Photoactive pigment—enzyme complexes of chlorophyll precursor in plant leaves

Abstract

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.

Ultrafast catalytic processes and conformational changes in the light-driven enzyme protochlorophyllide oxidoreductase (POR)

“…However, in the present review, we focus on another enzyme, POR, which requires light for both catalysis and to induce a conformational change, which switches the enzyme from an inactive into a fully active state. A detailed review on POR, including studies on the POR A and B isoforms from green plants, can be found in [21][22][23]. In the present study, we provide a limited overview of previous spectroscopic investigations of the isolated POR complex from cyanobacteria, with a particular focus on recent time-resolved transient absorption experiments.…”

Response to the Comments by L. O. Björn on our Paper “Catalytic Efficiency of a Photoenzyme—An Adaptation to Natural Light Conditions”

“…Throughout the last decade a series of such studies have been reported. [22] In a similar manner, the enzymes obtained from the extraction of etiolated plant material, PChlide holochromes, form high molecular weight complexes (~600 kDa), too. [1][2][3][4][5][6][7] In addition, the thermodynamics of substrate/coenzyme binding as well as the complete POR catalytic cycle have been investigated by spectroscopic techniques in conjunction with low-temperature and stopped flow methods.…”

“…They all include recombinant POR enzymes, which were expressed in Escherichia coli and reconstituted to the ternary enzyme complexes by addition of the substrate and coenzyme, protochlorophyllide (PChlide) and NADPH, respectively. [22] In a similar manner, the enzymes obtained from the extraction of etiolated plant material, PChlide holochromes, form high molecular weight complexes (~600 kDa), too. All these experiments were also performed on recombinant enzymes expressed in Escherichia coli or on a pigment-free, monomeric enzyme isolated from etiolated oat (Avena sativa) seedlings.…”

“…[19][20][21] There is strong evidence that POR associates with membranes under natural conditions and exists in an aggregated state as dimer or larger sized oligomer. [22] In a similar manner, the enzymes obtained from the extraction of etiolated plant material, PChlide holochromes, form high molecular weight complexes (~600 kDa), too. [19,21,23] The photochemistry of aggregated PChlide-POR complexes is impaired by pigment-pigment interactions, energy transfer between PChlide and chlorophyllide (the product of the enzymatic reaction) and further affected by protein-lipid interactions.Nowadays, considerable effort has therefore been made to substitute the aggregated POR complexes by recombinant monomeric enzymes, which are catalytically active.…”

“…The enzymes were purified to homology and reconstituted with the substrate PChlide and the cofactor NADPH. [22,26,[28][29][30] In contrast, the absorbance spectrum shown for the reconstituted POR ternary complex in our paper is representative for a monomeric POR enzyme. PChlide, necessary for the reconstitution of the photoactive enzymes, was isolated from etiolated oat seedlings and finally purified by reversedphase HPLC to a chemically pure form.…”