Site-directed mutagenesis in hemoglobin: attempts to control the oxygen affinity with cooperativity preserved.

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Abstract
We synthesized three artificial human hemoglobin mutants in which Lys-66(E10)beta was replaced by Ser, Arg or Thr by site-directed mutagenic protein engineering. These engineered hemoglobins were designated as eHb K66betaS, eHb K66betaR and eHb K66betaT, respectively. By synthesizing these mutants we attempted to control the oxygen affinity of hemoglobin with cooperativity preserved and to clarify the functional role of Lys-66(E10)beta, as well. Such attempts may be useful for creating an oxygen carrier suitable as a blood substitute. The oxygen affinities of eHbs K66betaS, K66betaR and K66betaT were 1.3-, 1.5- and 2.3-fold, respectively, lower than that of reconstituted Hb A. Their allosteric properties such as the Bohr effect and the effect of inositol hexaphosphate were well preserved. Since the oxygen affinity of eHb K66betaT is comparable with that of red cells, it may be a potential candidate for a blood substitute. X-Ray crystallographic data for Hb Chico [Lys-66beta-->Thr], which is identical with eHb K66betaT, together with our computer simulation indicate that an interaction between the introduced Thr and the distal His via a water molecule lowers the oxygen affinity for the T state eHb K66betaT.

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