A large variety of metabolically important molecules exist as pairs of tautomers, in which the major tautomer (tantotautomer) is so favored energetically over the minor tautomer (tenutautomer) that the latter species cannot even be detected in the equilibrium mixture by any available spectroscopic means. Nevertheless, mechanistic studies have revealed that the tenutautomer is very often the "true" reactant in various chemical transformations. We have proposed that the same phenomenon may exist in biological systems, namely, that the tenutautomers of various metabolites may be the "true" ligands for some enzymes and receptors. This theory provides a new pathway for substrate activation; it also leads to the critical consequence that potential inhibitors should be designed as analogues of the tenutautomers, rather than the common practice of mimicking the tantotautomers. Since it is still quite impossible to examine the structure or conformation of a ligand within a binding site, arguments for the tenutautomer hypothesis must be based on evidence and inference from the behavior of stable analogues of the tenutautomer. This approach was highly successful and convincing in the case of tryptophan. We found that stable analogues of the indolenine tautomer having a tetrahedral carbon at C-3-e.g., 2,3-dihydro-L-tryptophan, oxindolyl-L-alanine and dioxindolyl-L-alanine- are potent competitive inhibitors of tryptophan synthase and tryptophanase, enzymes involved in the biosynthesis and degradation of tryptophan. Further, the two enzymes show "mirror-image" specificity, in that the 3R diastereoisomer of each analogue inhibits only tryptophanase, while the 3S diastereoisomer inhibits only tryptophan synthase. A logical extension of this series involves the syntheses of 3-azido analogues as photoaffinity labels, as well as tetrahedral analogues of 5-hydroxy and 5-methoxy bioindoles. Additional work is in progress on the synthesis of tenutautomer analogues of tyrosine and dihydroxyphenylalanine (DOPA), as well as analogues of histamine and histidine. The latter compounds are expected to inhibit histidine deaminase and urocanase.