Methionine--lyase (MGL) from Trichomonas vaginalis has been crystallised and the structure solved by molecular replacement at 2.2 A. The enzyme has a similar overall secondary structure arrangement as the search model, cystathionine -lyase from Escherichia coli, but differs in the active site. In addition to the holoenzyme, the structure of the enzyme in complex with the acetylenic suicide inhibitor L-propargylglycine has also been solved. This has revealed the mechanism of inhibition by this compound. It has also provided insights into the catalytic mechanism of the enzyme and allowed the postulation of a scheme for its action. Particularly, this predicts core roles for active site tyrosine and cysteine residues (111 and 113, respectively). To test the hypothesised mechanism of catalysis, seven site-directed mutants were produced. The activities of the mutants were determined and the structure solved for one of the most interesting. The resulting data confirm the mode of action of L-propargylglycine and also reveal a proton relay which activates the important active site cysteine which mediates much of the enzyme activity. This mechanism is substantially different from that proposed for other members of the same family of enzymes and explains the substrate specificity of MGL. The structure also reveals a potential role for the enzyme. The active site cysteine is positioned ideally to form an internal persulphide with a sulphur atom released from the substrate. This implies a possible role for the enzyme in iron-sulphur cluster formation in an analogous fashion to the enzyme, cysteine desulphurase. Cultivation of the parasite in medium supplemented with L-cysteine has revealed reduced expression of the enzyme. This is the first demonstration of part of the de novo synthesis of cysteine in the T. vaginalis.