Model Compounds for the T State of Hemoglobin

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 75; no. 2; pp. 564 - 568
• Main Authors: Collman, James P., Brauman, John I., Doxsee, Kenneth M., Halbert, Thomas R., Suslick, Kenneth S.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01.02.1978; National Acad Sciences
• Subjects: Cobalt; Hemoglobins; Imidazoles; Iron; Metalloporphyrins; Models, Chemical; Myoglobin; Oxygen; Oxygen - blood; Phosphates; Picket fences; Porphyrins; Porphyrins - metabolism; Protein Binding
• DOI: 10.1073/pnas.75.2.564
• ISSN: 0027-8424

O2binding to a series of ferrous and cobaltous ``picket fence'' porphyrins is reported. N-Methylimidazole and covalently attached imidazoles give O2binding 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 O2affinity than the analogous iron porphyrins when the axial base is hindered. Thermodynamic data are presented. The molecular mechanism of cooperativity in hemoglobin is discussed.