Structure, High Affinity, and Negative Cooperativity of the Escherichia coli Holo-(Acyl Carrier Protein):Holo-(Acyl Carrier Protein) Synthase Complex

An in-cluster Sfp-type phosphopantetheinyl transferase instead of the holo-ACP synthase activates the granaticin biosynthesis under natural physiological conditions

Bacterial aromatic polyketides are mainly biosynthesized by type II polyketide synthases (PKSs). The PKSs cannot be functional unless their acyl carrier proteins (ACPs) are phosphopantetheinylated by phosphopantetheinyl transferases (PPTases). Gra-ORF32 was identified as an in-cluster PPTase dedicated for granaticin biosynthesis in Streptomyces vietnamensis and the Arg- and Pro-rich N terminus was found to be crucial for catalytic activity. Overexpression of the encoding genes of the holo-ACP synthases of fatty acid synthases (FAS ACPSs) of both E. coli and S. vietnamensis could efficiently activate the production of granaticins in the Δgra-orf32 mutant, suggesting the ACP of granaticin (graACP) is an efficient substrate for FAS ACPSs. However, Gra-ORF32, the cognate PPTase of the graACP, could not compensate the conditional deficiency of ACPS in E. coli HT253, indicating that it has evolved to be functionally segregated from fatty acid biosynthesis. Nine out of eleven endogenous and all the tested exogenous non-cognate PPTases could activate the production of granaticins to varied extents when overexpressed in the Δgra-orf32 mutant, indicating that ACPs of type II PKSs could also be widely recognized as effective substrates by the Sfp-type PPTases. The exogenous PPTases of type II PKSs activated the production of granaticins with much higher efficiency, suggesting that the phylogenetically distant in-cluster PPTases of type II PKSs could share substrate preferences for the ACPs of type II PKSs. A significantly elevated production of granaticins was observed when the mutant Δgra-orf32 was cultivated on ISP2 plates, which was a consequence of crosstalk between the granaticin pathway and a kinamycin-like pathway as revealed by transcriptome analysis and pathway inactivations. Although the host FAS ACPS could efficiently activate the production of granaticins when overexpressed, only Gra-ORF32 activated the efficient production of granaticins under natural physiological conditions, indicating that the activity of the host FAS ACPS was strictly regulated, possibly by binding the FAS holo-ACP product with high affinity. Our findings would contribute to a more comprehensive understanding of how the ACPs of type II PKSs are activated and facilitate the future functional reconstitutions of type II PKSs in E. coli.

DltC acts as an interaction hub for AcpS, DltA and DltB in the teichoic acid D-alanylation pathway of Lactiplantibacillus plantarum

Teichoic acids (TA) are crucial for the homeostasis of the bacterial cell wall as well as their developmental behavior and interplay with the environment. TA can be decorated by different modifications, modulating thus their biochemical properties. One major modification consists in the esterification of TA by d-alanine, a process known as d-alanylation. TA d-alanylation is performed by the Dlt pathway, which starts in the cytoplasm and continues extracellularly after d-Ala transportation through the membrane. In this study, we combined structural biology and in vivo approaches to dissect the cytoplasmic steps of this pathway in Lactiplantibacillus plantarum, a bacterial species conferring health benefits to its animal host. After establishing that AcpS, DltB, DltC1 and DltA are required for the promotion of Drosophila juvenile growth under chronic undernutrition, we solved their crystal structure and/or used NMR and molecular modeling to study their interactions. Our work demonstrates that the suite of interactions between these proteins is ordered with a conserved surface of DltC1 docking sequentially AcpS, DltA and eventually DltB. Altogether, we conclude that DltC1 acts as an interaction hub for all the successive cytoplasmic steps of the TA d-alanylation pathway.

Identification, structure determination and analysis of Mycobacterium smegmatis acyl-carrier protein synthase (AcpS) crystallized serendipitously

The unintended crystallization of proteins which generally originate from the expression host instead of the target recombinant proteins is periodically reported. Despite the massive technological advances in the field, assigning a structural model to the corresponding diffraction data is not a trivial task. Here, the structure of acyl-carrier protein synthase (AcpS) from Mycobacterium smegmatis (msAcpS), which crystallized inadvertently in an experimental setup to grow crystals of a Mycobacterium tuberculosis protein using M. smegmatis as an expression system, is reported. After numerous unsuccessful attempts to solve the structure of the target protein by the molecular-replacement method no convincing solutions were obtained, indicating that the diffraction data may correspond to a crystal of an artifactual protein, which was finally identified by the Sequence-Independent Molecular replacement Based on Available Databases (SIMBAD) server. The msAcpS structure was solved at 2.27 Å resolution and structural analysis showed an overall conserved fold. msAcpS formed a trimeric structure similar to those of other reported structures of AcpS from various organisms; however, the residues involved in trimer formation are not strictly conserved. An unrelated metal ion (Ni2+), which was possibly incorporated during protein purification, was observed in the proximity of His49 and His116. Structural and sequence differences were observed in the loop connecting the α3 and α4 helices that is responsible for the open and closed conformations of the enzyme. Moreover, the structural analysis of msAcpS augments the current understanding of this enzyme, which plays a crucial role in the functional activation of acyl-carrier proteins in the fatty-acid biosynthesis pathway.

Engineered Chimeras Unveil Swappable Modular Features of Fatty Acid and Polyketide Synthase Acyl Carrier Proteins

The strategic redesign of microbial biosynthetic pathways is a compelling route to access molecules of diverse structure and function in a potentially environmentally sustainable fashion. The promise of this approach hinges on an improved understanding of acyl carrier proteins (ACPs), which serve as central hubs in biosynthetic pathways. These small, flexible proteins mediate the transport of molecular building blocks and intermediates to enzymatic partners that extend and tailor the growing natural products. Past combinatorial biosynthesis efforts have failed due to incompatible ACP-enzyme pairings. Herein, we report the design of chimeric ACPs with features of the actinorhodin polyketide synthase ACP (ACT) and of the Escherichia coli fatty acid synthase (FAS) ACP (AcpP). We evaluate the ability of the chimeric ACPs to interact with the E. coli FAS ketosynthase FabF, which represents an interaction essential to building the carbon backbone of the synthase molecular output. Given that AcpP interacts with FabF but ACT does not, we sought to exchange modular features of ACT with AcpP to confer functionality with FabF. The interactions of chimeric ACPs with FabF were interrogated using sedimentation velocity experiments, surface plasmon resonance analyses, mechanism-based cross-linking assays, and molecular dynamics simulations. Results suggest that the residues guiding AcpP-FabF compatibility and ACT-FabF incompatibility may reside in the loop I, α-helix II region. These findings can inform the development of strategic secondary element swaps that expand the enzyme compatibility of ACPs across systems and therefore represent a critical step toward the strategic engineering of "un-natural" natural products.

Chemical Labeling of Protein 4'-Phosphopantetheinylation

Nature uses a diverse array of protein post-translational modifications (PTMs) to regulate protein structure, activity, localization, and function. Among them, protein 4'-phosphopantetheinylation derived from coenzyme A (CoA) is an essential PTM for the biosynthesis of fatty acids, polyketides, and nonribosomal peptides in prokaryotes and eukaryotes. To explore its functions, various chemical probes mimicking the natural structure of 4'-phosphopantetheinylation have been developed. In this minireview, we summarize these chemical probes and describe their applications in direct and metabolic labeling of proteins in bacterial and mammalian cells.

Activity Mapping the Acyl Carrier Protein: Elongating Ketosynthase Interaction in Fatty Acid Biosynthesis

Elongating ketosynthases (KSs) catalyze carbon-carbon bond-forming reactions during the committed step for each round of chain extension in both fatty acid synthases (FASs) and polyketide synthases (PKSs). A small α-helical acyl carrier protein (ACP) shuttles fatty acyl intermediates between enzyme active sites. To accomplish this task, the ACP relies on a series of dynamic interactions with multiple partner enzymes of FAS and associated FAS-dependent pathways. Recent structures of the Escherichia coli FAS ACP, AcpP, in covalent complexes with its two cognate elongating KSs, FabF and FabB, provide high-resolution details of these interfaces, but a systematic analysis of specific interfacial interactions responsible for stabilizing these complexes has not yet been undertaken. Here, we use site-directed mutagenesis with both in vitro and in vivo activity analyses to quantitatively evaluate these contacting surfaces between AcpP and FabF. We delineate the FabF interface into three interacting regions and demonstrate the effects of point mutants, double mutants, and region deletion variants. Results from these analyses reveal a robust and modular FabF interface capable of tolerating seemingly critical interface mutations with only the deletion of an entire region significantly compromising activity. Structure and sequence analyses of FabF orthologs from related type II FAS pathways indicate significant conservation of type II FAS KS interface residues and, overall, support its delineation into interaction regions. These findings strengthen our mechanistic understanding of molecular recognition events between ACPs and FAS enzymes and provide a blueprint for engineering ACP-dependent biosynthetic pathways.

Gating mechanism of elongating β-ketoacyl-ACP synthases

Carbon-carbon bond forming reactions are essential transformations in natural product biosynthesis. During de novo fatty acid and polyketide biosynthesis, β-ketoacyl-acyl carrier protein (ACP) synthases (KS), catalyze this process via a decarboxylative Claisen-like condensation reaction. KSs must recognize multiple chemically distinct ACPs and choreograph a ping-pong mechanism, often in an iterative fashion. Here, we report crystal structures of substrate mimetic bearing ACPs in complex with the elongating KSs from Escherichia coli, FabF and FabB, in order to better understand the stereochemical features governing substrate discrimination by KSs. Complemented by molecular dynamics (MD) simulations and mutagenesis studies, these structures reveal conformational states accessed during KS catalysis. These data taken together support a gating mechanism that regulates acyl-ACP binding and substrate delivery to the KS active site. Two active site loops undergo large conformational excursions during this dynamic gating mechanism and are likely evolutionarily conserved features in elongating KSs.

Structure and dynamics of human and bacterial acyl carrier proteins and their interactions with fatty acid synthesis proteins

Acyl carrier protein (ACP) is highly conserved across taxa and plays key roles in the fatty acid synthesis system by mediating acyl group delivery and shuttling. Here, we compared the structural and dynamic features of human type Ι ACP (hACP) and Escherichia coli type II ACP (EcACP). Analysis of chemical shift perturbations upon octanoyl group attachment showed perturbations in hACP only near acyl-group attachment sites, whereas EcACP showed the perturbation at residues in the hydrophobic cavity. This difference confirmed that hACP does not sequester the acyl chain in the hydrophobic cavity, which is blocked by hydrophobic triad residues (L34, L39, and V64). Moreover, hACP showed more flexible backbone dynamics than EcACP, especially in the front of α₁α₂ loop. We further investigated the interactions of hACP with Streptomyces coelicolor ACP synthase (ScAcpS), which is used to convert apo mammalian ACP to the holo form. Similar to protein-protein interface (PPI) found in hACP-hAcpS crystal structure, docking simulation and binding affinity measurements showed that the hydrophobic residues in universal recognition helix II of hACP contribute mainly to ScAcpS binding with binding affinity of 9.2 ± 9.1 × 10⁴ M. In contrast, interaction found in EcACP-EcAcpS crystal structure is dominated by electrostatic interactions. These results suggest that ScAcpS has relatively relaxed substrate specificity and a similar charge distribution to hAcpS. These fundamental differences of the charge distribution in hAcpS, ScAcpS and EcAcpS largely affect the interaction with hACP. These findings can provide a useful resource for development of novel antibiotics inhibiting PPI in bacterial FAS proteins with specificity.

A back-door Phenylalanine coordinates the stepwise hexameric loading of acyl carrier protein by the fatty acid biosynthesis enzyme β-hydroxyacyl-acyl carrier protein dehydratase (FabZ)

The fatty acid biosynthesis pathway (FAS) was a fundamental procedure to generate a diversity of lipid components for cellular metabolism in bacteria, while the mechanism of substrate recognition remains unclear. The β-hydroxyacyl-acyl carrier protein dehydratase hexamer (FabZ) is an essential module in the elongation cycle of type-II FAS, catalyzing the dehydration of β-hydroxyacyl-lipid substrate carried by the holo form acyl carrier protein (holo-ACP). We previously elucidated an alternating seesaw-like ACP loading manner within a FabZ dimer subunits, mediated by a front-door residue Tyrosine (Tyr100). Here, we demonstrated that a back-door residue Phenylalanine (Phe83) of FabZ regulates the stepwise hexameric loading of ACP. Our finding represents clues as to the dynamic ACP recognition and catalysis mechanism of dehydratase in fatty acid biosynthesis, and provides critical information for developing antimicrobials targeting the dehydratase module in fatty acid biosynthesis pathway.

The Effect of Divalent Cations on the Thermostability of Type II Polyketide Synthase Acyl Carrier Proteins

The successful engineering of biosynthetic pathways hinges on understanding the factors that influence acyl carrier protein (ACP) stability and function. The stability and structure of ACPs can be influenced by the presence of divalent cations, but how this relates to primary sequence remains poorly understood. As part of a course-based undergraduate research experience, we investigated the thermostability of type II polyketide synthase (PKS) ACPs. We observed an approximate 40 °C range in the thermostability amongst the 14 ACPs studied, as well as an increase in stability (5 - 26 °C) of the ACPs in the presence of divalent cations. Distribution of charges in the helix II-loop-helix III region was found to impact the enthalpy of denaturation. Taken together, our results reveal clues as to how the sequence of type II PKS ACPs relates to their structural stability, information that can be used to study how ACP sequence relates to function.

An integrative, multi-omics approach towards the prioritization of Klebsiella pneumoniae drug targets

Klebsiella pneumoniae (Kp) is a globally disseminated opportunistic pathogen that can cause life-threatening infections. It has been found as the culprit of many infection outbreaks in hospital environments, being particularly aggressive towards newborns and adults under intensive care. Many Kp strains produce extended-spectrum β-lactamases, enzymes that promote resistance against antibiotics used to fight these infections. The presence of other resistance determinants leading to multidrug-resistance also limit therapeutic options, and the use of 'last-resort' drugs, such as polymyxins, is not uncommon. The global emergence and spread of resistant strains underline the need for novel antimicrobials against Kp and related bacterial pathogens. To tackle this great challenge, we generated multiple layers of 'omics' data related to Kp and prioritized proteins that could serve as attractive targets for antimicrobial development. Genomics, transcriptomics, structuromic and metabolic information were integrated in order to prioritize candidate targets, and this data compendium is freely available as a web server. Twenty-nine proteins with desirable characteristics from a drug development perspective were shortlisted, which participate in important processes such as lipid synthesis, cofactor production, and core metabolism. Collectively, our results point towards novel targets for the control of Kp and related bacterial pathogens.

Acyl-coenzyme A:(holo-acyl carrier protein) transacylase enzymes as templates for engineering

This review will cover the structure, enzymology, and related aspects that are important for structure-based engineering of the transacylase enzymes from fatty acid biosynthesis and polyketide synthesis. Furthermore, this review will focus on in vitro characteristics and not cover engineering of the upstream or downstream reactions or strategies to manipulate metabolic flux in vivo. The malonyl-coenzyme A(CoA)-holo-acyl-carrier protein (holo-ACP) transacylase (FabD) from Escherichia coli serves as a model for this enzyme with thorough descriptions of structure, enzyme mechanism, and effects of mutation on substrate binding presented in the literature. Here, we discuss multiple practical and theoretical considerations regarding engineering transacylase enzymes to accept non-cognate substrates and form novel acyl-ACPs for downstream reactions.