Temperature-jump studies on hemoglobin. Kinetic evidence for a non-quaternary isomerization process in deoxy- and...

Temperature jumps on mixtures of hemoglobin and pH indicators give rise to relaxation signals in the microsecond range. The pH and concentration dependences of the reciprocal relaxation time, 1/tau, may be rationalized on the basis of a reaction scheme in which a slow isomerization process in the protein moiety is coupled to a rapid co-operative ionization of two protons. At 11 degrees C the rate constants of the isomerization are kr = 4.2(+/- 1.8) x 10(4) s-1 and kf = 1.3(+/- 0.1) x 10(4) s-1 in deoxyhemoglobin; in carbonmonoxyhemoglobin they are kr = 3.9(+/- 1.3) x 10(4) s-1 and kf = 5.3(+/- 1.8) x 10(3) s-1. The pKa values of the coupled ionizing groups are 5.3 in deoxy- and 6.0 in carbonmonoxyhemoglobin. Modification of the CysF9(93) beta sulfhydryl group with iodoacetamide abolishes the pH dependence of 1/tau, suggesting that this sulfhydryl is involved in the isomerization process. Consideration of the X-ray structure of oxyhemoglobin allows a structural interpretation of the results. It is concluded that the isomerization may be important for the physiological function of hemoglobin, but that it is not identical with the quaternary structure transition.