Assuming that blood retention and toxicity problems can be resolved by chemical or genetic cross-linking of hemoglobin, the next step in the design of a heme protein-based blood substitutes is to optimize oxygen transport and stability. Amino acids in and around the heme pocket of the hemoglobin molecule have been largely conserved throughout evolution with the exception of some human mutants and animal hemoglobins. We have compared large number of animal hemoglobins that are known to exhibit differences in their ligand affinity as well as chemical alterations of the heme environment a result of species differentiation. Studies conducted on animal hemoglobins appear to provide no clear and predictable correlation between and oxidation reactions, rather these reactions appear to be determined by the specific chemistry of the heme-protein, to provide a species adaptability to changing environments. Site directed mutagenesis is a potentially effective tool for the engineering hemoglobin because it allows the fine tuning of protein function and stability. At this point, myoglobin has provided a simple prototype for these experiments, and that indeed a number of myoglobin mutants have been prepared (at Rice University) that are have different ligand binding, autoxidation and stability. Sperm whale wild type myoglobin (His 64), single (V68F) (phenylalanine replaces valine) and double (L29F/H64Q) (phenylalanine replaces leucine; glutamine replaces histidine) mutants have been so far studied. Results up-to-date indicate clear differences among these myoglobins in terms of the rates of ferryl formation and its persistence in solutions. Differences in terms of the oxidative effects of HOOH treatment on both the heme and the proteins were also observed. We are currently probing the mechanism of HOOH mode of entry into the proteins. This will ultimately help in the design of a protein that can stereochemically restrict the entry of HOOH and minimize its oxidative effect.
