LIGHT-INDUCED SYNTHESIS OF PHEOPHYTIN A IN PLANT-LEAVES

Spectroscopic studies showed that pheophytin a and chlorophyll a are produced by etiolated plant leaves in the light from a common precursor, protochlorophyllide PChlde 655/650. The primary step of photoreduction of the precursor to protochlorophyllide (PChlde 655/650 --> Chlde 684/676) is shared by chlorophyll a and pheophytin a. Then their ways diverge: the main path initiated by the second photoreaction (Chide 684/676 --> Chlde 690/680) generates the pool of chlorophyll a through a series of well-investigated native forms of chlorophyllide (Shibata shift). The alternative reaction involving Chlde 684/676 in the dark or in low-intensity light generates a complex of pheophytin a-chlorophyll a (1:2): Chlde 684/676 --> (Pheo 679/675 + Chl 671/668). The relationship between the two pathways of biosynthesis and the amount of pheophytin accumulated are determined by the intensity and duration of exposure to light. The native complex of pheophytin a-chlorophyll a, including protochlorophyllide reductase and perhaps FAD, is supposed to be an immediate precursor of photosystem II reaction centers.