Kinetic studies on the five principal components of normal adult human hemoglobin

Abstract

The five principal components of human hemoglobin (Ala, Alb, Alc, Ao, and A2) have been isolated by column chromatography and by preparative isoelectric focusing in gels. The isoelectric points and a number of kinetic parameters have been determined for each hemoglobin. The greatest kinetic differences are found in the binding of CO to the deoxy conformation. At pH 7, A0 and A2 are nearly identical in their overall reaction with CO, whereas the initial lag phase characteristic of crude hemolysate and A0 is greatly reduced in Ala and Alc and is essentially absent in Alb. The general effect of p-mercuribenzoate bindind on CO association is to magnify kinetic differences among the hemoglobins, diminish the initial lag phase, and increase the overall rate of CO binding. Hemoglobin Ala is anomalous in that the overall CO binding rate actually decreases after reaction with the mercurial. In terms of an Adair model with four association constants the rate constant for the binding of the first molecule of CO (1l') showed the greatest variation among the five hemoglobins, with A0 having the smallest constant, and Alb the largest. For the native hemoglobins, 1l' for Alb was more than twice that for A0; for the mercurated hemoglobins, the difference was greater than threefold. Raising the pH form 7 to 8 increases 1l' for all hemoglobins, but Ala is anomalous in having a slower overall rate for CO binding at the higher pH. At pH 9, the time course of CO binding is biphasic for all hemoglobins, with A0, the fastest, and Ala, the slowest, differing by nearly threefold in rate. The equilibrium constant for the tetramer-dimer equilibrium was determined by flash photolysis. The largest dissociation constant occurs for Ala and is 4.4 times that for A0, and 5.6 times that for Alc, the least dissociated of the hemoglobins. The overall oxygen dissociation reaction is biphasic for Ala and Alb, with the two phases differing by a factor of 5; the dissociation reactions for the other three hemoglobins appear essentially monophasic. The kinetics of dissociation of the first oxygen molecule from oxyhemoglobin are very similar for all five hemoglobins, as are the association kinetics for CN-minus and N3-minus binding to the five methemoglobins.