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(Received for publication, February 25, 1997, and in revised form, April 8, 1997)
,
,
and
From the 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
Department of Zoophysiology,
Department of
Molecular Biology, University of Aarhus, 8000 Aarhus C, Denmark
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
1
2 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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