The structures of anthranilate synthase of Serratia marcescens crystallized in the presence of (i) its substrates, chorismate and glutamine, and a product, glutamate, and (ii) its end-product inhibitor, L-tryptophan

The crystal structure of anthranilate synthase (AS) from Serratia marcescen s, a mesophilic bacterium, has been solved in the presence of its substrate s, chorismate and glutamine, and one product, glutamate, at 1.95 A, and wit h its bound feedback inhibitor, tryptophan, at 2.4 W. In comparison with th e AS structure from the hyperthermophile Sulfolobus solfataricus, the S, ma rcescens structure shows similar subunit structures but a markedly differen t oligomeric organization. One crystal form of the S. marcescens enzyme dis plays a bound pyruvate as well as a putative anthranilate (the nitrogen gro up is ambiguous) in the TrpE subunit, It also confirms the presence of a co valently bound glutamyl thioester intermediate in the TrpG subunit. The try ptophan-bound form reveals that the inhibitor binds at a site distinct from that of the substrate, chorismate, Bound tryptophan appears to prevent cho rismate binding by a demonstrable conformational effect, and the structure reveals how occupancy of only one of the two feedback inhibition sites can immobilize the catalytic activity of both TrpE subunits. The presence of ef fecters in the structure provides a view of the locations of some of the am ino acid residues in the active sites. Our findings are discussed in terms of the previously described AS structure of S, solfataricus, mutational dat a obtained from enteric bacteria, and the enzyme's mechanism of action.