Plant Cytochrome P450 Monooxygenases

Plant systems utilize a diverse array of cytochrome P450 monooxygenases (P450s) in their biosynthetic and detoxification pathways. The classic forms of these enzymes are heme-dependent mixed function oxidases that utilize NADPH or NADH and molecular oxygen to produce functionalized organic products. The nonclassical forms are monooxygenases that either do not utilize flavoproteins for dioxygen activation or fail to incorporate molecular oxygen into their final product. Biosynthetic P450s play paramount roles in the synthesis of lignin intermediates, sterols, terpenes, flavonoids, isoflavonoids, furanocoumarins, and a variety of other secondary plant products. Other catabolic P450s metabolize toxic herbicides and insecticides into nontoxic products or, conversely, activate nontoxic substances into toxic products. Biochemical and molecular characterizations on a number of plant P450s have indicated that the relationships between these heme proteins and their substrates are at least as complex as those that exist in mammalian systems. Examples now exist of plant P450s that metabolize: a narrow range of substrates to yield different products, a single substrate to yield different products, multiple substrates to yield the same product, or a single substrate sequentially to yield discrete intermediates in the biosynthesis of a single product. Extensive divergence of catalytic site as well as noncatalytic site residues accounts for the high degree of primary structure variation in the P450 gene superfamily and the diverse array of substrates synthesized and/or detoxified by these proteins. Classic P450s still retain a highly conserved F-G-R-C-G motif in their catalytic site and conserved amino acids in their oxygen binding pocket; nonclassical P450s diverge at several of these positions. A broad range of cloning and transient expression strategies are suitable for plant P450 studies and these have allowed for the isolation and characterization of a number of P450 cDNAs and genes. Because many of these sequences have been cloned only recently, much remains to be learned about the substrate specificities of P450 reactions in plants and the mechanisms by which their genes are regulated.