The Kinetics of the Reactions of Leghemoglobin with Oxygen and Carbon Monoxide

Abstract

The values of the kinetic constants for the reactions of legume root nodule hemoglobin (leghemoglobin) with oxygen and carbon monoxide have been determined by stopped flow spectrophotometry. Leghemoglobin a and leghemoglobin c, which differ in amino acid sequence and peptide chain length, have very similar kinetic constants. The oxygen combination rate constant is the largest measured for any hemoglobin. The oxygen dissociation rate constant is similar to that of mammalian myoglobins. The oxygen affinity calculated by combining the kinetic constants agrees with the equilibrium value determined directly. The rate constant for combination with carbon monoxide is large, and is reflected in the very great affinity of leghemoglobin for carbon monoxide. The kinetic constants for the reactions of leghemoglobin with oxygen are particularly well suited to favor the facilitation of oxygen diffusion by leghemoglobin in the environment of very low mean oxygen pressure existing within the cells of the root nodule. It is suggested that leghemoglobin-facilitated oxygen diffusion serves not only to augment the influx of oxygen, but serves as well to make the oxygen pressure within the nodule or cell everywhere nearly the same.