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Volume 272, Number 25, Issue of June 20, 1997 pp. 15628-15635
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

The Anodic Hemoglobin of Anguilla anguilla
MOLECULAR BASIS FOR ALLOSTERIC EFFECTS IN A ROOT-EFFECT HEMOGLOBIN

(Received for publication, February 25, 1997, and in revised form, April 8, 1997)

Angela Fago , Emøke Bendixen , Hans Malte and Roy E. Weber

From the  Department of Zoophysiology, Institute of Biological Sciences, Building 131 and the  Department of Molecular Biology, University of Aarhus, 8000 Aarhus C, Denmark

The functional and structural basis for the Root effect has been investigated in the anodic hemoglobin of the European eel, Anguilla anguilla. This hemoglobin exhibits a large Bohr effect, which is accounted for by oxygen-linked binding of seven to eight protons in the presence of GTP at pH 7.5. Oxygen equilibrium curves show nonlinear lower asymptote of Hill plots, indicating the occurrence of heme-heme interactions within the T state. Analysis of the curves according to the co-operon model (Brunori, M., Coletta, M., and Di Cera, E. (1986) Biophys. Chem. 23, 215-222) reveals that T state cooperativity is positive at high pH and in the stripped hemoglobin (where the T  R allosteric transition is operative) and negative at low pH and in the presence of organic phosphate (where the molecule is locked in the low affinity structure), indicating site heterogeneity. The complete amino acid sequence of eel anodic hemoglobin has been established and compared with that of other fish hemoglobins. The presence of the Root effect correlates with a specific configuration of the ₁₂ switch interface, which at low pH would stabilize subunit ligation in the T state without changing the quaternary structure. We propose that the major groups involved in the binding of oxygen-linked protons in eel anodic hemoglobin are located on the  chain and comprise His-HC3 at the C terminus, His-FG4 at the switch interface, and Lys-EF6 and the N terminus at the phosphate-binding site.

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