Kinetics and thermodynamics of oxygen, CO, and azide binding by the subcomponents of soybean leghemoglobin

Kay D. Martin, Leonard Saari, Wang Guang-Xin, Thomas Wang, Lawrence J Parkhurst, Robert V. Klucas

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Abstract

Leghemoglobin shows extreme high affinity behavior in the binding of both oxygen and CO. We have determined the temperature dependence of the rate constants for ligation of oxygen and CO and from these data the thermodynamics (ΔG0, ΔH0, ΔS0) of ligation for the purified components of soybean leghemoglobin. X-ray crystallography has shown that the heme cavity can easily accommodate ligands the size of nicotinate, and analysis of extended x-ray absorption fine structure data has shown that the Fe atom is in the mean plane of the heme in the leghemoglobin-CO complex. Ligation of oxygen and CO are in accord with this picture in that the Ea for oxygen binding is that expected for a diffusion controlled reaction and ΔS0 for the ligation of both CO and oxygen is consistent with the simple immobilization of the ligand at the Fe, with no evidence for significant conformational changes in the protein or changes in solvation. At 20°C the rate constants for oxygen and CO binding vary by 26 - 44% among the eight leghemoglobin components. For azide binding the variation is a factor of 2. These variations appear to arise from amino acid substitutions outside either the heme cavity or the two major paths for ligand entry to the heme. The distribution of leghemoglobin components varies with the age of the soybean nodule during the growing season. The changes in composition alone, however, would only allow the concentration of free oxygen to vary by about 3%. This finding calls into question models that ascribe a significant functional role to changes in the distribution of leghemoglobin components in regulating oxygen concentration in the nodule.

Original languageEnglish (US)
Pages (from-to)19588-19593
Number of pages6
JournalJournal of Biological Chemistry
Volume265
Issue number32
StatePublished - Nov 15 1990

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Leghemoglobin
Azides
Carbon Monoxide
Thermodynamics
Soybeans
Oxygen
Kinetics
Heme
Ligation
Ligands
Rate constants
Niacin
X ray crystallography
X Ray Crystallography
Solvation
Amino Acid Substitution
Immobilization
Substitution reactions
X-Rays
Amino Acids

ASJC Scopus subject areas

  • Biochemistry

Cite this

Kinetics and thermodynamics of oxygen, CO, and azide binding by the subcomponents of soybean leghemoglobin. / Martin, Kay D.; Saari, Leonard; Guang-Xin, Wang; Wang, Thomas; Parkhurst, Lawrence J; Klucas, Robert V.

In: Journal of Biological Chemistry, Vol. 265, No. 32, 15.11.1990, p. 19588-19593.

Research output: Contribution to journalArticle

Martin, KD, Saari, L, Guang-Xin, W, Wang, T, Parkhurst, LJ & Klucas, RV 1990, 'Kinetics and thermodynamics of oxygen, CO, and azide binding by the subcomponents of soybean leghemoglobin', Journal of Biological Chemistry, vol. 265, no. 32, pp. 19588-19593.
Martin, Kay D. ; Saari, Leonard ; Guang-Xin, Wang ; Wang, Thomas ; Parkhurst, Lawrence J ; Klucas, Robert V. / Kinetics and thermodynamics of oxygen, CO, and azide binding by the subcomponents of soybean leghemoglobin. In: Journal of Biological Chemistry. 1990 ; Vol. 265, No. 32. pp. 19588-19593.
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