The type I 3-dehydroquinate dehydratase (DHQase) which catalyses the reversible dehydration of 3-dehydroquinic acid to 3-dehydroshikimic acid is involved in the shikimate pathway for the biosynthesis of aromatic compounds. The shikimate pathway is absent in mammals, which makes structural information about DHQase vital for the rational design of antimicrobial drugs and herbicides. The crystallographic structure of the type I DHQase from Salmonella typhi has now been determined for the native form at 1.78 Angstrom resolution (R=19.9%; R-free=24.7%). The structure of the modified enzyme to which the product has been covalently bound has also been determined but in a different crystal form (2.1 Angstrom resolution; R=17.7%; R-free=24.5%). An analysis of the three available crystal forms has provided information about the physiological dimer interface. The enzyme relies upon the closure of a lid-like loop to complete its active site. As the lid-loop tends to stay in the closed position, dimerization appears to play a role in biasing the arrangement of the loop towards its open position, thus facilitating substrate access.585798804