title: Conformational changes in the alpha-subunit coupled to binding of the beta 2-subunit of tryptophan synthase from Escherichia coli: crystal structure of the tryptophan synthase alpha-subunit alone.
abstract: When the tryptophan synthase alpha- and beta(2)-subunits combine to form the alpha(2)beta(2)-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 alpha- and beta-subunits alone and together with the alpha(2)beta(2)-complex. The crystal structures of the tryptophan synthase alpha(2)beta(2)-complex from Salmonella typhimurium (Stalpha(2)beta(2)-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 alpha-subunit alone from Escherichia coli (Ecalpha-subunit) was determined by an X-ray crystallographic analysis at 2.3 A, which is the first report on the subunits alone from the mesophiles. The biggest difference between the structures of the Ecalpha-subunit alone and the alpha-subunit in the Stalpha(2)beta(2)-complex (Stalpha-subunit) was as follows. Helix 2' in the Stalpha-subunit, including an active site residue (Asp60), was changed to a flexible loop in the Ecalpha-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 Stalpha(2)beta(2)-complex and interaction with the beta-subunit. These results suggest that the formation of helix 2'that is essential for the stimulation of the enzymatic activity of the alpha-subunit is constructed by the induced-fit mode involved in conformational changes upon interaction between the alpha- and beta-subunits. This also confirms the prediction of the conformational changes based on the thermodynamic analysis for the association between the alpha- and beta-subunits.
authors.affiliation: Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka 565-0871, Japan.
authors.forename: Kazuya
authors.initials: K
authors.lastname: Nishio
authors.forename: Yukio
authors.initials: Y
authors.lastname: Morimoto
authors.forename: Manabu
authors.initials: M
authors.lastname: Ishizuka
authors.forename: Kyoko
authors.initials: K
authors.lastname: Ogasahara
authors.forename: Tomitake
authors.initials: T
authors.lastname: Tsukihara
authors.forename: Katsuhide
authors.initials: K
authors.lastname: Yutani
mesh_terms: D000595:Amino Acid Sequence; D003460:Crystallization; D018360:Crystallography, X-Ray; D004789:Enzyme Activation; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D008969:Molecular Sequence Data; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D021122:Protein Subunits / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D012486:Salmonella typhimurium / Q000201:enzymology*; D017386:Sequence Homology, Amino Acid; D013816:Thermodynamics; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D029968:Escherichia coli Proteins; D021122:Protein Subunits; D014367:Tryptophan Synthase