PLANT CYTOCHROME-P450 MONOOXYGENASES

Plant systems utilize a diverse array of cytochrome P450 monooxygenase s (P450s) in their biosynthetic and detoxification pathways. The class ic forms of these enzymes are heme-dependent mixed function oxidases t hat utilize NADPH or NADH and molecular oxygen to produce functionaliz ed organic products. The nonclassical forms are monooxygenases that ei ther do not utilize flavoproteins for dioxygen activation or fail to i ncorporate molecular oxygen into their final product. Biosynthetic P45 0s play paramount roles in the synthesis of lignin intermediates, ster ols, terpenes, flavonoids, isoflavonoids, furanocoumarins, and a varie ty of other secondary plant products. Other catabolic P450s metabolize toxic herbicides and insecticides into nontoxic products or, converse ly, 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 substrate s are at least as complex as those that exist in mammalian systems. Ex amples now exist of plant P450s that metabolize: a narrow range of sub strates to yield different products, a single substrate to yield diffe rent products, multiple substrates to yield the same product, or a sin gle substrate sequentially to yield discrete intermediates in the bios ynthesis of a single product. Extensive divergence of catalytic site a s well as noncatalytic site residues accounts for the high degree of p rimary structure variation in the P450 gene superfamily and the divers e array of substrates synthesized and/or detoxified by these proteins. Classic p450s still retain a highly conserved F--G-R-C-G motif in the ir catalytic site and conserved amino acids in their oxygen binding po cket; nonclassical P450s diverge at several of these positions. A broa d range of cloning and transient expression strategies are suitable fo r plant P450 studies and these have allowed for the isolation and char acterization of a number of P450 cDNAs and genes. Because many of thes e 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.