In plants, fungi and bacteria, aromatic amino acids are synthesized via the shikimate pathway. The pathway consists of seven enzymecatalyzed reactions ending with the production of chorismate, a precursor of the amino acids phenylalanine, tryptophan and tyrosine, as well as a number of other aromatic compounds. The fourth step of the pathway, in which dehydroshikimate is reduced to shikimate, is catalyzed by shikimate dehydrogenase (SDH). In bacteria, kinetic and phylogenetic analyses have identified five SDH functional classes, annotated AroE, YdiB, RifI, SdhL and most recently, Ael1. Representative crystal structures have been determined for all classes except RifI. We are the first to present the crystal structure of the novel Ael1 homolog from Pseudomonas putida. SDH structures share a high degree of similarity and conservation of key residues involved in catalysis. 1 However, each SDH class has a distinct biochemical profile. While AroE is the archetypal SDH, YdiB has dual substrate specificity, accepting both shikimate and quinate. 2 RifI is predicted to accept amino-dehydroshikimate during the biosynthesis of the antibiotic rifamycin B. 3 The biological substrate of SdhL is unknown. Ael1 can catalyze the reduction of dehydroshikimate with a higher binding affinity but lower turnover rate than the AroE homolog. The natural enzymatic diversity observed among SDH classes provides an ideal system for studying modes of substrate selectivity.

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