Kinetic inequivalence between ? and ? subunits of ligand dissociation from ferrous nitrosylated human haptoglobin:hemoglobin complexes. A comparison with O2 and CO dissociation

Haptoglobin (Hp) counterbalances the adverse effects of extra-erythrocytic hemoglobin (Hb) by trapping ?? dimers Hb in bloodstream. In turn, Hp:Hb complexes display Hb-like reactivity. Here, kinetics NO dissociation from ferrous nitrosylated (i.e., Hp1–1:Hb(II)-NO and Hp2–2:Hb(II)-NO, respectively) are reported at pH 7.0 20.0 °C. Hp:Hb(II)-NO has been followed replacing with CO. Denitrosylation Hp2–2:Hb(II)-NO biphasic, relative amplitude fast slow phase being 0.495 ± 0.015 0.485 0.025, respectively. Values koff(NO)1 koff(NO)2 (6.4 0.8) × 10?5 s?1 (3.6 0.6) for (5.8 (3.1 Hp2–2:Hb(II)-NO) unaffected allosteric effectors correspond to those ? ? subunits tetrameric Hb(II)-NO isolated ?(II)-NO ?(II)-NO chains, This highlights view that conformation ?1?1 ?2?2 matches high affinity conformation. Moreover, observed functional heterogeneity reflects variation energy barriers ligand detachment exit pathway(s) associated different structural arrangement two R-state. Noteworthy, extent inequivalence chains is closely similar O2, CO R-state, suggesting it solely determined difference between subunits.