3-DIMENSIONAL STRUCTURE OF ESCHERICHIA-COLI DIHYDRODIPICOLINATE REDUCTASE

Dihydrodipicolinate reductase is an enzyme found in bacteria and highe r plants involved in the biosynthesis of diaminopimelic acid and lysin e. Because these pathways are unique to bacteria and plants, they may represent attractive targets for new antimicrobial or herbicidal compo unds. The three-dimensional structure of Escherichia coli dihydrodipic olinate reductase, complexed with NADPH, has been determined and refin ed to a crystallographic R-factor of 18.6% with diffraction data to 2. 2 Angstrom resolution. The refined model contains the complete protein chain, the cofactor NADPH, and 55 water molecules. The enzyme is comp osed of two domains. The dinucleotide binding domain has a central sev en-stranded parallel beta-sheet surrounded by four alpha-helices, with the cofactor binding site located at the carboxyterminal edge of the sheet. The second domain contains four beta-strands and two alpha-heli ces that form an open mixed beta-sandwich. A possible binding site for dihydrodipicolinate has been identified in this second domain, about 12 Angstrom away from the dinucleotide binding site. This would imply that the protein must undergo some conformational change in order to p erform catalysis. In the crystal, the native enzyme is a homotetramer generated by a 222 crystallographic axis. Implications of the tetramer ic structure for the enzyme function are presented. Dihydrodipicolinat e reductase uses both NADH and NADPH as cofactors, and analysis of its cofactor binding site allows for a molecular understanding of the enz yme's dual specificity.