The Pathway of Chlorophyll Degradation: Catabolites, Enzymes and Pathway Regulation

During leaf senescence and fruit ripening, chlorophyll is broken down to colorless linear tetrapyrroles, which are stored in the vacuoles of degreened cells. The pathway of chlorophyll degradation that is active in these developmental processes is fairly well known regarding its biochemistry and cell biology. It comprises at least six enzymatic and one non-enzymatic reaction and the chemical structures of several intermediary and final chlorophyll catabolites have been elucidated. In the last few years, genes coding for a number of chlorophyll catabolic enzymes have been characterized and mutants in these genes have been analyzed. This includes pheophorbide a oxygenase (PAO), the key enzyme of the pathway, which is responsible for opening of the chlorine macrocycle present in chlorophyll, thereby providing the characteristic structural basis of all further downstream breakdown products. The pathway is therefore nowadays termed the 'PAO pathway'. This review summarizes information on the structures of chlorophyll breakdown products and the reactions involved in their formation. In addition cell biological and regulatory aspects of the PAO pathway are discussed.