During leaf senescence and fruit ripening chlorophyll is degraded to colorless tetrapyrroles termed NCCs. Structures of NCCs from different species point to a common pathway of chlorophyll breakdown (termed PAO pathway), whose key enzyme, PAO, is responsible for the opening of the macrocycle ring of chlorophyll. Most of the intermediates of the PAO pathway are devoid of the phytyl-moiety characteristic for chlorophyll, indicating that dephytylation is an early reaction in the pathway. Pheophytinase (PPH) was recently shown to catalyze the dephytylation of Mg-free chlorophyll (pheophytin) to pheophorbide. PPH is specific for pheophytin and does not accept chlorophyll as substrate. One aim of this project is to biochemically characterize PPH in more detail and to crystallize the protein to get an insight into its intriguing specificity. We, furthermore, aim to elucidate the mechanism of chlorophyll breakdown during fruit ripening in detail, in particular with respect to the involvement of PPH. Finally we plan to investigate evolutionary aspects of the PAO pathway.