Ii. Changes in Conformation and Chemical Reactivity Linked to Reaction with a Dissociable Low Molecular Weight Ligand, X*

Leghemoglobin may be isolated from soybean root nodules as a complex, leghemoglobin a-X of ferric leghemoglobin with a low molecular weight ligand X. That complex dissociates at neutral or alkaline pH and the ligand X may be separated from free leghemoglobin by ultrafiltration. The complex is regenerated by combining ferric leghemoglobin with X. The affinity of ferrous leghemoglobin for X is very much less than the affinity of ferric leghemoglobin for X. The optical spectrum of leghemoglobin a-X closely resembles that of a number of b-cytochromes. The evidence does not distinguish whether X binds to leghemoglobin at the heme iron atom or elsewhere. Reaction of ferric leghemoglobin a-X with dithionite yields ferrous leghemoglobin a-X. Ferrous leghemoglobin a-X dissociates slowly into free ferrous leghemoglobin and X; first order rate constant = 0.036 s-l. Ferrous leghemoglobin a-X reacts with carbon monoxide only after prior dissociation of the ligand X.