Pressure dissociation and conformational drift of the beta dimer of tryptophan synthase

Micromolar solutions of tryptophan synthase beta 2 dimer dissociate into monomers in the pressure range of 800-1600 bars as shown by studies of the spectral shift of the intrinsic fluorescence and of the fluorescence polarization of dansyl conjugates. At 25 degrees C the standard change in volume on dissociation (dV0) of the holoprotein was -162 mL mol-1, and the dissociation constant at 1 bar was K0 = 3.7 10(-10) M. Pyridoxal-reduced holoprotein and apoprotein had, within 10%, the same dV0, but K0 was decreased in the reduced protein (6 X 10(-11) M) and increased in the apoprotein (3.6 X 10(-9) M). At 4 degrees C the free energy of association of the holoprotein was reduced by 1.4 kcal mol-1, but dV0 was unchanged. In all the protein forms the decompression curves differed from the respective compression curves, indicating the loss of some free energy of association following separation of the monomers. This hysteretic behavior was largest in the apoprotein and amounted to a loss of 2.6 kcal mol-1 in the free energy of association. When the pressure was rapidly raised to 2.2 kbars, half-dissociation of the reduced pyridoxal beta 2 dimer took approximately 12 min. Upon return to atmospheric pressure reassociation was complete in 2-3 min and half of the enzyme activity was regained in 10 min; pyridoxal fluorescence recovered more slowly with a biphasic course. The independent return of these properties and the hysteretic behavior indicate that subunit separation is followed by a conformational drift like that observed in lactate dehydrogenase dissociated by either pressure or temperature or in enolase dissociated by dilution.