Solution Structure and Conformational Flexibility of a Polyketide Synthase Module

Structural dynamics of human fatty acid synthase in the condensing cycle

Long-chain fatty acids are the building blocks of fat in human bodies. In mammals, fatty acid synthase (FASN) contains multiple enzymatic domains to catalyse all chemical reactions needed for de novo fatty acid synthesis1. Although the chemical reactions carried out by these enzymatic domains are well defined, how the dimeric FASN with an open architecture continuously catalyses such reactions to synthesize a complete fatty acid remains elusive. Here, using a strategy of tagging and purifying endogenous FASN in HEK293T cells for single-particle cryo-electron microscopy studies, we characterized the structural dynamics of endogenous human FASN. We captured conformational snapshots of various functional substates in the condensing cycle and developed a procedure to analyse the particle distribution landscape of FASN with different orientations between its condensing and modifying wings. Together, our findings reveal that FASN function does not require a large rotational motion between its two main functional domains during the condensing cycle, and that the catalytic reactions in the condensing cycle carried out by the two monomers are unsynchronized. Our data thus provide a new composite view of FASN dynamics during the fatty acid synthesis condensing cycle.

Snapshots of acyl carrier protein shuttling in human fatty acid synthase

The mammalian fatty acid synthase (FASN) enzyme is a dynamic multienzyme that belongs to the megasynthase family. In mammals, a single gene encodes six catalytically active domains and a flexibly tethered acyl carrier protein (ACP) domain that shuttles intermediates between active sites for fatty acid biosynthesis1. FASN is an essential enzyme in mammalian development through the role that fatty acids have in membrane formation, energy storage, cell signalling and protein modifications. Thus, FASN is a promising target for treatment of a large variety of diseases including cancer, metabolic dysfunction-associated fatty liver disease, and viral and parasite infections2,3. The multi-faceted mechanism of FASN and the dynamic nature of the protein, in particular of the ACP, have made it challenging to understand at the molecular level. Here we report cryo-electron microscopy structures of human FASN in a multitude of conformational states with NADPH and NADP+ plus acetoacetyl-CoA present, including structures with the ACP stalled at the dehydratase (DH) and enoyl-reductase (ER) domains. We show that FASN activity in vitro and de novo lipogenesis in cells is inhibited by mutations at the ACP-DH and ACP-ER interfaces. Together, these studies provide new molecular insights into the dynamic nature of FASN and the ACP shuttling mechanism, with implications for developing improved FASN-targeted therapeutics.

Architecture of full-length type I modular polyketide synthases revealed by X-ray crystallography, cryo-electron microscopy, and AlphaFold2

Covering: up to the end of 2023Type I modular polyketide synthases construct polyketide natural products in an assembly line-like fashion, where the growing polyketide chain attached to an acyl carrier protein is passed from catalytic domain to catalytic domain. These enzymes have immense potential in drug development since they can be engineered to produce non-natural polyketides by strategically adding, exchanging, and deleting individual catalytic domains. In practice, however, this approach frequently results in complete failures or dramatically reduced product yields. A comprehensive understanding of modular polyketide synthase architecture is expected to resolve these issues. We summarize the three-dimensional structures and the proposed mechanisms of three full-length modular polyketide synthases, Lsd14, DEBS module 1, and PikAIII. We also describe the advantages and limitations of using X-ray crystallography, cryo-electron microscopy, and AlphaFold2 to study intact type I polyketide synthases.

How Acyl Carrier Proteins (ACPs) Direct Fatty Acid and Polyketide Biosynthesis

Megasynthases, such as type I fatty acid and polyketide synthases (FASs and PKSs), are multienzyme complexes responsible for producing primary metabolites and complex natural products. Fatty acids (FAs) and polyketides (PKs) are built by assembling and modifying small acyl moieties in a stepwise manner. A central aspect of FA and PK biosynthesis involves the shuttling of substrates between the domains of the multienzyme complex. This essential process is mediated by small acyl carrier proteins (ACPs). The ACPs must navigate to the different catalytic domains within the multienzyme complex in a particular order to guarantee the fidelity of the biosynthesis pathway. However, the precise mechanisms underlying ACP-mediated substrate shuttling, particularly the factors contributing to the programming of the ACP movement, still need to be fully understood. This Review illustrates the current understanding of substrate shuttling, including concepts of conformational and specificity control, and proposes a confined ACP movement within type I megasynthases.

Module-Based Polyketide Synthase Engineering for de Novo Polyketide Biosynthesis

Polyketide retrobiosynthesis, where the biosynthetic pathway of a given polyketide can be reversibly engineered due to the colinearity of the polyketide synthase (PKS) structure and function, has the potential to produce millions of organic molecules. Mixing and matching modules from natural PKSs is one of the routes to produce many of these molecules. Evolutionary analysis of PKSs suggests that traditionally used module boundaries may not lead to the most productive hybrid PKSs and that new boundaries around and within the ketosynthase domain may be more active when constructing hybrid PKSs. As this is still a nascent area of research, the generality of these design principles based on existing engineering efforts remains inconclusive. Recent advances in structural modeling and synthetic biology present an opportunity to accelerate PKS engineering by re-evaluating insights gained from previous engineering efforts with cutting edge tools.

Discovery and Characterization of Antibody Probes of Module 2 of the 6-Deoxyerythronolide B Synthase

Fragment antigen-binding domains of antibodies (F_abs) are powerful probes of structure-function relationships of assembly line polyketide synthases (PKSs). We report the discovery and characterization of F_abs interrogating the structure and function of the ketosynthase-acyltransferase (KS-AT) core of Module 2 of the 6-deoxyerythronolide B synthase (DEBS). Two F_abs (AC2 and BB1) were identified to potently inhibit the catalytic activity of Module 2. Both AC2 and BB1 were found to modulate ACP-mediated reactions catalyzed by this module, albeit by distinct mechanisms. AC2 primarily affects the rate (k_cat), whereas BB1 increases the K_M of an ACP-mediated reaction. A third F_ab, AA5, binds to the KS-AT fragment of DEBS Module 2 without altering either parameter; it is phenotypically reminiscent of a previously characterized F_ab, 1B2, shown to principally recognize the N-terminal helical docking domain of DEBS Module 3. Crystal structures of AA5 and 1B2 bound to the KS-AT fragment of Module 2 were solved to 2.70 and 2.65 Å resolution, respectively, and revealed entirely distinct recognition features of the two antibodies. The new tools and insights reported here pave the way toward advancing our understanding of the structure-function relationships of DEBS Module 2, arguably the most well-studied module of an assembly line PKS.

Enzymology of assembly line synthesis by modular polyketide synthases

Modular polyketide synthases (PKSs) run catalytic reactions over dozens of steps in a highly orchestrated manner. To accomplish this synthetic feat, they form megadalton multienzyme complexes that are among the most intricate proteins on earth. Polyketide products are of elaborate chemistry with molecular weights of usually several hundred daltons and include clinically important drugs such as erythromycin (antibiotic), rapamycin (immunosuppressant) and epothilone (anticancer drug). The term 'modular' refers to a hierarchical structuring of modules and domains within an overall assembly line arrangement, in which PKS organization is colinearly translated into the polyketide structure. New structural information obtained during the past few years provides substantial direct insight into the orchestration of catalytic events within a PKS module and leads to plausible models for synthetic progress along assembly lines. In light of these structural insights, the PKS engineering field is poised to enter a new era of engineering.

Characterization of Unexpected Self-Acylation Activity of Acyl Carrier Proteins in a Modular Type I Apicomplexan Polyketide Synthase

Natural products play critical roles as antibiotics, anticancer therapeutics, and biofuels. Polyketides are a distinct natural product class of structurally diverse secondary metabolites that are synthesized by polyketide synthases (PKSs). The biosynthetic gene clusters that encode PKSs have been found across nearly all realms of life, but those from eukaryotic organisms are relatively understudied. A type I PKS from the eukaryotic apicomplexan parasite Toxoplasma gondii,TgPKS2, was recently discovered through genome mining, and the functional acyltransferase (AT) domains were found to be selective for malonyl-CoA substrates. To further characterize TgPKS2, we resolved assembly gaps within the gene cluster, which confirmed that the encoded protein consists of three distinct modules. We subsequently isolated and biochemically characterized the four acyl carrier protein (ACP) domains within this megaenzyme. We observed self-acylation─or substrate acylation without an AT domain─for three of the four TgPKS2 ACP domains with CoA substrates. Furthermore, CoA substrate specificity and kinetic parameters were determined for all four unique ACPs. TgACP2-4 were active with a wide scope of CoA substrates, while TgACP1 from the loading module was found to be inactive for self-acylation. Previously, self-acylation has only been observed in type II systems, which are enzymes that act in-trans with one another, and this represents the first report of this activity in a modular type I PKS whose domains function in-cis. Site-directed mutagenesis of specific TgPKS2 ACP3 acidic residues near the phosphopantetheinyl arm demonstrated that they influence self-acylation activity and substrate specificity, possibly by influencing substrate coordination or phosphopantetheinyl arm activation. Further, the lack of TgPKS2 ACP self-acylation with acetoacetyl-CoA, which is utilized by previously characterized type II PKS systems, suggests that the substrate carboxyl group may be critical for TgPKS2 ACP self-acylation. The unexpected properties observed from T. gondii PKS ACP domains highlight their distinction from well-characterized microbial and fungal systems. This work expands our understanding of ACP self-acylation beyond type II systems and helps pave the way for future studies on biosynthetic enzymes from eukaryotes.

Small-angle X-ray scattering studies of enzymes

Enzyme function requires conformational changes to achieve substrate binding, domain rearrangements, and interactions with partner proteins, but these movements are difficult to observe. Small-angle X-ray scattering (SAXS) is a versatile structural technique that can probe such conformational changes under solution conditions that are physiologically relevant. Although it is generally considered a low-resolution structural technique, when used to study conformational changes as a function of time, ligand binding, or protein interactions, SAXS can provide rich insight into enzyme behavior, including subtle domain movements. In this perspective, we highlight recent uses of SAXS to probe structural enzyme changes upon ligand and partner-protein binding and discuss tools for signal deconvolution of complex protein solutions.

Recent advances in the structural biology of modular polyketide synthases and nonribosomal peptide synthetases

Polyketides and nonribosomal peptides are an important class of natural products with useful bioactivities. These compounds are similarly biosynthesized using enzymes with modular structures despite having different physicochemical properties. These enzymes are attractive targets for bioengineering to produce "unnatural" natural products owing to their modular structures. Therefore, their structures have been studied for a long time; however, the main focus was on truncated-single domains. Surprisingly, there is an increasing number of the structures of whole modules reported, most of which have been enabled through the recent advances in cryogenic electron microscopy technology. In this review, we have summarized the recent advances in the structural elucidation of whole modules.

Solution structure of the type I polyketide synthase Pks13 from Mycobacterium tuberculosis

BACKGROUND

Type I polyketide synthases (PKSs) are multifunctional enzymes responsible for the biosynthesis of a group of diverse natural compounds with biotechnological and pharmaceutical interest called polyketides. The diversity of polyketides is impressive despite the limited set of catalytic domains used by PKSs for biosynthesis, leading to considerable interest in deciphering their structure-function relationships, which is challenging due to high intrinsic flexibility. Among nineteen polyketide synthases encoded by the genome of Mycobacterium tuberculosis, Pks13 is the condensase required for the final condensation step of two long acyl chains in the biosynthetic pathway of mycolic acids, essential components of the cell envelope of Corynebacterineae species. It has been validated as a promising druggable target and knowledge of its structure is essential to speed up drug discovery to fight against tuberculosis.

RESULTS

We report here a quasi-atomic model of Pks13 obtained using small-angle X-ray scattering of the entire protein and various molecular subspecies combined with known high-resolution structures of Pks13 domains or structural homologues. As a comparison, the low-resolution structures of two other mycobacterial polyketide synthases, Mas and PpsA from Mycobacterium bovis BCG, are also presented. This study highlights a monomeric and elongated state of the enzyme with the apo- and holo-forms being identical at the resolution probed. Catalytic domains are segregated into two parts, which correspond to the condensation reaction per se and to the release of the product, a pivot for the enzyme flexibility being at the interface. The two acyl carrier protein domains are found at opposite sides of the ketosynthase domain and display distinct characteristics in terms of flexibility.

CONCLUSIONS

The Pks13 model reported here provides the first structural information on the molecular mechanism of this complex enzyme and opens up new perspectives to develop inhibitors that target the interactions with its enzymatic partners or between catalytic domains within Pks13 itself.