Polymerization of recombinant Hb S-Kempsey (deoxy-R state) and Hb S-Kansas (oxy-T state)

In order to investigate the role of the R (relaxed) to T (tense) structural transition in facilitating polymerization of deoxy-Hb S, we have engineered and expressed two Hb S variants which destabilize either T state (Hb S-Kempsey, alpha 2 beta 2 Val-6,Asn-99) or R state structures (Hb S-Kansas, alpha 2 beta 2 Val-6, Thr-102). Polymerization of deoxy-Hb S-Kempsey, which shows high oxygen affinity and increased dimer dissociation, required about 2- and 6-fold higher hemoglobin concentrations than deoxy-Hb S for polymerization in low and high phosphate concentrations, and its kinetic pattern of polymerization was biphasic. In contrast, oxy- or CO Hb S-Kansas, which shows low oxygen affinity and increased dimer dissociation, polymerized at a slightly higher critical concentration than that required for polymerization of deoxy-Hb S in both low and high phosphate buffers. Polymerization of oxy- and CO Hb S-Kansas was linear and showed no delay time, which is similar to oversaturated oxy- or CO Hb S. These results suggest that nuclei formation, which occurs during the delay time prior to deoxy-Hb S polymerization, does not occur in T state oxy-Hb S-Kansas, even though the critical concentration for polymerization of T state oxy-Hb S-Kansas is similar to that of T state deoxy-Hb S.