Abstract
Fluorescence studies of transthyretin (TTR) were conducted to detect structural changes associated with the environment of its two tryptophans, induced by binding of thyroxine (T 4). Non-radiative tryptophans relaxation rate has an activation energy of 6.4 kcal/mol for TTR, which is decreased to 4.4 kcal/mol for TTR-T 4 complex. The maximum fluorescence wavelength was red-shifted as the excitation wavelength was increased. T 4 changed the magnitude of this shift. T 4 binding per se changed the emission maximum reflecting different environments of the tryptophans. Double-quenching experiments also showed that T 4 produces changes in the tryptophans environments. These findings were interpreted as the result of structural alterations in the protein matrix induced by T 4 which contribute in part to explain the negative cooperativity associated with the occupancy of the second binding site.
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