Protochlorophyllide photoreduction

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Chlorophylls play a fundamental role in the absorption of light energy and its conversion into chemical energy for use by all organisms living on the Earth. The formation of these compounds occurs by a complex series of reactions carried out throughout the lifetime of the plant. One step of this reaction series, the conversion of protochlorophyllide into chlorophyllide, is unique in its requirement for light. The reduction of protochlorophyllide to chlorophyllide is catalyzed by the nuclear-encoded enzyme NADPH: protochlorophyllide oxidoreductase (POR). Recently, genes encoding two different POR proteins, designated PORa and PORb, have been identified in vascular plants. Despite a high degree of amino acid sequence similarity, the roles played by these different gene products appear to be quite distinct during photomorphogenesis. While PORb is present and active throughout the life of the plant, PORa appears to be present and functional only in the first few hours of greening following the onset of illumination of etiolated (or dark-adapted) plants. Consistent with this, analysis of gene expression patterns showed that the transcription of the PORa and PORb encoding genes are controlled differently by light and plant developmental stage. PORa transcription is negatively photoregulated by light. Phytochrome, mainly Phytochrome a, is responsible for light control of the gene transcription. Import of PORa but not PORb into plastids also appears to be differentially regulated, with pPORa translocation across the chloroplast envelope being controlled by substrate (protochlorophyllide) availability. In etiolated plants, POR forms large aggregates located in tube-like structures termed, ’prolamellar bodies‘. Some amount of POR was detected in prothylakoids. In light-adapted vascular plants, green algae and cyanobacteria, POR and protochlorophyllide were detected in chloroplast envelope and stroma membranes. Photoactive enzyme complexes are identified mainly in stroma membranes near polyribosomes. The unique feature of POR is that it is a photo-enzyme whose catalytic activity depends on light. It also requires NADPH and undergoes conformational changes near Cys groups in the course of catalytic activity. The first step of the reaction is photo-induced electron transfer to protochlorophyllide that leads to the formation of a nonfluorescent ion-radical. This step is followed by hydrogen transfer from NADPH. Degradation of PORa is specifically controlled and performed by the nuclear encoded proteases. The mechanism of the light activation of these proteases expression is unknown, but they appear only after onset of illumination. Their activity also depends on the susceptibility of POR after its conformational change followed by protochlorophyllide photoreduction. Analysis of chlorophyll biosynthesis in chloroplasts of light-adapted plants revealed a new mechanism of protochlorophyllide photo-reduction. The reaction differs from that previously observed in etiolated plants by the initial photoactive protochlorophyllide form, intermediate steps and the final product. The rates of chlorophyll accumulation through these two reactions also differ. The same reaction was also detected in etiolated plants in parallel to the main reaction of protochlorophyllide photoreduction. Study of different photosynthetic organisms reveals the presence of POR in all of them including higher plants, green algae and cyanobacteria, which possess for light-independent chlorophyll synthesis, except for photosynthetic bacteria. Among those, PORa is present only in angiosperms, which lost the ability to synthesize chlorophyll in the dark. Sequence similarity reveals the evolutionary origin of POR from short-chain alcohol dehydrogenases.

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