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The effects of domain assembly on the conformation of the F1 (N-terminal) and F2 (C-terminal) domains of the β2 subunit of Escherichia coli tryptophan synthase (EC 4.2.1.20) were analysed using six monoclonal antibodies which recognize six different epitopes of the native β2 subunit (five carried by the F1 domain and one carried by the F2 domain). For this purpose, the affinity constant of each monoclonal antibody for the isolated domains F1 or F2, the associated domains in the trypsin-nicked apo-β2 and in the native apo-β2 subunits were determined, both with the intact immunoglobulin and the Fab fragment. It was found that the association of the F1 and F2 domains within β2 is accompanied by structural changes of the two domains, as detected by variations of their affinity constants for the monoclonal antibodies.