Evidence for cooperative hydrogen ligand binding to sperm whale metmyoglobin.
Potentiometric titrations of sperm whale metmyoglobin from pH 10 to 3 shows the well-known exposure of groups between pH 4.5 and 4. However the reverse titration, at low protein concentration, results in the regeneration of its ionic property in the form of a reversible hysteresis, which was obtained by titrating the same solution twice. The Soret intensity band indicates reversibility for about 94%. Apomyoglobin shows an acid-base reversible titration from pH 9.5 to 3. Since the analysis by electrostatic interaction models do not adequately describe the overall ionic equilibria of metmyoglobin, a cooperative model was introduced. The model exactly reproduces the titration curve, indicating that 6 groups ionize with a cooperative coefficient of 9. The cooperative model explains the anomalous behaviour toward hydrogen ions and the incomplete spectral reversibility of sperm whale metmyoglobin as a molecular mechanism with physiological significance.
Forlani
L
L
Dipartimento di Medicina Sperimentale, Facoltà di Medicina, Università La Sapienza, Rome.
Ioppolo
C
C
Evans
W J
WJ
Marini
M A
MA
D000818:Animals; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008786:Metmyoglobin / Q000737:chemistry / Q000378:metabolism*; D008962:Models, Theoretical; D010100:Oxygen; D011487:Protein Conformation; D014907:Whales / Q000378:metabolism*
D008024:Ligands; D008786:Metmyoglobin; D010100:Oxygen