Question

In the mutant hemoglobin known as HB Providence, an asparagine residue in the B-chain replaces Lys-82. In the normal hemoglobin, Lys-82 projects in the cental cavity of the hemoglobin molecule. Predict the effect of the Lys-Asn mutation of the affinity of allosteric modifiers (relative to normal Hb e.g HbA) and describe the effect the mutation would have on the function (oxygen binding) og HB Providence.

Answer 1

Hemoglobin providence Asn is a mutant or abnormal hemoglobin that arises from a single genetic change that substitutes asparagine (Asn) for lysine at position 82 in the beta chain of human hemoglobin. This substitution is notable because beta82 lysine is one of the residues involved in 2,3-diphosphoglycerate binding. In addition, beta82 lysine is typically invariant in hemoglobin beta chain sequences.

Lysine being a positively charged residue involves in the binding of anionic cofactors. When it is substituted by a neutral residue in Hb providence Asn, it results in differences in terms of kinetics and equilibria of ligand binding. Even though the homotropic interactions are normal, the heterotrophic interactions are reduced greatly. Thus, the absence of positive residue (lysine) at beta82 resulted in the reduction of anion sensitivity. It has also lead to the reduction of net positive charge at the central cavity.

Relative to Hb A, hemoglobin providence shows decreased affinity for oxygen at neutral pH in the absence of cofactors. This shows that binding of anionic cofactors in Hb A directly influences the affinity for oxygen by neutralizing the charged groups of the diphosphoglycerate binding site.

Thus, a single amino acid substitution can modify hemoglobin’s anionic interactions, pH, affinity for allosteric modifiers, and affinity for oxygen without altering the cooperative interactions between hemoglobin subunits.