![]() From this perspective, cytochromes P450 (P450s) are a prototypical example of ubiquitous enzymes encoded by a gene superfamily that can carry out multiple types of reactions, among them hydroxylation, epoxidation, oxygenation, dealkylation, decarboxylation, C-C cleavage, and ring opening ( Bak et al., 2011 Guengerich and Munro, 2013). This singular complexity of compounds results from an evolutionary process that involves a dramatic diversification of plant metabolic pathways and the genes encoding the associated metabolic enzymes. With more than 200,000 distinct molecules, the specialized metabolism of plants constitutes one of the main sources of bioactive natural compounds, with this large number reflecting the capacity of these sessile organisms to adapt to and interact with the environment. Such high specialization of class II CPRs in planta highlights the evolutionary strategy that ensures an efficient reduction of P450s in specialized metabolism. Direct assays of interaction and reduction of P450s in vitro, however, showed that both classes of CPR performed equally well. ![]() Global analyses of gene expression correlation combined with transcript localization in specific leaf tissues and gene-silencing experiments of both classes of CPR all point to the strict requirement of class II CPRs for monoterpene indole alkaloid biosynthesis with a minimal or null role of class I. By studying the role of CPRs in the biosynthesis of monoterpene indole alkaloids, we provide compelling evidence of an operational specialization of CPR isoforms in Catharanthus roseus (Madagascar periwinkle). CPR isoforms usually group into two distinct classes with different proposed roles, namely involvement in primary and basal specialized metabolisms for class I and inducible specialized metabolism for class II. For catalysis, P450s require a two-electron transfer catalyzed by shared cytochrome P450 oxidoreductases (CPRs), making these auxiliary proteins an essential component of specialized metabolism. Expansion of the biosynthesis of plant specialized metabolites notably results from the massive recruitment of cytochrome P450s that catalyze multiple types of conversion of biosynthetic intermediates.
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