Equilibria and Kinetics of Ligand Binding by Leghemoglobin from Soybean Root Nodules

Abstract

Abstract The equilibrium between carbon monoxide and leghemoglobin from soybean root nodules shows that the protein is half-saturated with carbon monoxide at only about 0.00074 mm Hg pco at 20°. This value varies only slightly with changes in salt or protein concentration. The value is essentially independent of pH (no Bohr effect). The temperature dependence measurements show a rather large van't Hoff apparent enthalpy, -18.9 kcal per mole, which reflects the very high free energy of ligand binding indicated by the equilibrium. Leghemoglobin is half-saturated with oxygen at about 0.047 mm Hg. The kinetic dissociation constant for oxygen, koo2 off, is 11 sec-1 at 25°. This constant is believed to play a controlling role in the facilitation of the diffusion of oxygen by mammalian myoglobins which have koo2 ≅ 10 sec-1. This close similarity suggests that a primary role of leghemoglobin may also be that of the facilitation of the diffusion of oxygen within the nodule. The kinetic on constant in leghemoglobin appears to be primarily responsible for the very high affinity of ligand binding.