Most thiol oxidoreductases contain the well conserved thioredoxin fold. This fold is present in both oxidases and disulfide reductases. I intend to address how can different members of the thioredoxin family though they all have the same fold differ so greatly in their chemistry and reactivity. I will force thiol thiol reductases into thiol oxidases by selecting for mutants that are able to complement a thiol oxidase deficiency. The results garnered from this work will allow us to investigate how structure and function are related and how. Diseases such as sickle cell anemia and cystic fibrosis are caused by small mutations in a general fold that renders the protein inactive. Our work will investigate how small mutational changes can affect directionality of a reaction, which will help elucidate how small mutations within normal proteins can cause them to become diseased. [unreadable] [unreadable]