Conformational Changes in Hemoglobin S (βE6V) Imposed by Mutation of the βGlu7–βLys132 Salt Bridge and Detected by UV Resonance Raman Spectroscopy

Abstract

The impact upon molecular structure of an additional point mutation adjacent to the existing E6V mutation in sickle cell hemoglobin was probed spectroscopically. The UV resonance Raman results show that the conformational consequences of mutating the salt bridge pair, betaGlu(7)-betaLys(132), are dependent on which residue of the pair is modified. The betaK132A mutants exhibit the spectroscopic signatures of the R --> T state transition in both the "hinge" and "switch" regions of the alpha(1)beta(2) interface. Both singly and doubly mutated hemoglobin (Hb) betaepsilon7Alpha exhibit the switch region signature for the R --> T quaternary state transition but not the hinge signature. The absence of this hinge region-associated quaternary change is the likely origin of the observed increased oxygen binding affinity for the Hb betaepsilon7Alpha mutants. The observed large decrease in the W3 alpha14beta15 band intensity for doubly mutated Hb betaepsilon7Alpha is attributed to an enhanced separation in the A helix-E helix tertiary contact of the beta subunits. The results for the Hb A betaGlu(7)-betaLys(132) salt bridge mutants demonstrate that attaining the T state conformation at the hinge region of the alpha(1)beta(2) dimer interface can be achieved through different intraglobin pathways; these pathways are subject to subtle mutagenic manipulation at sites well removed from the dimer interface.