This work will examine the factors which govern the rate of transition between equilibrium states of an allosteric protein. It is an attempt to determine if the factors which are significant in determining the free energy of the equilibrium states are equally significant in determining the rates of transition between those states. Hemoglobin has been selected as a model allosteric protein because it has already been extensively described in structural and thermodynamic terms. The forward and reverse rates of transition between the T and R conformations of hemoglobin are to be measured using the method of modulated excitation spectroscopy. Conformational change will be monitored by optical probes sensitive to the environment of the heme and to the environment of the subunit interfaces, and by the use of a DPG analog whose fluorescence is quenched upon binding between the Beta subunits. Measurements will be performed as a function of pH and temperature. Although the best signals are available for three ligands bound, we will also extend the technique to measurements with one ligand bound. In addition to studies with carbon monoxide as a ligand, we will also gather data for conformational change rates with oxygen bound. Experiments will also be performed on Trout I and IV hemoglobins. These experiments will further serve to characterize the transition state between the alternative hemoglobin structures, and relate it to molecular descriptions of the allosteric change. The work done here also generates rate pairs, so that equilibrium data about relative stabilities of structures is a natural byproduct. This equilbrium information can be used to test various thermodynamic theories describing hemoglobin function. Finally, by multi-spectral studies this work hopes to further identify the species which give rise to the various structural probes.
| Martino, A J; Ferrone, F A (1989) Rate of allosteric change in hemoglobin measured by modulated excitation using fluorescence detection. Biophys J 56:781-94 |
