The CO-binding kinetics and the optical spectra of the NO derivative of the homodimeric hemoglobin from Scapharca inaequivalvis have been investigated over the range between pH 7.0 and 2.0. In the deoxygenated derivative, protonation of the proximal imidazole at very low pH values and the consequent cleavage of the Fe-N(ε) bond result in a ≃50-fold enhancement of the rate constant for CO binding, as found in other hemo-proteins. However, in the case of the hemoglobin from S. inaequivalvis, the pH profile displays a cooperative behavior (n = 1.8 ± 0.1), a unique feature that differentiates this protein from any other hemoprotein investigated thus far. Cleavage of the proximal bond in the NO derivative of S. inaequivalvis hemoglobin likewise displays a very steep pH transition. The mode of assembly of the homodimer, in which the heme-carrying E and F helices provide the subunit interface and bring the hemes at a much shorter distance (18.4 Å) than in vertebrate hemoglobins, is likely to provide the structural basis for this unique behavior.
|Number of pages||3|
|Journal||Journal of Biological Chemistry|
|Publication status||Published - 1990|
ASJC Scopus subject areas