Characterization of hemoglobin Würzburg (α2ß2 4(A1)Thr → Asn), a new electrophoretically silent variant, by mass spectrometry and molecular modeling studies

The first hemoglobin (Hb) variant carrying a mutation at ß4 was identified as ß4(A1)Thr → Asn or Hb Würzburg and constituted 38% of the total hemoglobin. It showed a slightly elevated oxygen affinity and a slightly decreased cooperativity index (n50 = 2.3 versus n50 = 2.8). The analysis of the electrostatic potential showed an increased negative charge at the site of the mutation with a displacement of ß6(A3)Glu by 1.3 Å. The replacement of threonine by asparagine seems to stabilize the R conformation. This may explain partially both the high affinity and the reduction in cooperativity.