A Cytochrome P450 Enzyme Catalyses Oxetane Ring Formation in Paclitaxel Biosynthesis

Oxetane synthase (TmCYP1), a novel cytochrome P450 enzyme from Taxus×media cell cultures, has been functionally characterized to efficiently catalyse the formation of the oxetane ring in tetracyclic taxoids. Transient expression of TmCYP1 in Nicotiana benthamiana using 2α,5α,7β,9α,10β,13α-hexaacetoxytaxa-4(20),11(12)-diene (1) as a substrate led to the production of a major oxetane derivative, 1β-dehydroxybaccatin IV (1 a), and a minor 4β,20-epoxide derivative, baccatin I (1 b). However, feeding the substrate decinnamoyltaxinine J (2), a 5-deacetylated derivative of 1, yielded only 5α-deacetylbaccatin I (2 b), a 4β,20-epoxide. A possible reaction mechanism was proposed on the basis of substrate-feeding, 2H and 18O isotope labelling experiments, and density functional theory calculations. This reaction could be an intramolecular oxidation-acetoxyl rearrangement and the construction of the oxetane ring may occur through a concerted process; however, the 4β,20-epoxide might be a shunt product. In this process, the C5-O-acetyl group in substrate is crucial for the oxetane ring formation but not for the 4(20)-epoxy ring formation by TmCYP1. These findings provide a better understanding of the enzymatic formation of the oxetane ring in paclitaxel biosynthesis.

Intricate Metabolic Network for Paclitaxel Biosynthesis

Paclitaxel is a renowned broad-spectrum anticancer drug. With the establishment of a chromosome-level high-quality reference genome map of Taxus, recent research on paclitaxel biosynthesis has flourished. The oxetane ring is a distinctive chemical moiety of paclitaxel, and three recent studies have proposed different enzymes involved in its formation, reflecting divergent opinions on whether the pathway proceeds via acetylation followed by epoxidation or vice versa. Subsequently, researchers have elucidated gene clusters responsible for the biosynthesis of the key intermediate baccatin III. Despite varying reports, two studies successfully achieved heterologous biosynthesis of baccatin III by transient expression in tobacco. Taxadiene 5α-hydroxylase (T5αH), the first cytochrome P450 in the pathway, exhibited varied product profiles upon heterologous expression systems, contrasting with observations in native Taxus species, probably due to differences in partner proteins or cellular microenvironments. Further elucidation of biosynthesis mechanisms, including the reaction order and the promiscuity of key enzymes, is anticipated through collaborative efforts among botanists, chemists, and synthetic biologists.

Characterization and heterologous reconstitution of Taxus biosynthetic enzymes leading to baccatin III

Paclitaxel is a well known anticancer compound. Its biosynthesis involves the formation of a highly functionalized diterpenoid core skeleton (baccatin III) and the subsequent assembly of a phenylisoserinoyl side chain. Despite intensive investigation for half a century, the complete biosynthetic pathway of baccatin III remains unknown. In this work, we identified a bifunctional cytochrome P450 enzyme [taxane oxetanase 1 (TOT1)] in Taxus mairei that catalyzes an oxidative rearrangement in paclitaxel oxetane formation, which represents a previously unknown enzyme mechanism for oxetane ring formation. We created a screening strategy based on the taxusin biosynthesis pathway and uncovered the enzyme responsible for the taxane oxidation of the C9 position (T9αH1). Finally, we artificially reconstituted a biosynthetic pathway for the production of baccatin III in tobacco.

Oxetane Ring Formation in Taxol Biosynthesis Is Catalyzed by a Bifunctional Cytochrome P450 Enzyme

Taxol is a potent drug used in various cancer treatments. Its complex structure has prompted extensive research into its biosynthesis. However, certain critical steps, such as the formation of the oxetane ring, which is essential for its activity, have remained unclear. Previous proposals suggested that oxetane formation follows the acetylation of taxadien-5α-ol. Here, we proposed that the oxetane ring is formed by cytochrome P450-mediated oxidation events that occur prior to C5 acetylation. To test this hypothesis, we analyzed the genomic and transcriptomic information for Taxus species to identify cytochrome P450 candidates and employed two independent systems, yeast (Saccharomyces cerevisiae) and plant (Nicotiana benthamiana), for their characterization. We revealed that a single enzyme, CYP725A4, catalyzes two successive epoxidation events, leading to the formation of the oxetane ring. We further showed that both taxa-4(5)-11(12)-diene (endotaxadiene) and taxa-4(20)-11(12)-diene (exotaxadiene) are precursors to the key intermediate, taxologenic oxetane, indicating the potential existence of multiple routes in the Taxol pathway. Thus, we unveiled a long-elusive step in Taxol biosynthesis.

Oxetane-containing metabolites: origin, structures, and biological activities

Cyclobutanes containing one oxygen atom in a molecule are called oxetane-containing compounds (OCC). More than 600 different OCC are found in nature; they are produced by microorganisms, and also found in marine invertebrates and algae. The greatest number of them is found in plants belonging to the genus Taxus. Oxetanes are high-energy oxygen-containing non-aromatic heterocycles that are of great interest as new potential pharmacophores with a significant spectrum of biological activities. The biological activity of OCC that is produced by bacteria and Actinomycetes demonstrates antineoplastic, antiviral (arbovirus), and antifungal activity with confidence an angiogenesis stimulator, respiratory analeptic, and antiallergic activity dominate with confidence from 81 to 99%.

Enzymatic Formation of Oxygen-Containing Heterocycles in Natural Product Biosynthesis

Oxygen-containing heterocycles are widely encountered in natural products that display diverse pharmacological properties and have potential benefits to human health. The formation of O-heterocycles catalyzed by different types of enzymes in the biosynthesis of natural products not only contributes to the structural diversity of these compounds, but also enriches our understanding of nature's ability to construct complex molecules. This minireview focuses on the various modes of enzymatic O-heterocyclization identified in natural product biosynthesis and summarizes the possible mechanisms involved in ring closure.

Ring-Size Effects on the Stability and Spectral Shifts of Hydrogen Bonded Cyclic Ethers Complexes

In order to investigate ring-size effects on the stability and spectral shifts of hydrogen bonded cyclic ethers complexes, the strength of hydrogen bonds in gas phase complexes formed between 2,2,2-trifluoroethanol (TFE) and selected cyclic ethers were examined using FTIR spectroscopy. TFE was chosen as hydrogen bond donor in these complexes, while trimethylene oxide (TMO), tetrahydrofuran (THF) and tetrahydropyran (THP) were selected as hydrogen bond acceptors. Comparable OH-stretching red shifts were observed in the three kinds of complexes. The difference of red shifts is so small (<7 cm⁻¹) for TFE−TMO/THF/THP complexes that one can conclude that their stabilities and the strength of the hydrogen bonds are nearly similar and do not show any marked dependence with the ring size of the hydrogen bond acceptor. The equilibrium constants for the complexation were determined, and atoms-in-molecules (AIM) and natural bond orbital (NBO) analyses were performed to further investigate the intermolecular interactions. Regardless of the ring size, hydrogen bonds in the complexes showed similar strength, in agreement with the observed OH-stretching red shifts.

Oxidative Cyclization in Natural Product Biosynthesis

Oxidative cyclizations are important transformations that occur widely during natural product biosynthesis. The transformations from acyclic precursors to cyclized products can afford morphed scaffolds, structural rigidity, and biological activities. Some of the most dramatic structural alterations in natural product biosynthesis occur through oxidative cyclization. In this Review, we examine the different strategies used by nature to create new intra(inter)molecular bonds via redox chemistry. This Review will cover both oxidation- and reduction-enabled cyclization mechanisms, with an emphasis on the former. Radical cyclizations catalyzed by P450, nonheme iron, α-KG-dependent oxygenases, and radical SAM enzymes are discussed to illustrate the use of molecular oxygen and S-adenosylmethionine to forge new bonds at unactivated sites via one-electron manifolds. Nonradical cyclizations catalyzed by flavin-dependent monooxygenases and NAD(P)H-dependent reductases are covered to show the use of two-electron manifolds in initiating cyclization reactions. The oxidative installations of epoxides and halogens into acyclic scaffolds to drive subsequent cyclizations are separately discussed as examples of "disappearing" reactive handles. Last, oxidative rearrangement of rings systems, including contractions and expansions, will be covered.

Biosynthesis via carbocations: theoretical studies on terpene formation

This review describes applications of quantum chemical calculations in the field of terpene biosynthesis, with a focus on insights into the mechanisms of terpene-forming carbocation rearrangements arising from theoretical studies.

Synthesis and In Vitro Evaluation of Taxol oxetane ring D precursors

A series of potential taxoid substrates was prepared in radiolabelled form to probe in vitro for the oxirane formation step and subsequent ring expansion step to the oxetane (ring D) presumably involved in the biosynthesis of the anticancer agent Taxol. None of the taxoid test substrates underwent transformation in cell-free systems from Taxus suggesting that these surrogates bore substitution patterns inappropriate for recognition or catalysis by the target enzymes, or that taxoid oxiranes and oxetanes arise by independent biosynthetic pathways.