Kinetics of cyanide and carbon monoxide dissociation from ferrous human haptoglobin:hemoglobin(II) complexes

Haptoglobin (Hp) counterbalances the adverse effects of extra-erythrocytic hemoglobin (Hb) trapping the alpha beta dimers of Hb. In turn, the Hp:Hb complexes display heme-based reactivity. Here, the kinetics of cyanide and carbon monoxide dissociation from ferrous-ligated Hp:Hb complexes are reported at pH 7.0 and 20.0 degrees C. Cyanide dissociation from Hp1-1:Hb(II)-CN- and Hp2-2:Hb-CN- has been followed upon the dithionite-mediated conversion of ferric to ferrous-ligated Hp:Hb complexes. Values of k(on) for the dithionite-mediated reduction of Hp1-1:Hb(III)-CN- and Hp2-2:Hb(III)-CN- are (7.3 +/- 1.1) x 10(6) M-1 s(-1) and (6.2 +/- 1.0) x 10(6) M-1 s(-1), respectively. Values of the first-order rate constant (i.e., h) for cyanide dissociation from Hp1-1:Hb(II)-CN- and Hp2-2:Hb(II)-CN- are (1.2 +/- 0.2) x 10(-1) s(-1) and (1.3 +/- 0.2) x 10(-1) s(-1), respectively. CO dissociation from Hp:Hb(II)-CO complexes has been followed by replacing CO with NO. Values of the first-order rate constant (i.e., l) for CO dissociation from Hp1-1:Hb(II)-CO are (1.4 +/- 0.2) x 10(-2) s(-1) and (6.2 +/- 0.8) x 10(-3) s(-1), and those from Hp2-2:Hb(II)-CO are (1.3 +/- 0.2) x 10(-2) s(-1) and (7.3 +/- 0.9) x 10(-3) s(-1). Values of k(on), h, and l correspond to those reported for the R-state of tetrameric Hb and isolated alpha and beta chains. This highlights the view that the conformation of the Hb alpha beta-dimers bound to Hp1-1 and Hp2-2 matches that of the R-state of the Hb tetramer. Furthermore, unlike ferric Hb(III), ligated ferrous Hb(II) does not show an assembly-linked structural change. Graphic abstract