Allosteric properties of carbamylated hemoglobins

The spectra, ligand-binding properties, and conformational change of carbamylated human hemoglobin A, specifically modified at its alpha-NH2 groups, have been compared with those of hemoglobin A. In the Soret region, the spectra of all deoxyhemoglobins are identical. The Soret bands of carbonmonoxy alpha 2c beta 2 and alpha 2 beta 2c are shifted in opposite directions relative to hemoglobin A, alpha 2c beta 2 having its Soret band at the longer wavelength. Carbamylation of either chain slows oxygen dissociation from the alpha chain in the R-state, although the effect is larger for alpha 2c beta 2 and alpha 2c beta 2c than alpha 2 beta 2c. Carbamylation of either chain also slowed oxygen binding to the alpha chain while the rate for the beta chain is not changed significantly, indicating that the chains influence each other in the R-state. The rates of oxygen binding after full and partial laser photolysis of oxygen-saturated alpha 2c beta 2 appear identical. The rate of oxygen rebinding for alpha 2 beta 2c, after partial (10%) photolysis is the same as that for alpha 2c beta 2, but following full photolysis, biphasic rebinding was observed due to the appearance of T-state molecules. Carbon monoxide binding studies under photostationary conditions suggested an L value for alpha 2c beta 2c smaller than that for alpha 2 beta 2c and hemoglobin A. The R to T transition of the deoxy form of alpha 2c beta 2 is abut 10 times slower than hemoglobin A. Carbamylation of the beta chain does not slow the transition, indicating that carbamylation of the chain affects the allosteric equilibrium in deoxyhemoglobin. To a reasonable approximation with minimum assumptions, all of the observations made in this study may be rationalized using the two-state model of Monod, J., Wyman, J., and Changeux, J. P. ((1965) J. Mol. Biol. 12, 88-118).