Molecular aspects of the high oxygen afinity of non-hypertensive hexa pegylated hemoglobin, [(SP-PEG5K)6-Hb]

The development of hexaPEGylated Hb, (SP-PEG5K)6-Hb, using the newly designed thiolation-mediated maleimide chemistry based PEGylation, has validated the concept that engineering 'plasma volume expander' -like properties to Hb neutralizes its vasoactivity. The high O2 affinity of hexaPEGylated Hb has been attributed to the two PEG-5K chains on its two Cys-93(β) residues. In an attempt to map the influence of the additional four PEG-5K chains of HexaPEGylated Hb on the O2 affinity, we have now investigated the influence of PEGylation of the surface amino groups alone on the subunit interface interactions and O2 affinity of Hb using rHb(βC93A). The molecular radius of PEGylated rHb(βC93A) was slightly smaller than that of (SP-PEG5K)6-Hb, and the overall site-selectivity of PEGylation in the PEGylated rHb(βC93A) at Lys-residues was comparable to that of (SP-PEG5K)6-Hb. Proton NMR studies have shown that the conjugation of the protein with PEG-5K does not have any significant influence on its subunit interface interactions. Surprisingly, the influence of PEGylation on the O2 affinity and Bohr effect of HbA and rHb(βC93A) is also nearly the same. Apparently, conjugation of PEG-chains to Lys residues of Hb by the thiolation mediated PEGylation induces unique changes in the structure of the hydration shell of Hb (layer of tightly bound water molecules), which, in turn, induces constraints in its R to T conformational transition to favor the more hydrated R-state.