Triterpene Structural Diversification by Plant Cytochrome P450 Enzymes

Abstract

Cytochrome P450 monooxygenases (P450s) represent the largest enzyme family of the plant metabolism. Plants typically devote about 1% of the protein-coding genes for the P450s to execute primary metabolism and also to perform species-specific specialized functions including metabolism of the triterpenes, isoprene-derived 30-carbon compounds. Triterpenes constitute a large and structurally diverse class of natural products with various industrial and pharmaceutical applications. P450-catalyzed structural modification is crucial for the diversification and functionalization of the triterpene scaffolds. In recent times, a remarkable progress has been made in understanding the function of the P450s in plant triterpene metabolism. So far, ∼80 P450s are assigned biochemical functions related to the plant triterpene metabolism. The members of the subfamilies CYP51G, CYP85A, CYP90B-D, CYP710A, CYP724B, and CYP734A are generally conserved across the plant kingdom to take part in plant primary metabolism related to the biosynthesis of essential sterols and steroid hormones. However, the members of the subfamilies CYP51H, CYP71A,D, CYP72A, CYP81Q, CYP87D, CYP88D,L, CYP93E, CYP705A, CYP708A, and CYP716A,C,E,S,U,Y are required for the metabolism of the specialized triterpenes that might perform species-specific functions including chemical defense toward specialized pathogens. Moreover, a recent advancement in high-throughput sequencing of the transcriptomes and genomes has resulted in identification of a large number of candidate P450s from diverse plant species. Assigning biochemical functions to these P450s will be of interest to extend our knowledge on triterpene metabolism in diverse plant species and also for the sustainable production of valuable phytochemicals.

Multi‐omics data‐driven investigations of metabolic diversity of plant triterpenoids

“…CYP716As are conserved in higher plants and catalyze a consecutive three‐step oxidation at C28 of α‐amyrin, lupel and β‐amyrin (Carelli et al ., ; Fukushima et al ., ; Misra et al ., ; Tamura et al ., ). Other CYP716 subfamily members CYP716C/‐E/‐S/‐Y also participate in oxidation of the pentacyclic triterpene scaffolds (Ghosh, ). Interestingly, although the CYP716 family is important for triterpene metabolism, this family is not found in monocots (Nelson and Werck‐Reichhart, ).…”

“…Interestingly, although the CYP716 family is important for triterpene metabolism, this family is not found in monocots (Nelson and Werck‐Reichhart, ). Instead, the CYP51H family was recruited from CYP51G, the family involved in the primary metabolic pathway, to modify the pentacyclic triterpene scaffolds as a possible way to counteract loss of the CYP716s in monocots (Ghosh, ). For example, CYP51H10 is a β‐amyrin oxidase in oat, leading to the formation of 12,13β‐epoxy‐16β‐hydroxy‐β‐amyrin (Qi et al ., ; Kunii et al ., ; Geisler et al ., ).…”

“…Except for the CYP716s, the CYP88D and CYP87D subfamilies within the CYP85 clan are also pentacyclic terpene oxidases (Ghosh, ). CYP88D6 catalyzes two consecutive oxidations of β‐amyrin at C11 in Glycyrrhiza uralensis (Seki et al ., ), while CYP87D16 is a C16 hydrolase of β‐amyrin in Msesa lanceolate (Moses et al ., ).…”

“…Interestingly, CYP716Y1 in Bupleurum falcatum has the same capability as CYP87D16, although their sequences are less similar to each other (Moses et al ., ), indicating that independent evolution may occur in these species. Subfamilies CYP93E, CYP71A, CYP71D, CYP81Q and CYP705A within the CYP71 clan are also known for the biosynthesis of triterpenoids and have diverged remarkably during evolution (Ghosh, ). For example, while widely distributed in angiosperm species, CYP72A subfamily members that participate in the pentacyclic triterpene modifications are not found in gymnosperms, mosses or ferns (Prall et al ., ) and the C24 hydroxylases of β‐amyrin, CYP93Es, have only been identified in legumes (Moses et al ., ).…”

“…For instance, CYP705 in the CYP71 clan and CYP716 in the CYP85 clan are found mainly coupled with OSCs in triterpene biosynthesis, and consist of a core component from which other gene clusters may be derived (Boutanaev et al ., ). To date, about 60 P450s involved in plant triterpenoid pathways have been identified and characterized, 55 of which participate in modifications of the pentacyclic terpene scaffold (Ghosh, ). Compared with the large number of P450s found in plant genomes this number is very limited and is insufficient to unveil the complete evolutionary routes of P450s and the divergence of natural plant products (Table ).…”