Studies on ligand binding to sulphaemoglobin.

Equilibrium studies show that, at low protein concentrations, sulphaemoglobin, in the ferrous form, binds carbon monoxide at pH 6.0 in a non co-operative manner with a Hill coefficient of 1.15 and an affinity constant of 8 . 10(-7) M. At both pH 6.0 and pH 9.0 the kinetics of CO binding show the presence of a simple mono-exponential process with a second-order rate constant of 8 . 10(3) M-1 . s-1. The rate of dissociation of CO from sulphaemoglobin is approx. 0.01 s-1. The activation energy of the binding process is calculated as 40 kJ . mol-1. A comparison is presented between the CO binding properties of sulphaemoglobin, myoglobin and haemoglobin and a mechanism whereby the CO binding parameters of sulphaemoglobin are modified is proposed.