Specific interactions of the allosteric effector 2,3-bisphosphoglycerate with human hemoglobin--a difference FTIR study.

The specific interactions of 2,3-bisphosphoglycerate (BPG) with human deoxy-hemoglobin (deoxy-HbA) are studied by difference FTIR spectroscopy. Due to these interaction effects the O2 affinity of the hemoglobins is regulated. In deoxy-HbA a NH+ ... -OOC hydrogen bond is formed between beta 82 Lys and the carboxylate group of BPG. The phosphate groups of BPG are completely deprotonated causing a strong electrostatic stress within the Hb molecule. The aminotermini (beta 1 Val) interact with the phosphate groups but no hydrogen bonds are formed. The interaction is limited to an electrostatic interaction between the NH3+ and the negatively charged phosphate group, i.e. only ionic bonds are built up. The histidines beta 2 and beta 143 of the two beta-chains form hydrogen bonds with the phosphates. To each phosphate two bonds are formed. These bonds polarize each other and hence only the polar structures NH+ ... -OP of the hydrogen bonds are realized. This follows since no protons are present at the BPG molecules. Thus, caused by the hydrogen bonds and the electrostatic interaction the conformation of HbA is changed by BPG in the way that the T-structure is favored and hence the affinity for oxygen is decreased.