Alkaloid biosynthesis

Tropane alkaloid biosynthesis: a centennial review

Covering: 1917 to 2020Tropane alkaloids (TAs) are a remarkable class of plant secondary metabolites, which are characterized by an 8-azabicyclo[3.2.1]octane (nortropane) ring. Members of this class, such as hyoscyamine, scopolamine, and cocaine, are well known for their long history as poisons, hallucinogens, and anaesthetic agents. Since the structure of the tropane ring system was first elucidated in 1901, organic chemists and biochemists have been interested in how these mysterious tropane alkaloids are assembled in vitro and in vivo. However, it was only in 2020 that the complete biosynthetic route of hyoscyamine and scopolamine was clarified, and their de novo production in yeast was also achieved. The aim of this review is to present the innovative ideas and results in exploring the story of tropane alkaloid biosynthesis in plants from 1917 to 2020. This review also highlights that Robinson's classic synthesis of tropinone, which is one hundred years old, is biomimetic, and underscores the importance of total synthesis in the study of natural product biosynthesis.

NMR-based metabolomic profiling of Peganum harmala L. reveals dynamic variations between different growth stages

Xerophytes play an active role in preventing soil denudation and desertification in arid and semi-arid areas. Peganum harmala L. (Zygophyllaceae family), a seasonally growing, poisonous and drought-tolerant plant, is widely distributed in the Xinjiang Uygur Autonomous Region and used as a traditional herbal medicine as well as, in winter, a fodder source. Previous research has focused on the pharmacological activity of isolated compounds and stress responses to growth environments. However, the metabolic profile of P. harmala and variations in its metabolites, including medicinally active and stress resistance components, have not been illustrated during different growth stages. Here, we collected plant samples in May, August, October and December. We determined the metabolic composition of methanol extracts using NMR spectroscopy, and comparisons of four growth stages were accomplished by applying statistical analysis. The results showed that vasicine, choline and sucrose were significantly elevated in samples harvested in May. Significantly higher amounts of betaine, lysine, 4-hydroxyisoleucine and proline were found in samples collected in August than in samples collected in other months, and the concentrations of phosphorylcholine, glucose, acetic acid and vasicinone were highest in December. The relationships between differential biomarkers and plant physiological states affected by diverse growth environmental factors were discussed. Our result deepened the understanding of metabolic mechanisms in plant development and confirmed the advantage of using NMR-based metabolomic treatments in quality evaluation when P. harmala is used for different purposes.

Noscapine comes of age

Noscapine is a phthalideisoquinoline alkaloid, which represents a class of plant specialized metabolites within the large and structurally diverse group of benzylisoquinoline alkaloids. Along with the narcotic analgesic morphine, noscapine is a major alkaloid in the latex of opium poppy (Papaver somniferum) that has long been used as a cough suppressant and has undergone extensive investigation as a potential anticancer drug. Cultivated opium poppy plants remain the only commercial source of noscapine. Despite its isolation from opium more than two centuries ago, the almost complete biosynthesis of noscapine has only recently been established based on an impressive combination of molecular genetics, functional genomics, and metabolic biochemistry. In this review, we provide a historical account of noscapine from its discovery through to initial investigations of its formation in opium poppy. We also describe recent breakthroughs that have led to an elucidation of the noscapine biosynthetic pathway, and we discuss the pharmacological properties that have prompted intensive evaluation of the potential pharmaceutical applications of noscapine and several semi-synthetic derivatives. Finally, we speculate on the future potential for the production of noscapine using metabolic engineering and synthetic biology in plants and microbes.

Benzylisoquinoline alkaloid metabolism: a century of discovery and a brave new world

Benzylisoquinoline alkaloids (BIAs) are a structurally diverse group of plant specialized metabolites with a long history of investigation. Although the ecophysiological functions of most BIAs are unknown, the medicinal properties of many compounds have been exploited for centuries. These include the narcotic analgesics codeine and morphine, the antimicrobial agents sanguinarine and berberine, and the antitussive and anticancer drug noscapine. BIA biosynthesis involves a restricted number of enzyme types that catalyze landmark coupling reactions and subsequent functional group modifications. A pathogenesis-related (PR)10/Bet v1 'Pictet-Spenglerase', several O-methyl-, N-methyl- and O-acetyltransferases, cytochromes P450, FAD-dependent oxidases, non-heme dioxygenases and NADPH-dependent reductases have been implicated in the multistep pathways leading to structurally diverse alkaloids. A small number of plant species, including opium poppy (Papaver somniferum) and other members of the Ranunculales, have emerged as model systems to study BIA metabolism. The expansion of resources to include a wider range of plant species is creating an opportunity to investigate previously uncharacterized BIA pathways. Contemporary knowledge of BIA metabolism reflects over a century of research coupled with the development of key innovations such as radioactive tracing, enzyme isolation and molecular cloning, and functional genomics approaches such as virus-induced gene silencing. Recently, the emergence of transcriptomics, proteomics and metabolomics has expedited the discovery of new BIA biosynthetic genes. The growing repository of BIA biosynthetic genes is providing the parts required to apply emerging synthetic biology platforms to the development of production systems in microbes as an alternative to plants as a commecial source of valuable BIAs.

Secondary metabolite biosynthesis: the first century

An account of work on the biosynthesis of secondary metabolites up to 1965 is presented. The earliest suggestions for three of the four major pathways were speculative; for the isoprene rule, hypotheses date to 1877, for the polyketide rule to 1907, and for a role for amino acids in alkaloid biosynthesis to 1910. The fourth major pathway based on intermediates of the shikimic acid pathway has a much shorter history because shikimic acid itself was only identified as a primary metabolite in 1951. In addition to speculation, biomimetic syntheses were carried out in which chemists attempted to duplicate possible biosynthetic pathways in vitro. The classic example was Robinson's synthesis of tropinone in 1917. Direct examination of secondary metabolite biosynthesis was possible with the use of the isotopic tracer technique. This methodology, applied extensively to primary metabolism beginning in 1935 and to secondary metabolism from about 1950, was facilitated by the increasing availability of the 14C isotope. With the use of isotopes as tracers, the broad outlines of secondary metabolite biosynthesis, reviewed here, were established in the period 1950 to 1965.

THE BIOGENESIS OF ALKALOIDS: XXIV. THE ROLE OF CITRIC ACID IN THE BIOGENESIS OF HYOSCYAMINE

It is known that ornithine is a precursor of the pyrrolidine ring present in hyoscyamine. It was assumed that citric acid might be the precursor of the remaining three carbons that complete the tropine ring structure. To test this assumption 3-14C-citric acid was fed to Daturastramonium plants. The hyoscyamine isolated from these plants was radioactive, but the activity was very low. It must be concluded that citric acid, although it can be used to a small extent, is not the normal precursor of the chain of three carbons attached to the pyrrolidine ring in hyoscyamine.