Laser photolysis study of conformational change rates for hemoglobin in viscous solutions

Rates for the R leads to T conformational change of deoxyhemoglobin formed by laser photolysis of carboxyhemoglobin were determined from CO rebinding observed in three solution systems with viscosities between 1 and 6 cP. Experiments were carried out at 20 degrees C and pH 8.3 in solutions consisting of borate buffer containing various amounts of sucrose, glycerol, or ethylene glycol. As in the case of earlier experiments in borate buffer (Sawicki and Gibson, 1976, J. Biol. Chem., 251:1533-1542), a simple two-state allosteric model which takes into account tetramer-dimer dissociation was found to give a good description of all experimental results. Using measured values for the R- and T-state CO-binding rate constants and the tetramer-dimer dissociation constant, values for the conformational change rate were determined by fitting this model to the experimental data. These rates were compared with Gavish's transient strain model (Gavish, 1978, Biophys. Struct. Mech., 4:37-52), which predicts an inverse dependence of conformational change rate on viscosity. Although fair agreement is found for hemoglobin in sucrose/borate solutions, in glycerol/borate and ethylene glycol/borate solutions, conformational change rate falls off much more rapidly with increasing viscosity than predicted by the model.