Equilibrium and kinetic measurements of the binding of pyridoxal 5'-phosphate to hybrid tryptophan synthase from Escherichia coli

Influence of cofactor pyridoxal 5'-phosphate on reversible high-pressure denaturation of isolated beta 2 dimer of tryptophan synthase bienzyme complex from Escherichia coli

High hydrostatic pressure has been shown to cause reversible dissociation of the isolated apo beta 2 dimer of tryptophan synthase from Escherichia coli into enzymatically inactive monomers [Seifert, T., Bartholmes, P., & Jaenicke, R. (1982) Biophys. Chem. 15, 1-8]. Addition of the coenzyme pyridoxal 5'-phosphate affects the structural stability, as well as the kinetics of dissociation and deactivation. The apo beta 2 dimer is deactivated faster than the holoenzyme by a factor of 10. The midpoints of the corresponding equilibrium transition curves are observed at 690 and 870 bar, respectively. As shown by hybridization of native and chemically modified beta chains, the loss of enzymatic activity is accompanied by subunit dissociation. An additional deactivating effect is produced by the pressure-induced release of the cofactor from the holoenzyme. Renaturation after decompression has been monitored by circular dichroism and intrinsic fluorescence emission. Alterations of the dichroic absorption at 222 nm reflect the recovery of the native secondary structure, while tryptophan fluorescence represents a specific probe for the native tertiary structure in the immediate neighborhood of the active center of the enzyme. By application of both methods to monitor the reconstitution of the apo beta 2 dimer, two first-order processes may be separated along the time scale. The faster phase (k1 = 1.2 X 10(-2) s-1) yields a "structured monomer" with 85% native secondary structure and the tryptophan side chain buried in its native hydrophobic environment. As indicated by sodium borohydride reduction, this intermediate is able to interact with the coenzyme pyridoxal 5'-phosphate in the correct way; however, it does not show enzymatic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Reconstitution of the isolated beta2-subunit of tryptophan synthase from Escherichia coli after dissociation induced by high hydrostatic pressure. Equilibrium and kinetic studies

The isolated beta2-subunit of Escherichia coli tryptophan synthase can be reversibly dissociated into enzymically inactive monomers under high hydrostatic pressure. Deactivation at 1.5 kbar which shows a half-time of 11 min (rate constant k=10 (-3) s (-1) is paralleled by dissociation with a small lag phase of about 5 min. Pressure release leads to 95 +/- 5% recovery of specific activity and complete restoration of the hydrodynamic and spectral properties which specify the native dimer. Over the concentration range 1-100 micrograms/ml (0.02-2.3 micrograms M) the kinetics of reactivation can be fitted by one apparent first-order rate constant (k=6.5 +/- 0.6 X 10 (-4) s (-1), half-time = 17.5 min). The reconstitution of catalytic activity is paralleled by alterations in tryptophan fluorescence at 327 nm, thus presenting direct evidence for conformational changes in the direct vicinity of the active center (k1 = 1.9 X 10 (-3) s(-1), k2 = 6.5 +/- 0.6 X 10(-4) s (-1) ). On the other hand, a definite mechanism of reactivation requires the association of the refolding monomers to be included. The kinetics of dimerization have been followed via hybridization between native and chemically modified beta-chains, yielding an apparent first-order rate constant of 6.3 +/- 0.6 X 10 (-4) s (-1). As a consequence, we propose a sequential uni-uni-bimolecular mechanism, which is characterized by a minimum of two conformational changes in substantially structured monomers followed by a fast dimerization reaction to yield the active beta2-subunit.