The Transition State for the Change in Hemoglobin Quaternary Structure is T-Lilte

Abstract

Using modulated excitation of the carboxy derivative, and monitoring the structural change in the Soret band, we have measured the rate of change between the oxy (R) and deoxy (T) quarternary structure for hemoglobin A and hemoglobin Kansas with three CO ligands bound. Taken with other available data, the data from HbA suggests that the transition state is T like, i.e. the R → T rate changes much less than the R-T equilibrium changes as ligands are added. This may arise from the control of the transition state by a barrier unconnected with stability of either state, as proposed by Baldwin and Chothia. Alternatively it may be due to the intersection between the potential wells describing the R and the T states being very close to the latter along the reaction coordinate. Because Hb Kansas has a destabilized T state, measurement of its structural kinetics provides a test to distinguish between the two hypotheses. We find that the Kansas mutation changes the equilibrium constant (relative to HbA) by much more than it alters the R → T rate, in contradiction to the conjecture of Baldwin and Chothia. Thus is appears that the transition state is governed by the very same factor which affect the relative stabilities of the R and T structures.