Ligand-dependent behavior of the hemoglobin from the ascarid Parascaris equorum

Abstract

Hemoglobin from the ascarid Parascaris equorum is made of eight identical subunits, each containing protoheme as a prosthetic group. The thermodynamic and kinetic properties of the binding of this protein with three ligands, i.e., oxygen, carbon monoxide and ethyl isocyanide, have been investigated. The reaction with ethyl isocyanide is characterized by a hyperbolic binding curve ( n = 1), whereas binding of CO displays a weak but definite cooperativity ( n = 1.3). Displacement experiments suggest that the noncooperative binding of ethyl isocyanide occurs fully in the high-affinity state, related to a very early quaternary switch. All three ligands display binding kinetics similar to those of T-state HbA. Dissociation rate constants for CO and ethyl isocyanide fall in the same range observed for other hemoglobins. On the other hand, oxygen affinity is much higher than in human HbA and, similarly to Ascaris lumbricoides Hb, it has to be attributed to an unusually slow dissociation rate constant. In this paper it is shown that such a slow kinetic process is not affected by: (i) pH; (ii) guanidine-HCl; (iii) a large excess of p-chloromercuribenzoate; (iv) the lifetime of the oxy-Hb complex.