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Polyketides are a diverse class of molecule composed of multiple ketide groups (adjacent methylene and carbonyl groups) or their derivatives and are biologically synthesized via polyketide synthases, which are derived from polyunsaturated fatty acid synthases (Staunton and Weissman 2001). In fatty acid synthesis, addition of acetyl groups to the growing fatty acid is followed by full reduction of the acetyl carbonyl to an alkyl group. In contrast, during polyketide synthesis carbonyls can be fully reduced, partially reduced to a hydroxy group, or not reduced at all, which results in a huge variety of polyketides. Polyketide synthases frequently contain more than one “module” that add and modify acetyl monomers, and multiple polyketide synthases often work in conjuction to create large polyketides. Intended as chemical weapons against other organisms by the organisms that produce them, many polyketides have found applications as therapeutic drugs, including antibiotic, antifungal, and anti-cancer drugs. Fatty acid synthases and polyketide synthases are composed of several enzymes that perform the various reactions involved in the biosynthesis. One of these is an acyl carrier protein (ACP) that carries the growing molecule from one enzymatic active site to the next. Most Type I polyketide synthases and fatty acid synthases have just one ACP. However, some fatty acid synthases have several ACP domains (together called a tandem ACP domain), which has been correlated with increased production of fatty acids (Jiang et al. 2008). This indicates that one of the rate-limiting factors of polyketide biosynthesis is the availability of substrates made readily accessible by being bound to ACP domains.
Research into polyketides is of great interest but is hampered by the low levels at which they tend to be produced by organisms, inhibiting the ability of researchers to obtain the large quantities necessary for their work. Wang et al. (2018) approached this problem by developing a method to create polyketide synthases that have increased production capabilities by integrating multiple ACP domains into a single polyketide synthase. The authors expressed both module 6-TE, a simple polyketide synthase developed from the well-characterized 6-deoxyerythronolide B synthase (DEBS), responsible for synthesizing the macrolide ring of erythromycin, and moldule 6-3ACP-TE, a version of module 6-TE containing three ACP domains rather than one. Liquid chromatography-mass spectrometry was used to assess the polyketide production capacities of the isolated module 6-TE and module 6-3ACP-TE proteins.
Module 6-TE synthesizes both triketide lactone and triketide ketolactone under normal conditions. The creation of triketide lactone is a result of a ketoreductase domain in the synthase that uses NADPH as a cofactor to reduce one of the carbonyls to a hydroxl group. The major significant finding of this study is that Wang et al. determined that their incorporation of two additional ACP domains increased polyketide production by a factor of about 2.5. This finding provides the opportunity to overcome a major challenge in the efforts to create polyketide-based therapeutics, which could open the door to a faster development process.
Avenues for further work are determining the impact of larger tandem ACP domains on polyketide biosynthesis rates and developing more complex multi-module polyketide synthases to experiment with increasing the production of more complex and therapeutically interesting polketides. Future applications of engineered polyketide synthases are hinted at by the most surprising result discovered by the author: while module 6-TE produced a mixture of triketide lactone and triketide ketolactone, module 6-3ACP-TE solely synthesized triketide ketolactone despite the presence of NADPH. As of yet the authors have not determined an explanation for this result. This is an avenue for further exploration, since determining what caused the specificity in products could allow for control of the mechanism governing it. Being able to increase the rate of polyketide production is not as beneficial as it could be if increasing the rate of biosynthesis also decreases the diversity of compounds that can be produced. Figuring out issues such as why module 6-3ACP-TE does not display any ketoreductase activity would not only restore the benefits of increased production rate, but would also provide the benefit of greater control over the polyketides being synthesized.
Natural products or their derivatives are often developed into therapeutic drugs. However, creating total syntheses, which are complete syntheses from commercially available precursors, for natural products is a notorious challenge. For instance, the synthesis of erythromycin, a widely used polyketide antibiotic that is on the World Health Organization’s list of the most safe and effective medicines necessary for a basic health-care system, was worked on for more than a decade by over a dozen different research groups (“WHO Model List of Essential Medicines,” Mulzer 1991). A popular potential alternative to performing difficult laboratory syntheses is biosynthesis using bacteria, where bacteria are engineered to produce large quantities of pharmaceutical drugs. The work presented by Wang and colleagues is a step toward the large-scale aspect of commercially biosynthesizing complex drugs. Work on why module 6-3ACP-TE exclusively produces triketide ketolactone would contribute to the capacity to readily produce complex drug candidates that differ at specific locations in order to develop compounds with the desired biological properties.
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