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

• Published in: Biochemistry (Easton) Vol. 44; no. 4; pp. 1184 - 1192
• Main Authors: Nishio, Kazuya, Morimoto, Yukio, Ishizuka, Manabu, Ogasahara, Kyoko, Tsukihara, Tomitake, Yutani, Katsuhide
• Format: Journal Article
• Language: English
• Published: United States 01.02.2005
• Subjects: Amino Acid Sequence; Crystallization; Crystallography, X-Ray; Enzyme Activation; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - isolation & purification; Escherichia coli Proteins - metabolism; Molecular Sequence Data; Protein Binding; Protein Conformation; Protein Structure, Secondary; Protein Subunits - chemistry; Protein Subunits - isolation & purification; Protein Subunits - metabolism; Salmonella typhimurium - enzymology; Sequence Homology, Amino Acid; Thermodynamics; Tryptophan Synthase - chemistry; Tryptophan Synthase - isolation & purification; Tryptophan Synthase - metabolism
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi047927m

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.