Energetic adaptation of ligand binding to subunit structure of tryptophan synthase from Escherichia coli

The binding of indole and L-serine to the isolated alpha and beta 2 subunits and the native alpha 2 beta 2 complex of tryptophan synthase from Escherichia coli was investigated by direct microcalorimetry to reveal the energetic adaptation of ligand binding to the subunit structure of a multienzyme complex. In contrast to the general finding that negative heat capacity changes are associated with ligand binding to proteins, complex formation of indole and the alpha subunit involves a small positive change in heat capacity. This unusual result was considered as being indicative of a loosening of the protein structure. Such an interpretation is in good agreement with results of chemical accessibility studies (Freedberg & Hardman, 1971). Whereas the thermodynamic parameters of indole binding are not influenced by the subunit interaction, the large negative change in heat capacity of -6.5 kJ/(K X mol of beta 2) measured for the binding of L-serine to the isolated beta 2 subunit disappears completely when serine interacts with the tetrameric complex. These data demonstrate that the energy transduction pattern and therefore the functional roles of the substrates indole and L-serine vary strongly with the subunit structure of tryptophan synthase.