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Using a novel technique of modulated photo-dissociation of carbon monoxide from hemoglobin, we have obtained the rates for conversion between the two quaternary states, R, and T, at 3-fold ligation. Our measurements at pH 7 and 22 degrees give rates of 780 +/- 40 sec-1 for going from R to T, and 2500 +/- 200 sec-1 from T to R. This yields an equilibrium constant of 0.31 +/- 0.04, which is in good agreement with previous estimates. The degree of agreement between this equilibrium constant and that predicted from the allosteric model provides a new, quantitative test of the allosteric description. A sequential model for the change in structure was found incompatible with the data, even if kinetic subunit inequivalence was assumed. The technique described here is quite general and can be used as long as the system under investigation can be repetitively excited in a regime in which it responds linearly to the excitation.

Rate of quaternary structure change in hemoglobin measured by modulated excitation.