Abstract
The reactions of 13 isonitriles with deoxyhemoglobin have been examined and characterized at pH 7, 20 degrees C. Kinetic studies have shown that these ligands can be divided into two mechanistic classes based on their size and stereochemistry. The larger and branched compounds (isopropyl, all butyl isomers, n-pentyl, n-hexyl, cyclohexyl, and benzyl isocyanides) exhibit biphasic time courses at all ligand concentrations as a result of intrinsic differences between the reactivities of the alpha and beta subunits. In contrast, the smaller isonitriles (methyl, ethyl, and n-propyl isocyanides) exhibit monophasic, and often accelerating, time courses at high ligand concentrations. At low concentrations, the smaller isonitriles also exhibit biphasic time courses; however, in this case, the two phases are due to marked differences between the dissociation rate constants of the high and low affinity quaternary conformations of the protein. Sets of equilibrium and kinetic data for the binding of 11 of the isonitriles were fitted to an expanded version of the two-state allosteric model first described by Monad, Wyman, and Changeux (Monod, J., Wyman, J., and Changeux, J.-P. (1965) J. Mol. 12, 108 118). The resultant rate and equilibrium co...Continue Reading