Recent developments in the production of novel polyketides by combinatorial biosynthesis

Animal Fatty Acid Synthase: A Chemical Nanofactory

Fatty acids are crucial molecules for most living beings, very well spread and conserved across species. These molecules play a role in energy storage, cell membrane architecture, and cell signaling, the latter through their derivative metabolites. De novo synthesis of fatty acids is a complex chemical process that can be achieved either by a metabolic pathway built by a sequence of individual enzymes, such as in most bacteria, or by a single, large multi-enzyme, which incorporates all the chemical capabilities of the metabolic pathway, such as in animals and fungi, and in some bacteria. Here we focus on the multi-enzymes, specifically in the animal fatty acid synthase (FAS). We start by providing a historical overview of this vast field of research. We follow by describing the extraordinary architecture of animal FAS, a homodimeric multi-enzyme with seven different active sites per dimer, including a carrier protein that carries the intermediates from one active site to the next. We then delve into this multi-enzyme's detailed chemistry and critically discuss the current knowledge on the chemical mechanism of each of the steps necessary to synthesize a single fatty acid molecule with atomic detail. In line with this, we discuss the potential and achieved FAS applications in biotechnology, as biosynthetic machines, and compare them with their homologous polyketide synthases, which are also finding wide applications in the same field. Finally, we discuss some open questions on the architecture of FAS, such as their peculiar substrate-shuttling arm, and describe possible reasons for the emergence of large megasynthases during evolution, questions that have fascinated biochemists from long ago but are still far from answered and understood.

Diversity oriented biosynthesis via accelerated evolution of modular gene clusters

Erythromycin, avermectin and rapamycin are clinically useful polyketide natural products produced on modular polyketide synthase multienzymes by an assembly-line process in which each module of enzymes in turn specifies attachment of a particular chemical unit. Although polyketide synthase encoding genes have been successfully engineered to produce novel analogues, the process can be relatively slow, inefficient, and frequently low-yielding. We now describe a method for rapidly recombining polyketide synthase gene clusters to replace, add or remove modules that, with high frequency, generates diverse and highly productive assembly lines. The method is exemplified in the rapamycin biosynthetic gene cluster where, in a single experiment, multiple strains were isolated producing new members of a rapamycin-related family of polyketides. The process mimics, but significantly accelerates, a plausible mechanism of natural evolution for modular polyketide synthases. Detailed sequence analysis of the recombinant genes provides unique insight into the design principles for constructing useful synthetic assembly-line multienzymes.

Reengineering polyketide synthase encoding genes to produce analogues of natural products can be slow and low-yielding. Here the authors use accelerated evolution to recombine the gene cluster for rapid production of rapamycin-related products.

The crystal structure and mechanism of an unusual oxidoreductase, GilR, involved in gilvocarcin V biosynthesis

GilR is a recently identified oxidoreductase that catalyzes the terminal step of gilvocarcin V biosynthesis and is a unique enzyme that establishes the lactone core of the polyketide-derived gilvocarcin chromophore. Gilvocarcin-type compounds form a small distinct family of anticancer agents that are involved in both photo-activated DNA-alkylation and histone H3 cross-linking. High resolution crystal structures of apoGilR and GilR in complex with its substrate pregilvocarcin V reveals that GilR belongs to the small group of a relatively new type of the vanillyl-alcohol oxidase flavoprotein family characterized by bicovalently tethered cofactors. GilR was found as a dimer, with the bicovalently attached FAD cofactor mediated through His-65 and Cys-125. Subsequent mutagenesis and functional assays indicate that Tyr-445 may be involved in reaction catalysis and in mediating the covalent attachment of FAD, whereas Tyr-448 serves as an essential residue initiating the catalysis by swinging away from the active site to accommodate binding of the 6R-configured substrate and consequently abstracting the proton of the hydroxyl residue of the substrate hemiacetal 6-OH group. These studies lay the groundwork for future enzyme engineering to broaden the substrate specificity of this bottleneck enzyme of the gilvocarcin biosynthetic pathway for the development of novel anti-cancer therapeutics.

The structural biology of type II fatty acid biosynthesis

The type II fatty acid synthetic pathway is the principal route for the production of membrane phospholipid acyl chains in bacteria and plants. The reaction sequence is carried out by a series of individual soluble proteins that are each encoded by a discrete gene, and the pathway intermediates are shuttled between the enzymes as thioesters of an acyl carrier protein. The Escherichia coli system is the paradigm for the study of this system, and high-resolution X-ray and/or NMR structures of representative members of every enzyme in the type II pathway are now available. The structural biology of these proteins reveals the specific three-dimensional features of the enzymes that explain substrate recognition, chain length specificity, and the catalytic mechanisms that define their roles in producing the multitude of products generated by the type II system. These structures are also a valuable resource to guide antibacterial drug discovery.

Mining the microbial metabolome: a new frontier for natural product lead discovery

Traditionally, natural products have been important sources of new leads for the pharmaceutical industry, but with discovery rates of novel structural classes in decline, the need to bioprospect alternate sources of chemical diversity is evident. Microbial genome sequencing projects have revealed the presence of 'silent' biosynthetic gene clusters where there is no current detectable product. Likewise, culture-independent techniques have provided access to the collective genomes of environmental microflora. Both sources of molecular diversity could encode potentially valuable metabolites. The ability to measure the entire complement of metabolites within microorganisms that are used as surrogate hosts to express such gene clusters will be crucial to the exploitation of these yet untapped reservoirs of metabolic diversity for future natural product drug discovery.