A study of the mechanisms of slow religation to sickle cell hemoglobin polymers following laser photolysis.

Time-resolved linear dichroism (TRLD) measurements are conducted on gels of sickle cell hemoglobin following laser photolysis of the carbonyl adduct to monitor religation kinetics to hemoglobin S polymers. The return of the polymer phase to its equilibrium ligation state has been found to be about 1000 times slower than that of the solution phase hemoglobin tetramers. Several mechanisms describing this slow religation to the polymer were proposed: (1) religation occurs through a biomolecular process involving all polymer hemes, (2) religation occurs through a bimolecular process in which only hemoglobin molecules at the polymer ends can participate, and (3) religation occurs through the exchange of ligated hemoglobin molecules in the monomer phase with unligated ones in the polymer phase. To test these mechanisms, measurements are performed on gels having different domain sizes. The results show no relation between domain size and religation kinetics. The independence of religation kinetics and domain size is most consistent with the first of the three mechanisms described above (bimolecular recombination involving all polymer hemes). This result is discussed in terms of a model in which diffusion of the ligand is inhibited in the polymer phase. An understanding of the ligand binding kinetics of sickle hemoglobin polymers could have pathophysiological significance in its relevance to polymer formation and melting during red blood cell circulation.