Enzymes in natural product total synthesis

Natural product biological and structural diversity has long fascinated chemists and thus spurred innovative research over many decades. The often densely functionalized, stereochemically rich and complex molecular scaffolds designed by Nature pose enormous challenges on the structure elucidation process and the development of synthetic routes. Inspired by Nature’s chemistry, chemists over the last century have developed a fascinating arsenal of reagents and strategies to synthesize challenging natural product structures. Iconic molecules such as strychnine, taxol, tetracyline, vancomycin, brevetoxin, and palau’amine have all pushed organic chemists to streamline syntheses in terms of steps, efficiencies, and waste. Today, the advances in the field of synthetic organic chemistry have progressed to a point where even molecules of such exceptional complexity can be made in the gram scale.

Nature as a chemist has, however, been making these same molecules since the dawn of time, not in a fume hood, but in a cell. While isotope tracer studies were instrumental in mapping cellular metabolism and unmasking the major branches of biosynthetic pathways, the logic of biosynthesis only became apparent about 25 years ago with advances in genomics that established the genetic code of biosynthesis. Researchers in the field have since uncovered astonishing new enzyme mechanisms that have evolved to selectively assemble even the most complex chemical structures. Many such biosynthetic transformations are still unmet, even with modern synthesis. While biosynthetic enzymes have long been studied (almost exclusively) to illuminate biochemical mechanisms, this newfound knowledge is now increasingly being tasked to apply biosynthetic enzymes as biocatalysts for the synthesis of complex natural products. For this themed collection entitled Enzymes in Natural Product Total Synthesis, we commissioned representative review articles that summarize many of the exciting directions that this emerging field has taken in recent years to fuse enzymatic and synthetic organic transformations.

The integration of pathway-specific enzymes into chemo-enzymatic approaches towards the original natural products and analogs is summarized in a series of articles. This includes the chemo-enzymatic synthesis of complex alkaloids such as saframycin A using multimodular enzymes (Oguri and Oikawa et al., DOI: 10.1039/C9NP00073A), the application of tailoring enzymes for biocatalytic polyketide assembly exemplified by the ambruticins and jerangolids (Hahn et al., DOI: 10.1039/D0NP00012D), the development of streamlined chemo-enzymatic routes to complex bacterial meroterpenoids (Moore and George et al., DOI: 10.1039/D0NP00018C), as well as approaches towards non-natural terpene analogs accessible by applying terpene synthases (Kirschning and Dickschat et al., DOI: 10.1039/C9NP00055K). Renata et al. highlight the application of iron- and α-ketoglutarate-dependent dioxygenases in the chemo-enzymatic synthesis of a suite of structurally diverse natural products (DOI: 10.1039/C9NP00075E).

Large-scale and industrial applications of biosynthetic enzymes can be rather limited by natural substrate scopes, limited turnover or enzyme stability. Galanie et al. summarize methods for enzyme engineering to facilitate biocatalytic synthesis of natural products for a commercial setting (DOI: 10.1039/C9NP00071B). An important additional aspect to be taken into consideration when developing chemo-enzymatic syntheses is the supply of energy. Many enzymes utilize high-energy cofactors to fuel substrate turnover, including NAD(P) or ATP. Other cofactors are required for activation or tethering (e.g., coenzyme A) or are consumed as substrates in transfer reactions (e.g., SAM). Costs for such cofactors can be significant. It is thus often important to integrate efficient cofactor regeneration strategies, which are summarized in an article by Mordhorst and Andexer (DOI: 10.1039/D0NP00004C).

As evident from these reviews, the potential of the implementation of powerful biosynthetic enzymes in chemo-enzymatic synthesis is enormous. Even the application of large, multimodular polyketide synthases and non-ribosomal peptide synthetases has proven possible. Such approaches are producing impressive molecular complexity in one-pot biocatalytic transformations from the simplest biogenic precursors but often are extremely challenging to develop, given the susceptibility of large biosynthetic proteins in in vitro setups. Enzymatic reactions that efficiently and selectively install desired functionality and reactivity in a readily accessible synthetic substrate to enable a streamlined overall synthetic process not currently viable with synthetic chemistry alone are highly valuable additions to the biocatalytic toolbox. Such an amalgamation of chemical synthesis incorporating key enzymatic transformations certainly has the potential to transform the field of total synthesis in the near future.

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