Novel Insights into the Enzymology, Regulation and Physiological Functions of Light-dependent Protochlorophyllide Oxidoreductase in Angiosperms

The reduction of protochlorophyllide (Pchlide) is a key regulatory step in the biosynthesis of chlorophyll in phototrophic organisms. Two distinct enzymes catalyze this reduction; a light-dependent NADPH:protochlorophyllide oxidoreductase (POR) and light-independent Pchlide reductase (DPOR). Both enzymes are widely distributed among phototrophic organisms with the exception that only POR is found in angiosperms and only DPOR in anoxygenic photosynthetic bacteria. Consequently, angiosperms become etiolated in the absence of light, since the reduction of Pchlide in angiosperms is solely dependent on POR. In eukaryotic phototrophs, POR is a nuclear-encoded single polypeptide and post-translationally imported into plastids. POR possesses unique features, its light-dependent catalytic activity, accumulation in plastids of dark-grown angiosperms (etioplasts) via binding to its substrate, Pchlide, and cofactor, NADPH, resulting in the formation of prolamellar bodies (PLBs), and rapid degradation after catalysis under subsequent illumination. During the last decade, considerable progress has been made in the study of the gene organization, catalytic mechanism, membrane association, regulation of the gene expression, and physiological function of POR. In this review, we provide a brief overview of DPOR and then summarize the current state of knowledge on the biochemistry and molecular biology of POR mainly in angiosperms. The physiological and evolutional implications of POR are also discussed.