance of the alpha-chains in determining the rate of ligand combination with the desaturated tetramer. It is suggested that faster rate of release of O2 from Hb B may be due to its having lysine at position HCl in the beta-chain whereas the other haemoglobins have arginine.