Cytochrome c peroxidase (CCP) is a 34 kDa heme enzyme that has been extensively studied, both functionally and structurally. The ease of crystallization of CCP and its structural and biochemical similarities to other heme proteins make CCP an ideal paradigm for heme function. The H175G mutant, in which there is no longer a protein ligand to the heme, has been constructed and shown to bind numerous small molecules in the created proximal cavity. High resolution structures of this mutant with and without bound ligands will aid our understanding of heme enzyme function.