Multiple T state conformations in a fish hemoglobin. Carbon monoxide binding to hemoglobin of Thunnus thynnus.

The blood of the Atlantic bluefin tuna (Thunnus thynnus) contains four hemoglobin components separable by chromatography on diethylaminoethylcellulose. These components are stable and functionally identical in their reactions with carbon monoxide. At low pH they remain in the T state even when liganded, and show two kinetic components in binding and in dissociation, with rates of 1) 1.2 microM-1 s-1 and 0.095 s-1 and 2) 0.013 microM-1 s-1 and 0.195 s-1, respectively, at pH 6, 20 degrees C, 0.1 m KPi. These components have difference spectra separated by more than 2 nm and are present in equal amounts. After CO has bound, there is a conformation change to an altered T state, in terms of the model of Monod et al. (Monod, J., Wyman, J., and Changeux, J. P. (1965) J. Mol. Biol. 12, 88-118), with a half-time of 65 s. At equilibrium, one-third of the slow kinetic component is changed into the new conformer, which binds CO at a rate of 0.14 microM-1 s-1.