Diffraction images for 5-aminolevulinic acid dehydratase (ALAD) from E. coli complexed with porphobilinogen.

The diffraction images which allowed the 2.1 Angstrom resolution structure determination of Escherichia coli ALAD co-crystallised with a non-covalently bound moiety of the product, porphobilinogen (PBG), are presented. The structure revealed that the pyrrole side chain amino group is datively bound to the active site zinc ion and that the PBG carboxylates interact with the enzyme via hydrogen bonds and salt-bridges with invariant residues. A number of hydrogen bond interactions that were previously observed in the structure of yeast ALAD with a cyclic intermediate resembling the product PBG appear to be weaker in the new structure suggesting that these interactions are only optimal in the transition state.