It has been found that at low temperatures (77K-153K) a long-lived (at these temperatures) singlet ESR signal induced by intensive light appears in etiolated leaves of plants and in model systems including both the monomeric and aggregated protochlorophyll.Comparison of the results of ESR, fluorescence and absorption spectra measurements made it possible to suggest that at the initial stages of the protochlorophyll(ide) photoreduction process at least two paramagnetic non-fluorescent intermediates are formed, one of which seems to be identical to the previously found intermediate with absorption maximum at 690 nm. On the strength of the obtained results a conclusion can be drawn that photoreduction of the semi-isolated double-c=c-bond of the chlorophyll precursor molecule in etiolated leaves and in model systems is actualized via at least two stages of free radicals formation. A scheme of the primary reactions of chlorophyllide biosynthesis has been proposed.

This review summarizes contemporary data on structure and function of photoactive pigment--enzyme complexes of the chlorophyll precursor that undergoes photochemical transformation to chlorophyllide. The properties and functions of the complex and its principal components are considered including the pigment (protochlorophyllide), the hydrogen donor (NADPH), and the photoenzyme protochlorophyllide oxidoreductase (POR) that catalyzes the photochemical production of chlorophyllide. Chemical variants of the chlorophyll precursor are described (protochlorophyllide, protochlorophyll, and their mono- and divinyl forms). The nature and photochemical activity of spectrally distinct native protochlorophyllide forms are discussed. Data are presented on structural organization of the photoenzyme POR, its substrate specificity, localization in etioplasts, and heterogeneity. The significance of different POR forms (PORA, PORB, and PORC) in adaptation of chlorophyll biosynthesis to various illumination conditions is considered. Attention is paid to structural and functional interactions of three main constituents of the photoactive complex and to possible existence of additional components associated with the pigment-enzyme complex. Historical aspects of the problem and the prospects of further investigations are outlined.

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