Conformational Changes in the α-Subunit Coupled to Binding of the β₂-Subunit of Tryptophan Synthase from Escherichia coli:  Crystal Structure of the Tryptophan Synthase α-Subunit Alone^†,‡

When the tryptophan synthase α- and β₂-subunits combine to form the α₂β₂-complex, the enzymatic activity of each subunit is stimulated by 1−2 orders of magnitude. To elucidate the structural basis of this mutual activation, it is necessary to determine the structures of the α- and β-subunits alone and together with the α₂β₂-complex. The crystal structures of the tryptophan synthase α₂β₂-complex from Salmonella typhimurium (Stα₂β₂-complex) have already been reported. However, the structures of the subunit alone from mesophiles have not yet been determined. The structure of the tryptophan synthase α-subunit alone from Escherichia coli (Ecα-subunit) was determined by an X-ray crystallographic analysis at 2.3 Å, which is the first report on the subunits alone from the mesophiles. The biggest difference between the structures of the Ecα-subunit alone and the α-subunit in the Stα₂β₂-complex (Stα-subunit) was as follows. Helix 2‘ in the Stα-subunit, including an active site residue (Asp60), was changed to a flexible loop in the Ecα-subunit alone. The conversion of the helix to a loop resulted in the collapse of the correct active site conformation. This region is also an important part for the mutual activation in the Stα₂β₂-complex and interaction with the β-subunit. These results suggest that the formation of helix 2‘ that is essential for the stimulation of the enzymatic activity of the α-subunit is constructed by the induced-fit mode involved in conformational changes upon interaction between the α- and β-subunits. This also confirms the prediction of the conformational changes based on the thermodynamic analysis for the association between the α- and β-subunits.