A high energy structure change in hemoglobin studied by difference hydrogen exchange.

The hydrogen exchange behavior of a small allosterically responsive set of exchanging hydrogens was studied in hemoglobin A and in some chemically modified hemoglobins. The set experiences an exceptionally large change in exchange rate through hemoglobin's allosteric transition. This indicates, according to the local unfolding model of H-exchange, that a large change in allosteric free energy impinges on the opening segment that exposes these protons to exchange. In oxyhemoglobin the set consists of 5 to 6 protons which exchange with a half-time of 20 s at pH 7.4 and 0 degrees C. In deoxyhemoglobin the set splits into a slower and a faster half. The slower 3 protons exchange more slowly than in oxyhemoglobin by a factor of 5000 (26 h half-time) and are 5-fold slower still in the presence of pyrophosphate or inositol hexaphosphate (136 h half-time). The other 2 to 3 protons exchange about 20-fold faster in both cases (about 2 h and 10 h half-times). The effect of some chemical modifications was tested, including reaction with iodoacetamide and N-ethylmaleimide and cleavage with carboxypeptidases A and B. In all cases the 3 slower protons continue to behave as a cohesive set and in the various modified deoxyhemoglobins their exchange is accelerated by factors ranging between 1 and 3 decades. These factors correlate with the effect of the different modifications on hemoglobin cooperativity.