Enzymatic syntheses of 13C-enriched geranylgeranyl diphosphate and casbene from 13C-labeled isopentenyl diphosphate

Collective Total Synthesis of Casbane Diterpenes: One Strategy, Multiple Targets

Of the more than 100 casbane diterpenes known to date, only the eponymous parent hydrocarbon casbene itself has ever been targeted by chemical synthesis. Outlined herein is a conceptually new approach that brings not a single but a variety of casbane derivatives into reach, especially the more highly oxygenated and arguably more relevant members of this family. The key design elements are a catalyst‐controlled intramolecular cyclopropanation with or without subsequent equilibration, chain extension of the resulting stereoisomeric cyclopropane building blocks by chemoselective hydroboration/cross‐coupling, and the efficient closure of the strained macrobicyclic framework by ring‐closing alkyne metathesis. A hydroxy‐directed catalytic trans‐hydrostannation allows for late‐stage diversity. These virtues are manifested in the concise total syntheses of depressin, yuexiandajisu A, and ent‐pekinenin C. The last compound turned out to be identical to euphorhylonal A, the structure of which had clearly been misassigned.

Engineered isoprenoid pathway enhances astaxanthin production in Escherichia coli

The isoprenoid pathway is a versatile biosynthetic network leading to over 23,000 compounds. Similar to other biosynthetic pathways, the production of isoprenoids in microorganisms is controlled by the supply of precursors, among other factors. To engineer a host that has the capability to supply geranylgeranyl diphosphate (GGPP), a common precursor of isoprenoids, we cloned and overexpressed isopentenyl diphosphate (IPP) isomerase (encoded by idi) from Escherichia coli and GGPP synthase (encoded by gps) from the archaebacterium Archaeoglobus fulgidus. The latter was shown to be a multifunctional enzyme converting dimethylallyl diphosphate (DMAPP) to GGPP. These two genes and the gene cluster (crtBIYZW) of the marine bacterium Agrobacterium aurantiacum were introduced into E. coli to produce astaxanthin, an orange pigment and antioxidant. This metabolically engineered strain produces astaxanthin 50 times higher than values reported before. To determine the rate-controlling steps in GGPP production, the IDI-GPS pathway was compared with another construct containing idi, ispA (encoding farnesyl diphosphate (FPP) synthase in E. coli), and crtE (encoding GGPP synthase from Erwinia uredovora). Results show that the conversion from FPP to GGPP is the first bottleneck, followed sequentially by IPP isomerization and FPP synthesis. Removal of these bottlenecks results in an E. coli strain providing sufficient precursors for in vivo synthesis of isoprenoids.