The Bohr effect of hemoglobin intermediates and the role of salt bridges in the tertiary/quaternary transitions

Understanding mechanisms in cooperative proteins requires the analysis of t he intermediate ligation states. The release of hydrogen ions at the interm ediate states of native and chemically modified hemoglobin, known as the Bo hr effect, is an indicator of the protein tertiary/ quaternary transitions, useful for testing models of cooperativity, The Bohr effects due to ligati on of one subunit of a dimer and two subunits across the dimer interface ar e not additive. The reductions of the Bohr effect due to the chemical modif ication of a Bohr group of one and two alpha or beta subunits are additive, The Bohr effects of monoliganded chemically modified hemoglobins indicate the additivity of the effects of ligation and chemical modification with th e possible exception of ligation and chemical modification of the ct subuni ts, These observations suggest that ligation of a subunit brings about a te rtiary structure change of hemoglobin in the T quaternary structure, which breaks some salt bridges, releases hydrogen ions, and is signaled across th e dimer interface in such a way that ligation of a second subunit in the ad jacent dimer promotes the switch from the T to the R quaternary structure. The rupture of the salt bridges per se does not drive the transition.