AtPPH

AtPPH, short for Arabidopsis thaliana pheophytinase, is a plastid-localized enzyme that participates in chlorophyll degradation during leaf senescence. It hydrolyzes pheophytin a to form pheophorbide a, a reaction that removes the magnesium-containing porphyrin center and prepares chlorophyll-derived molecules for further breakdown by the pheophorbide a oxygenase (PAO) pathway. This step is considered an early, rate-limiting event in the chlorophyll catabolic cycle, and is essential for efficient pigment loss in aging leaves. The protein is predicted to belong to the alpha/beta-hydrolase fold superfamily and is targeted to chloroplasts, consistent with its role in chlorophyll metabolism.

Expression of AtPPH is upregulated during natural leaf senescence and darkness-induced senescence, aligning with the timing of chlorophyll breakdown. Genetic studies in Arabidopsis show that loss-of-function mutants of AtPPH exhibit delayed chlorophyll degradation, retain higher chlorophyll content, and show greener leaves during senescence compared with wild-type, whereas overexpression strains display accelerated pigment loss. These phenotypes underscore AtPPH's contribution to chlorophyll catabolism and nutrient remobilization during aging.

Orthologous genes are found across higher plants, suggesting a conserved function. In the broader chlorophyll degradation pathway, AtPPH acts upstream of PAO; together with other enzymes such as pheophorbide a oxygenase, the pathway leads to non-green catabolites that are eventually stored as nonfluorescent chlorophyll catabolites in plant tissues.