Signal transmission between subunits in the hemoglobin T-state

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.