Open AccessModel compounds for the T state of hemoglobin.
Author(s) -
James P. Collman,
John I. Brauman,
Kenneth M. Doxsee,
Thomas R. Halbert,
Kenneth S. Suslick
Publication year - 1978
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.75.2.564
Subject(s) - cooperativity , myoglobin , hemoglobin , chemistry , ferrous , cobalt , yield (engineering) , crystallography , stereochemistry , inorganic chemistry , organic chemistry , thermodynamics , biochemistry , physics
O2 binding to a series of ferrous and cobaltous "picket fence" porphyrins is reported. N-Methylimidazole and covalently attached imidazoles gives O2 binding to ferrous porphyrins with deltaH degrees =-16.2 kcal/mol (-67.7 kJ/mol) and deltaS degrees =-40 eu (standard state, 1 atmosphere O2). Similar studies with cobaltous porphyrins yield deltaH degrees =- 12.8 kcal/mol (-53.5 kJ/mol) and deltaS 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.