Enzymes containing pyridoxal-5'-phosphate (PLP) are ubiquitous in biology, performing essential reactions in metabolism of amino acids and amines. These enzymes catalyze a wide variety of reactions, including racemization, transamination, alpha- and beta-decarboxylation, retro-aldol cleavage, beta- and gamma-elimination and substitution. The chemistry of these enzymatic reactions is controlled by the PLP cofactor, and PLP alone has been shown to perform many of these reactions, albeit at very low rates. Therefore, it is the protein environment that confers the reaction specificity and enormous rate accelerations typical of these enzymes. The carbon-carbon lyase, tyrosine phenol-lyase (EC 4.1.99.2, TPL), catalyzes the hydrolytic beta-elimination of L-tyrosine to give phenol and ammonium pyruvate. Methionine-gamma-lyase (EC 4.4.1.11, MGL) catalyzes gamma-elimination reactions of L-methionine to give methylmercaptan and ammonium alpha-ketobutyrate. The main goal of our work is comparative analysis on the molecular level of the principal structure-activity relationships which are characteristic for beta-eliminating (TPL) and gamma-eliminating (MGL) lyases. The most intriguing problem seems to be the transfer of the reaction center from the alpha- to beta-carbon of the substrate. To achieve this goal, a number of experiments will be carried out. Three dimensional structures of intermediates in the reactions of TPL will be determined. Thorough analysis of the catalytic mechanisms of TPL and MGL, depending on the substrate structures, will be performed, using steady state and stopped-flow techniques and multiple isotope effect studies. The enzymes under study are potential drug targets in pathogens. In addition, both TPL and MGL may be useful in cancer therapy. Thus, knowledge of the structure and mechanism of these enzymes may lead to new pharmaceuticals.