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Model Compounds for the T State of Hemoglobin

James P. Collman, John I. Brauman, Kenneth M. Doxsee, Thomas R. Halbert and Kenneth S. Suslick
Proceedings of the National Academy of Sciences of the United States of America
Vol. 75, No. 2 (Feb., 1978), pp. 564-568
Stable URL: http://www.jstor.org/stable/67825
Page Count: 5
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Model Compounds for the T State of Hemoglobin
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Abstract

O2 binding to a series of ferrous and cobaltous ``picket fence'' porphyrins is reported. N-Methylimidazole and covalently attached imidazoles give O2 binding to ferrous porphyrins with Δ H degrees = -16.2 kcal/mol (-67.7 kJ/mol) and Δ S degrees = -40 eu (standard state, 1 atmosphere O2). Similar studies with cobaltous porphyrins yield Δ H degrees = -12.8 kcal/mol (-53.5 kJ/mol) and Δ S degrees = -39 eu. These values match well those of myoglobin and isolated subunits of hemoglobin and their cobalt reconstituted analogues. 1,2-Dimethylimidazole has been successfully used to mimic the presumed restraint of T state hemoglobin. In direct analogy to the decreased cooperativity shown by cobalt-substituted hemoglobin, model cobalt porphyrins show a smaller decrease in O2 affinity than the analogous iron porphyrins when the axial base is hindered. Thermodynamic data are presented. The molecular mechanism of cooperativity in hemoglobin is discussed.

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