Directed Biosynthesis of Mitragynine Stereoisomers

Chemical, pharmacological properties and biosynthesis of opioid mitragynine in Mitragyna speciosa (kratom)

Mitragynine, an alkaloid found in Mitragyna speciosa (kratom), shows promise as a potential alternative to opioids owing to its distinctive indole alkaloid structure and its capacity for pain relief, alleviation of opioid withdrawal symptoms, and anti-inflammatory effects. Recently the intricate process of mitragynine biosynthesis from the precursor strictosidine was elucidated, providing insights into the complex pathways responsible for synthesizing this opioid compound and its related diastereomers. As the search continues for the authentic hydroxylase and methyltransferase crucial for mitragynine formation, leveraging enzymes from other species and exploiting enzyme promiscuity has facilitated heterologous mitragynine biosynthesis in microbes. This highlights the extraordinary flexibility of enzymes in generating a spectrum of variations and analogs of kratom opioids within alternative biological systems.

Cutting-edge plant natural product pathway elucidation

Plant natural products (PNPs) play important roles in plant physiology and have been applied across diverse fields of human society. Understanding their biosynthetic pathways informs plant evolution and meanwhile enables sustainable production through metabolic engineering. However, the discovery of PNP biosynthetic pathways remains challenging due to the diversity of enzymes involved and limitations in traditional gene mining approaches. In this review, we will summarize state-of-the-art strategies and recent examples for predicting and characterizing PNP biosynthetic pathways, respectively, with multiomics-guided tools and heterologous host systems and share our perspectives on the systematic pipelines integrating these various bioinformatic and biochemical approaches.

An insight review on the neuropharmacological effects, mechanisms of action, pharmacokinetics and toxicity of mitragynine

Mitragynine is one of the main psychoactive alkaloids in Mitragyna speciosa Korth. (kratom). It has opium-like effects by acting on μ-, δ-, and κ-opioid receptors in the brain. The compound also interacts with other receptors, such as adrenergic and serotonergic receptors and neuronal Ca2+ channels in the central nervous system to have its neuropharmacological effects. Mitragynine has the potential to treat diseases related to neurodegeneration such as Alzheimer's disease and Parkinson's disease, as its modulation on the opioid receptors has been reported extensively. This review aimed to provide an up-to-date and critical overview on the neuropharmacological effects, mechanisms of action, pharmacokinetics and safety of mitragynine as a prospective psychotropic agent. Its multiple neuropharmacological effects on the brain include antinociceptive, anti-inflammatory, antidepressant, sedative, stimulant, cognitive, and anxiolytic activities. The potential of mitragynine to manage opioid withdrawal symptoms related to opioid dependence, its pharmacokinetics and toxic effects were also discussed. The interaction of mitragynine with various receptors in the brain produce diverse neuropharmacological effects, which have beneficial properties in neurological disorders. However, further studies need to be carried out on mitragynine to uncover its complex mechanisms of action, pharmacokinetics, pharmacodynamic profiles, addictive potential, and safe dosage to prevent harmful side effects.

De novo biosynthesis of antiarrhythmic alkaloid ajmaline

The antiarrhythmic drug ajmaline is a monoterpenoid indole alkaloid (MIA) isolated from the Ayurvedic plant Rauvolfia serpentina (Indian Snakeroot). Research into the biosynthesis of ajmaline and another renowned MIA chemotherapeutic drug vinblastine has yielded pivotal advancements in the fields of plant specialized metabolism and engineering over recent decades. While the majority of vinblastine biosynthesis has been recently elucidated, the quest for comprehending ajmaline biosynthesis remains incomplete, marked by the absence of two critical enzymes. Here, we show the discovery and characterization of these two elusive reductases, alongside the identification of two physiologically relevant esterases that complete the biosynthesis of ajmaline. We show that ajmaline biosynthesis proceeds with vomilenine 1,2(R)-reduction followed by its 19,20(S)-reduction. This process is further modulated by two root-expressing esterases that deacetylate 17-O-acetylnorajmaline. Expanding upon the successful completion of the ajmaline biosynthetic pathway, we engineer the de novo biosynthesis of ajmaline in Baker's yeast.

Engineered biosynthesis of plant heteroyohimbine and corynantheine alkaloids in Saccharomyces cerevisiae

Monoterpene indole alkaloids (MIAs) are a class of natural products comprised of thousands of structurally unique bioactive compounds with significant therapeutic values. Due to difficulties associated with isolation from native plant species and organic synthesis of these structurally complex molecules, microbial production of MIAs using engineered hosts are highly desired. In this work, we report the engineering of fully integrated Saccharomyces cerevisiae strains that allow de novo access to strictosidine, the universal precursor to thousands of MIAs at 30-40 mg/L. The optimization efforts were based on a previously reported yeast strain that is engineered to produce high titers of the monoterpene precursor geraniol through compartmentalization of mevalonate pathway in the mitochondria. Our approaches here included the use of CRISPR-dCas9 interference to identify mitochondria diphosphate transporters that negatively impact the titer of the monoterpene, followed by genetic inactivation; the overexpression of transcriptional regulators that increase cellular respiration and mitochondria biogenesis. Strain construction included the strategic integration of genes encoding both MIA biosynthetic and accessory enzymes into the genome under a variety of constitutive and inducible promoters. Following successful de novo production of strictosidine, complex alkaloids belonging to heteroyohimbine and corynantheine families were reconstituted in the host with introduction of additional downstream enzymes. We demonstrate that the serpentine/alstonine pair can be produced at ∼5 mg/L titer, while corynantheidine, the precursor to mitragynine can be produced at ∼1 mg/L titer. Feeding of halogenated tryptamine led to the biosynthesis of analogs of alkaloids in both families. Collectively, our yeast strain represents an excellent starting point to further engineer biosynthetic bottlenecks in this pathway and to access additional MIAs and analogs through microbial fermentation.

ONE SENTENCE SUMMARY

An Saccharomyces cerevisiae-based microbial platform was developed for the biosynthesis of monoterpene indole alkaloids, including the universal precursor strictosidine and further modified heteroyohimbine and corynantheidine alkaloids.

Biosynthesis of kratom opioids

Mitragynine, an analgesic alkaloid from the plant Mitragyna speciosa (kratom), offers a safer alternative to clinical opioids such as morphine, owing to its more favorable side effect profile. Although kratom has been traditionally used for stimulation and pain management in Southeast Asia, the mitragynine biosynthesis pathway has remained elusive. We embarked on a search for mitragynine biosynthetic genes from the transcriptomes of kratom and other members of the Rubiaceae family. We studied their functions in vitro and in vivo. Our investigations led to the identification of several reductases and an enol methyltransferase that forms a new clade within the SABATH methyltransferase family. Furthermore, we discovered a methyltransferase from Hamelia patens (firebush), which catalyzes the final step. With the tryptamine 4-hydroxylase from the psychedelic mushroom Psilocybe cubensis, we accomplished the four-step biosynthesis for mitragynine and its stereoisomer, speciogynine in both yeast and Escherichia coli when supplied with tryptamine and secologanin. Although we have yet to pinpoint the authentic hydroxylase and methyltransferase in kratom, our discovery completes the mitragynine biosynthesis. Through these breakthroughs, we achieved the microbial biosynthesis of kratom opioids for the first time. The remarkable enzyme promiscuity suggests the possibility of generating derivatives and analogs of kratom opioids in heterologous systems.

Discovering Dynamic Plant Enzyme Complexes in Yeast for Kratom Alkaloid Pathway Identification

Discovering natural product biosynthetic pathways of medicinal plants is challenging and laborious. Capturing the coregulation patterns of pathway enzymes, particularly transcriptomic regulation, has proven an effective method to accelerate pathway identification. In this study, we developed a yeast-based screening method to capture the protein-protein interactions (PPI) between plant enzymes, which is another useful pattern to complement the prevalent approach. Combining this method with plant multiomics analysis, we discovered four enzyme complexes and their organized pathways from kratom, an alkaloid-producing plant. The four pathway branches involved six enzymes, including a strictosidine synthase, a strictosidine β-D-glucosidase (MsSGD), and four medium-chain dehydrogenase/reductases (MsMDRs). PPI screening selected six MsMDRs interacting with MsSGD from 20 candidates predicted by multiomics analysis. Four of the six MsMDRs were then characterized as functional, indicating the high selectivity of the PPI screening method. This study highlights the opportunity of leveraging post-translational regulation features to discover novel plant natural product biosynthetic pathways.

Total Synthesis and Structural Plasticity of Kratom Pseudoindoxyl Metabolites

Mitragynine pseudoindoxyl, a kratom metabolite, has attracted increasing attention due to its favorable side effect profile as compared to conventional opioids. Herein, we describe the first enantioselective and scalable total synthesis of this natural product and its epimeric congener, speciogynine pseudoindoxyl. The characteristic spiro-5-5-6-tricyclic system of these alkaloids was formed through a protecting-group-free cascade relay process in which oxidized tryptamine and secologanin analogues were used. Furthermore, we discovered that mitragynine pseudoindoxyl acts not as a single molecular entity but as a dynamic ensemble of stereoisomers in protic environments; thus, it exhibits structural plasticity in biological systems. Accordingly, these synthetic, structural, and biological studies provide a basis for the planned design of mitragynine pseudoindoxyl analogues, which can guide the development of next-generation analgesics.

Metabolite and Molecular Characterization of Mitragyna speciosa Identifies Developmental and Genotypic Effects on Monoterpene Indole and Oxindole Alkaloid Composition

The monoterpene indole alkaloid (MIA) mitragynine has garnered attention as a potential treatment for pain, opioid use disorder, and opioid withdrawal because of its combined pharmacology at opioid and adrenergic receptors in humans. This alkaloid is unique to Mitragyna speciosa (kratom), which accumulates over 50 MIAs and oxindole alkaloids in its leaves. Quantification of 10 targeted alkaloids from several tissue types and cultivars of  M. speciosa revealed that mitragynine accumulation was highest in leaves, followed by stipules and stems, but was absent, along with other alkaloids, in roots. While mitragynine is the predominant alkaloid in mature leaves, juvenile leaves accumulate higher amounts of corynantheidine and speciociliatine. Interestingly, corynantheidine has an inverse relationship with mitragynine accumulation throughout leaf development. Characterization of various cultivars of M. speciosa indicated altered alkaloidal profiles ranging from undetectable to high levels of mitragynine. DNA barcoding and phylogenetic analysis using ribosomal ITS sequences revealed polymorphisms leading M. speciosa cultivars having lower mitragynine content to group with other mitragyna species, suggesting interspecific hybridization events. Root transcriptome analysis of low- and high-mitragynine-producing cultivars indicated significant differences in gene expression and revealed allelic variation, further supporting that hybridization events may have impacted the alkaloid profile of M. speciosa.