Bleomycin hydrolase (BH) is a cysteine peptidase that hydrolyzes the anti-tumor drug bleomycin thus limiting its use for treatment of human cancers. The central theme of this proposal is to gain an understanding of the mechanisms of action of BH and to learn how to control the activity of this enzyme. The combined biochemical, mutagenesis, and structural approaches developed during the previous grant period will be employed to study structure-function relationships of bleomycin hydrolase. . The crystal structure of the human form of BH (hBH) has been recently determined by this investigator and will serve as the starting point for developing inhibitors of this enzyme (Specific aim 1). An iterative approach will be applied to search for the best inhibitor of hBH activity, and the crystal structure of a complex derived from the most favorable inhibitor and the enzyme will be determined. Specific aim 2 is concerned with determining the mechanisms of hydrolysis of bleomycin by hBH, and with determining the co-crystal structure of a bleomycin-protein complex. Specific aim 3 is concerned with structure-function studies of hBH and the yeast bleomycin hydrolase Ga16, with a focus on the unique role of the C-terminal of this cysteine peptidase.