The domain structure of tryptophan synthase. A neutron scattering study

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Abstract
Tryptophan synthase from Escherichia coli is a complex of two .alpha. subunits and two .beta. subunits. Small-angle neutron scattering involving deuterium-labelled isomers revealed the quaternary structure of the enzyme at the level of the .beta.2 subunit and the two structural domains P1 and P2 which constitute the .alpha. subunits. Within the .alpha.2.beta.2 complex, the two .alpha. subunits are completely separated. They are situated on opposite sides of the .beta.2 subunit. The most probable distance between the two .alpha. protomers is 10.5 .+-. 1 nm; the nearest distance is 5.8 .+-. 0.5 nm, and the largest distance is 13.5 .+-. 0.5 nm. The two domains of the same .alpha. subunit are intimately juxtaposed. The distances between two like or unlike domains belonging to opposite .alpha. subunits are roughly equal. All domains exhibit about equal distances to the .beta.2 subunit which is situated in the centre of the complex. Thus the cleft between P1 and P2, which probably contains the active site of the .alpha. subunit, makes intimate contact with the .beta.2 subunit. Neutron scattering allows us to determine the shape of the .beta.2 subunit within the complex. Comparison with the free dimer suggests a conformational change, upon assembly, from an elongated into a more compact form.

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