Fluorometric Analysis of Carrier-Protein-Dependent Biosynthesis through a Conformationally Sensitive Solvatochromic Pantetheinamide Probe.

Carrier proteins (CPs) play a fundamental role in the biosynthesis of fatty acids, polyketides, and non-ribosomal peptides, encompassing many medicinally and pharmacologically relevant compounds. Current approaches to analyze novel carrier-protein-dependent synthetic pathways are hampered by a lack of activity-based assays for natural product biosynthesis. To fill this gap, we turned to 3-methoxychromones, highly solvatochromic fluorescent molecules whose emission intensity and wavelength are heavily dependent on their immediate molecular environment. We have developed a solvatochromic carrier-protein-targeting probe which is able to selectively fluoresce when bound to a target carrier protein. Additionally, the probe displays distinct responses upon CP binding in carrier-protein-dependent synthases. This discerning approach demonstrates the design of solvatochromic fluorophores with the ability to identify biosynthetically active CP-enzyme interactions.

Cross-Linking of the Nonribosomal Peptide Synthetase Adenylation Domain with a Carrier Protein Using a Pantetheine-Type Probe.

Adenylation domains (A-domains) are responsible for the selective incorporation of carboxylic acid substrates in the biosynthesis of nonribosomal peptides and related natural products. The A-domain transfers an acyl substrate onto its cognate carrier protein (CP). The proper interactions between an A-domain and the cognate CP are important for functional substrate transfer. To stabilize the transient interactions sufficiently for structural analysis of A-domain-CP complex, vinylsulfonamide adenosine inhibitors have been traditionally used as molecular probes. Recently, we have developed an alternative strategy using a synthetic pantetheine-type probe that enables site-specific cross-linking between an A-domain and a CP. In this chapter, we describe the laboratory protocols for this cross-linking reaction.

Amelioration of hydrolyzed guar gum on high-fat diet-induced obesity: Integrated hepatic transcriptome and metabolome.

Hydrolyzed guar gum has gained attention as an anti-obesity agent; however, few studies have focused on its role in amelioration of hepatic-associated metabolic processes. Here, the anti-obesity effect of low molecular weight hydrolyzed guar gum (GMLP, 1-10 kDa) on high-fat diet (HFD)-fed C57BL/6 J mice was investigated via transcriptome and metabolome in liver. GMLP reduced body weight gain and hepatic lipid accumulation dose-dependently, regulated blood lipid levels, and improved liver damage in HFD-fed mice. Integrated transcriptome and metabolome indicated that GMLP mainly altered lipid metabolism pathways (glycerophospholipid metabolism, glycerolipid metabolism, and fatty acid degradation), reduced disease biomarkers of ethyl glucuronide and neopterin, and increased levels of choline, flavin adenine dinucleotide, and pantetheine metabolites. Real-time quantitative PCR showed that GMLP downregulated key genes involved in de novo lipogenesis and triacylglycerol synthesis, while promoting fatty acid oxidation and choline synthesis. This study provides a theoretical basis for GMLP treatment in future clinical applications.

Controlled co-immobilisation of proteins via 4'-phosphopantetheine-mediated site-selective covalent linkage.

In Escherichia coli, acyl carrier protein (ACP) is posttranslationally converted into its active holo-ACP form via covalent linkage of 4'-phosphopantetheine (4'-PP) to residue serine-36. We found that the long flexible 4'-PP arm could react chemoselectively with the iodoacetyl group introduced on solid supports with high efficiency under mild conditions. Based on this finding, we developed site-selective immobilisation of proteins via the active holo-ACP fusion tag, independently of the physicochemical properties of the protein of interest. Furthermore, the molecular ratios of co-immobilised proteins can be manipulated because the tethering process is predominantly directed by the molar concentrations of diverse holo-ACP fusions during co-immobilisation. Conveniently tuning the molecular ratios of co-immobilised proteins allows their cooperation, leading to a highly productive multi-protein co-immobilisation system. Kinetic studies of enzymes demonstrated that α-amylase (Amy) and methyl parathion hydrolase (MPH) immobilised via active tag holo-ACP had higher catalytic efficiency (kcat/Km) in comparison with their corresponding counterparts immobilised via the sulfhydryl groups (-SH) of these proteins. The immobilised holo-ACP-Amy also presented higher thermostability compared with free Amy. The enhanced α-amylase thermostability upon immobilisation via holo-ACP renders it more suitable for industrial application.

Squalene synthase predicts poor prognosis in stage I-III colon adenocarcinoma and synergizes squalene epoxidase to promote tumor progression.

Colon adenocarcinoma (COAD) is one of the most prevalent malignancies, with poor prognosis and lack of effective treatment targets. Squalene synthase (FDFT1) is an upstream enzyme of squalene epoxidase (SQLE) in cholesterol biosynthesis. In a previous study, we revealed that SQLE promotes colon cancer cell proliferation in vitro and in vivo. Here, we investigate the prognostic value of FDFT1 in stage I-III COAD and explore the potential underlying mechanisms. Squalene synthase was significantly upregulated in stage I-III COAD and positively correlated with poor differentiation and advanced tumor stage. High expression of FDFT1 was an independent predictor of overall and relapse-free survival, and the nomograms based on FDFT1 could effectively identify patients at high risk of poor outcome. Squalene synthase accelerated colon cancer cell proliferation and promoted tumor growth. Lack of FDFT1 resulted in accumulating NAT8 and D-pantethine to lower reactive oxygen species levels and inhibit colon cancer cell proliferation. Moreover, the combined inhibition of FDFT1 and SQLE induced a greater suppressive effect on cell proliferation and tumor growth than single inhibition. Taken together, these results indicate that FDFT1 predicts poor prognosis in stage I-III COAD and has the tumor-promoting effect on COAD through regulating NAT8 and D-pantethine. Targeting both FDFT1 and SQLE is a more promising therapy than their single inhibition for stage I-III COAD.

Characterization of Phosphopantetheinyl Hydrolase from Mycobacterium tuberculosis.

Phosphopantetheinyl hydrolase, PptH (Rv2795c), is a recently discovered enzyme from Mycobacterium tuberculosis that removes 4'-phosphopantetheine (Ppt) from holo-carrier proteins (CPs) and thereby opposes the action of phosphopantetheinyl transferases (PPTases). PptH is the first structurally characterized enzyme of the phosphopantetheinyl hydrolase family. However, conditions for optimal activity of PptH have not been defined, and only one substrate has been identified. Here, we provide biochemical characterization of PptH and demonstrate that the enzyme hydrolyzes Ppt in vitro from more than one M. tuberculosis holo-CP as well as holo-CPs from other organisms. PptH provided the only detectable activity in mycobacterial lysates that dephosphopantetheinylated acyl carrier protein M (AcpM), suggesting that PptH is the main Ppt hydrolase in M. tuberculosis. We could not detect a role for PptH in coenzyme A (CoA) salvage, and PptH was not required for virulence of M. tuberculosis during infection of mice. It remains to be determined why mycobacteria conserve a broadly acting phosphohydrolase that removes the Ppt prosthetic group from essential CPs. We speculate that the enzyme is critical for aspects of the life cycle of M. tuberculosis that are not routinely modeled. IMPORTANCE Tuberculosis (TB), caused by Mycobacterium tuberculosis, was the leading cause of death from an infectious disease before COVID, yet the in vivo essentiality and function of many of the protein-encoding genes expressed by M. tuberculosis are not known. We biochemically characterize M. tuberculosis's phosphopantetheinyl hydrolase, PptH, a protein unique to mycobacteria that removes an essential posttranslational modification on proteins involved in synthesis of lipids important for the bacterium's cell wall and virulence. We demonstrate that the enzyme has broad substrate specificity, but it does not appear to have a role in coenzyme A (CoA) salvage or virulence in a mouse model of TB.

Complex structure of the acyltransferase VinK and the carrier protein VinL with a pantetheine cross-linking probe.

Acyltransferases are responsible for the selection and loading of acyl units onto carrier proteins in polyketide and fatty-acid biosynthesis. Despite the importance of protein-protein interactions between the acyltransferase and the carrier protein, structural information on acyltransferase-carrier protein interactions is limited because of the transient interactions between them. In the biosynthesis of the polyketide vicenistatin, the acyltransferase VinK recognizes the carrier protein VinL for the transfer of a dipeptidyl unit. The crystal structure of a VinK-VinL covalent complex formed with a 1,2-bismaleimidoethane cross-linking reagent has been determined previously. Here, the crystal structure of a VinK-VinL covalent complex formed with a pantetheine cross-linking probe is reported at 1.95 Šresolution. In the structure of the VinK-VinL-probe complex, the pantetheine probe that is attached to VinL is covalently connected to the side chain of the mutated Cys106 of VinK. The interaction interface between VinK and VinL is essentially the same in the two VinK-VinL complex structures, although the position of the pantetheine linker slightly differs. This structural observation suggests that interface interactions are not affected by the cross-linking strategy used.

Natural Molecules and Neuroprotection: Kynurenic Acid, Pantethine and α-Lipoic Acid.

The incidence of neurodegenerative diseases has increased greatly worldwide due to the rise in life expectancy. In spite of notable development in the understanding of these disorders, there has been limited success in the development of neuroprotective agents that can slow the progression of the disease and prevent neuronal death. Some natural products and molecules are very promising neuroprotective agents because of their structural diversity and wide variety of biological activities. In addition to their neuroprotective effect, they are known for their antioxidant, anti-inflammatory and antiapoptotic effects and often serve as a starting point for drug discovery. In this review, the following natural molecules are discussed: firstly, kynurenic acid, the main neuroprotective agent formed via the kynurenine pathway of tryptophan metabolism, as it is known mainly for its role in glutamate excitotoxicity, secondly, the dietary supplement pantethine, that is many sided, well tolerated and safe, and the third molecule, α-lipoic acid is a universal antioxidant. As a conclusion, because of their beneficial properties, these molecules are potential candidates for neuroprotective therapies suitable in managing neurodegenerative diseases.

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.

Chemical Proteomic Profiling of Protein 4'-Phosphopantetheinylation in Mammalian Cells.

Protein 4'-phosphopantetheinylation is an essential post-translational modification (PTM) in prokaryotes and eukaryotes. So far, only five protein substrates of this specific PTM have been discovered in mammalian cells. These proteins are known to perform important functions, including fatty acid biosynthesis and folate metabolism, as well as ß-alanine activation. To explore existing and new substrates of 4'-phosphopantetheinylation in mammalian proteomes, we designed and synthesized a series of new pantetheine analogue probes, enabling effective metabolic labelling of 4'-phosphopantetheinylated proteins in HepG2 cells. In combination with a quantitative chemical proteomic platform, we enriched and identified all the currently known 4'-phosphopantetheinylated proteins with high confidence, and unambiguously determined their exact sites of modification. More encouragingly, we discovered, using targeted chemical proteomics, a potential 4'-phosphopantetheinylation site in the protein of mitochondrial dehydrogenase/reductase SDR family member 2 (DHRS2).

Shifting the Hydrolysis Equilibrium of Substrate Loaded Acyl Carrier Proteins.

Acyl carrier proteins (ACP)s transport intermediates through many primary and secondary metabolic pathways. Studying the effect of substrate identity on ACP structure has been hindered by the lability of the thioester bond that attaches acyl substrates to the 4'-phosphopantetheine cofactor of ACP. Here we show that an acyl acyl-carrier protein synthetase (AasS) can be used in real time to shift the hydrolysis equilibrium toward favoring acyl-ACP during solution NMR spectroscopy. Only 0.005 molar equivalents of AasS enables 1 week of stability to palmitoyl-AcpP from Escherichia coli. 2D NMR spectra enabled with this method revealed that the tethered palmitic acid perturbs nearly every secondary structural region of AcpP. This technique will allow previously unachievable structural studies of unstable acyl-ACP species, contributing to the understanding of these complex biosynthetic pathways.

Decrypting the oscillating nature of the 4'-phosphopantetheine arm in acyl carrier protein AcpM of Mycobacterium tuberculosis.

In Mycobacterium tuberculosis, acyl carrier protein (AcpM)-mediated fatty acid synthase type II is integral for the synthesis of mycolic acids. AcpM, designated as an atypical ACP, comprises of a putative 33 amino acid long C-terminal extension which is distinctive in nature. Here, we aimed at devising an 'easy-to-go' method for the generation of crypto-AcpM loaded with a solvatochromic probe 7-Nitrobenz-2-oxa-1,3-diazol-4-yl, which is linked to the 4'-phosphopantetheine (Ppant) prosthetic group of AcpM. The crypto-AcpM, coupled with fluorescence spectroscopy and molecular dynamics simulation studies, was employed to explore the elusive dynamics of Ppant arm in AcpM. This investigation establishes the role of the flexible C-terminal extension of AcpM in regulating the prosthetic group sequestration ability by modulating the 'Asp-Ser-Leu' motif.

Dipeptide synthesis by internal adenylation domains of a multidomain enzyme involved in nonribosomal peptide synthesis.

The adenylation domain of nonribosomal peptide synthetase (NRPS) is responsible for its selective substrate recognition and activation of the substrate (yielding an acyl-O-AMP intermediate) on ATP consumption. DhbF is an NRPS involved in bacillibactin synthesis and consists of multiple domains [adenylation domain, condensation domain, peptidyl carrier protein (PCP) domain, and thioesterase domain]; DhbFA1 and DhbFA2 (here named) are "internal" adenylation domains in the multidomain enzyme DhbF. We firstly succeeded in expressing and purifying the "internal" adenylation domains DhbFA1 and DhbFA2 separately. Furthermore, we initially demonstrated dipeptide synthesis by "internal" adenylation domains. When glycine and L-cysteine were used as substrates of DhbFA1, the formation of N-glycyl-L-cysteine (Gly-Cys) was observed. Furthermore, when L-threonine and L-cysteine were used as substrates of DhbFA2, N-L-threonyl-L-cysteine (Thr-Cys) was formed. These findings showed that both adenylation domains produced dipeptides by forming a carbon-nitrogen bond comprising the carboxyl group of an amino acid and the amino group of L-cysteine, although these adenylation domains are acid-thiol ligase using 4'-phosphopantetheine (bound to the PCP domain) as a substrate. Furthermore, DhbFA1 and DhbFA2 synthesized oligopeptides as well as dipeptides.

Pantethine Down-Regulates Leukocyte Recruitment and Inflammatory Parameters in a Mouse Model of Allergic Airway Inflammation.

BACKGROUND Migration of leukocytes into airways is the hallmark of allergic asthma. The aim of this study was to target the pathological process using pantethine, a pleiotropic natural compound which has been recently shown to down-regulate chemokine-driven T cell migration. MATERIAL AND METHODS Mice were sensitized to the Leishmania LACK antigen, then treated or not treated with pantethine and exposed to LACK or saline aerosol. After sacrifice of the animals, cells in the bronchoalveolar lavage were analyzed and inflammatory parameters were determined to evaluate inflammation seriousness. RESULTS As compared to untreated animals, pantethine-treated animals displayed a moderated response to the allergen, as documented by decreased infiltration of inflammatory cells (all types), in addition to reduced levels of lung Th2 cytokines and circulating LACK-specific IgE. CONCLUSIONS These data reveal the potential therapeutic importance of pantethine to moderate allergic asthma pathology. The compound has been previously shown to exert a broad range of protective activity in animals and in humans, with few or no adverse effects.

Ratiometric Fluorescent Probe for Imaging of Pantetheinase in Living Cells.

Pantetheinase, which catalyzes the cleavage of pantetheine to pantothenic acid (vitamin B5) and cysteamine, is involved in the regulation of oxidative stress, pantothenate recycling and cell migration. However, further elucidating the cellular function of this enzyme is largely limited by the lack of a suitable fluorescence imaging probe. By conjugating pantothenic acid with cresyl violet, herein we develop a new fluorescence probe CV-PA for the assay of pantetheinase. The probe not only possesses long analytical wavelengths but also displays linear ratiometric (I628/582 nm) fluorescence response to pantetheinase in the range of 5-400 ng/mL with a detection limit of 4.7 ng/mL. This probe has been used to evaluate the efficiency of different inhibitors and quantitatively detect pantetheinase in serum samples, revealing that pantetheinase in fetal bovine serum and new born calf serum is much higher than that in normal human serum. Notably, with the probe the ratiometric imaging and in situ quantitative comparison of pantetheinase in different living cells (LO2 and HK-2) have been achieved for the first time. It is found that the level of pantetheinase in LO2 cells is much larger than that in HK-2 cells, as further validated by Western blot analysis. The proposed probe may be useful to better understand the specific function of pantetheinase in the pantetheinase-related pathophysiological processes.

Atomic structure of the entire mammalian mitochondrial complex I.

Mitochondrial complex I (also known as NADH:ubiquinone oxidoreductase) contributes to cellular energy production by transferring electrons from NADH to ubiquinone coupled to proton translocation across the membrane. It is the largest protein assembly of the respiratory chain with a total mass of 970 kilodaltons. Here we present a nearly complete atomic structure of ovine (Ovis aries) mitochondrial complex I at 3.9 Å resolution, solved by cryo-electron microscopy with cross-linking and mass-spectrometry mapping experiments. All 14 conserved core subunits and 31 mitochondria-specific supernumerary subunits are resolved within the L-shaped molecule. The hydrophilic matrix arm comprises flavin mononucleotide and 8 iron-sulfur clusters involved in electron transfer, and the membrane arm contains 78 transmembrane helices, mostly contributed by antiporter-like subunits involved in proton translocation. Supernumerary subunits form an interlinked, stabilizing shell around the conserved core. Tightly bound lipids (including cardiolipins) further stabilize interactions between the hydrophobic subunits. Subunits with possible regulatory roles contain additional cofactors, NADPH and two phosphopantetheine molecules, which are shown to be involved in inter-subunit interactions. We observe two different conformations of the complex, which may be related to the conformationally driven coupling mechanism and to the active-deactive transition of the enzyme. Our structure provides insight into the mechanism, assembly, maturation and dysfunction of mitochondrial complex I, and allows detailed molecular analysis of disease-causing mutations.

Coenzyme A: to make it or uptake it?

The consensus has been that intracellular coenzyme A (CoA) is obtained exclusively by de novo biosynthesis via a universal, conserved five-step pathway in the cell cytosol. However, old and new evidence suggest that cells (and some microorganisms) have several strategies to obtain CoA, with 4'-phosphopantetheine (P-PantSH; the fourth intermediate in the canonical CoA biosynthetic pathway) serving as a 'nexus' metabolite.

Synthetic cycle of the initiation module of a formylating nonribosomal peptide synthetase.

Nonribosomal peptide synthetases (NRPSs) are very large proteins that produce small peptide molecules with wide-ranging biological activities, including environmentally friendly chemicals and many widely used therapeutics. NRPSs are macromolecular machines, with modular assembly-line logic, a complex catalytic cycle, moving parts and many active sites. In addition to the core domains required to link the substrates, they often include specialized tailoring domains, which introduce chemical modifications and allow the product to access a large expanse of chemical space. It is still unknown how the NRPS tailoring domains are structurally accommodated into megaenzymes or how they have adapted to function in nonribosomal peptide synthesis. Here we present a series of crystal structures of the initiation module of an antibiotic-producing NRPS, linear gramicidin synthetase. This module includes the specialized tailoring formylation domain, and states are captured that represent every major step of the assembly-line synthesis in the initiation module. The transitions between conformations are large in scale, with both the peptidyl carrier protein domain and the adenylation subdomain undergoing huge movements to transport substrate between distal active sites. The structures highlight the great versatility of NRPSs, as small domains repurpose and recycle their limited interfaces to interact with their various binding partners. Understanding tailoring domains is important if NRPSs are to be utilized in the production of novel therapeutics.

Structures of two distinct conformations of holo-non-ribosomal peptide synthetases.

Many important natural products are produced by multidomain non-ribosomal peptide synthetases (NRPSs). During synthesis, intermediates are covalently bound to integrated carrier domains and transported to neighbouring catalytic domains in an assembly line fashion. Understanding the structural basis for catalysis with non-ribosomal peptide synthetases will facilitate bioengineering to create novel products. Here we describe the structures of two different holo-non-ribosomal peptide synthetase modules, each revealing a distinct step in the catalytic cycle. One structure depicts the carrier domain cofactor bound to the peptide bond-forming condensation domain, whereas a second structure captures the installation of the amino acid onto the cofactor within the adenylation domain. These structures demonstrate that a conformational change within the adenylation domain guides transfer of intermediates between domains. Furthermore, one structure shows that the condensation and adenylation domains simultaneously adopt their catalytic conformations, increasing the overall efficiency in a revised structural cycle. These structures and the single-particle electron microscopy analysis demonstrate a highly dynamic domain architecture and provide the foundation for understanding the structural mechanisms that could enable engineering of novel non-ribosomal peptide synthetases.

Extracellular 4'-phosphopantetheine is a source for intracellular coenzyme A synthesis.

The metabolic cofactor coenzyme A (CoA) gained renewed attention because of its roles in neurodegeneration, protein acetylation, autophagy and signal transduction. The long-standing dogma is that eukaryotic cells obtain CoA exclusively via the uptake of extracellular precursors, especially vitamin B5, which is intracellularly converted through five conserved enzymatic reactions into CoA. This study demonstrates an alternative mechanism that allows cells and organisms to adjust intracellular CoA levels by using exogenous CoA. Here CoA was hydrolyzed extracellularly by ectonucleotide pyrophosphatases to 4'-phosphopantetheine, a biologically stable molecule able to translocate through membranes via passive diffusion. Inside the cell, 4'-phosphopantetheine was enzymatically converted back to CoA by the bifunctional enzyme CoA synthase. Phenotypes induced by intracellular CoA deprivation were reversed when exogenous CoA was provided. Our findings answer long-standing questions in fundamental cell biology and have major implications for the understanding of CoA-related diseases and therapies.