To study allosteric mechanism in hemoglobin, a hydrogen-exchange method was used to measure ligand-dependent changes in structural free energy at defined allosterically sensitive positions. When the two α-subunits are CN-met liganded, effects can be measured locally, within the α-subunit, and also remotely, within the β-subunit, even though the quaternary structure remains in the T conformation. When the two β-subunits are liganded, effects occur at the same positions. The effects seen are the same, independently of whether ligands occupy the α-chain hemes or the β-chain hemes. Control experiments rule out modes of energy transfer other than programmed cross-subunit interaction within the T-state. Cross-subunit transfer may depend on pulling the heme trigger (moving the heme iron into the heme plane) rather than on liganding alone.
Bibliographical noteFunding Information:
This work was supported by research grant DK11295 from the National Institutes of Health. We thank Drs A. Arnone and R. Noble for gifts of mutant hemoglobins, and G. Ackers, A. Arnone, J. Friedman, and R Noble for helpful comments on this work and on the manuscript.
- Energy transfer
- Hydrogen exchange
- Molecular switching code