This book provides, through X-ray crystallography, information about the structure and function relationships of urease and cytochrome c-553 from Bacillus pasteurii. Urease is a nickel enzyme involved in nitrogen turnover and in human and animal pathologies. The comparison between the native and the inhibited structures provides information about the active site and the inhibition mechanisms, and allows the proposal of a reaction mechanism which agrees with the available biochemical data. Cytochrome c-553 is a small electron transfer protein. The characterisation at atomic resolution (0.97 A) provides one of the most accurate heme structures to date. Providing a thoroughly written background and an accurate description of all the steps from bacterial growth to structure solution, this book is aimed at researchers interested in structural biology, in protein science and enzymatic catalysis. Useful for graduate and PhD students who are interested in protein crystallography. An easy to read background of protein crystallography is provided in the appendix for the less experienced reader.