Structure of shikimate kinase, an in vivo essential metabolic enzyme in the nosocomial pathogen Acinetobacter baumannii, in complex with shikimate

Inhibition of Shikimate Kinase from Methicillin-Resistant Staphylococcus aureus by Benzimidazole Derivatives. Kinetic, Computational, Toxicological, and Biological Activity Studies

Antimicrobial resistance (AMR) is one of the biggest threats in modern times. It was estimated that in 2019, 1.27 million deaths occurred around the globe due to AMR. Methicillin-resistant Staphylococcus aureus (MRSA) strains, a pathogen considered of high priority by the World Health Organization, have proven to be resistant to most of the actual antimicrobial treatments. Therefore, new treatments are required to be able to manage this increasing threat. Under this perspective, an important metabolic pathway for MRSA survival, and absent in mammals, is the shikimate pathway, which is involved in the biosynthesis of chorismate, an intermediate for the synthesis of aromatic amino acids, folates, and ubiquinone. Therefore, the enzymes of this route have been considered good targets to design novel antibiotics. The fifth step of the route is performed by shikimate kinase (SK). In this study, an in-house chemical library of 170 benzimidazole derivatives was screened against MRSA shikimate kinase (SaSK). This effort led to the identification of the first SaSK inhibitors, and the two inhibitors with the greatest inhibition activity (C1 and C2) were characterized. Kinetic studies showed that both compounds were competitive inhibitors with respect to ATP and non-competitive for shikimate. Structural analysis through molecular docking and molecular dynamics simulations indicated that both inhibitors interacted with ARG113, an important residue involved in ATP binding, and formed stable complexes during the simulation period. Biological activity evaluation showed that both compounds were able to inhibit the growth of a MRSA strain. Mitochondrial assays showed that both compounds modify the activity of electron transport chain complexes. Finally, ADMETox predictions suggested that, in general, C1 and C2 can be considered as potential drug candidates. Therefore, the benzimidazole derivatives reported here are the first SaSK inhibitors, representing a promising scaffold and a guide to design new drugs against MRSA.

Multiple Metabolic Engineering Strategies to Improve Shikimate Titer in Escherichia coli

Shikimate is a valuable chiral precursor for synthesizing oseltamivir (Tamiflu^®) and other chemicals. High production of shikimate via microbial fermentation has attracted increasing attention to overcome the unstable and expensive supply of shikimate extracted from plant resources. The current cost of microbial production of shikimate via engineered strains is still unsatisfactory, and thus more metabolic strategies need to be investigated to further increase the production efficiency. In this study, we first constructed a shikimate E. coli producer through the application of the non-phosphoenolpyruvate: carbohydrate phosphotransferase system (non-PTS) glucose uptake pathway, the attenuation of the shikimate degradation metabolism, and the introduction of a mutant of feedback-resistant 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase. Inspired by the natural presence of bifunctional 3-dehydroquinate dehydratase (DHD)-shikimate dehydrogenase (SDH) enzyme in plants, we then designed an artificial fusion protein of DHD-SDH to decrease the accumulation of the byproduct 3-dehydroshikimate (DHS). Subsequently, a repressed shikimate kinase (SK) mutant was selected to promote shikimate accumulation without the supplementation of expensive aromatic substances. Furthermore, EsaR-based quorum sensing (QS) circuits were employed to regulate the metabolic flux distribution between cell growth and product synthesis. The final engineered strain dSA10 produced 60.31 g/L shikimate with a yield of 0.30 g/g glucose in a 5 L bioreactor.

Molecular analysis and essentiality of Aro1 shikimate biosynthesis multi-enzyme in Candida albicans

In the human fungal pathogen Candida albicans, ARO1 encodes an essential multi-enzyme that catalyses consecutive steps in the shikimate pathway for biosynthesis of chorismate, a precursor to folate and the aromatic amino acids. We obtained the first molecular image of C. albicans Aro1 that reveals the architecture of all five enzymatic domains and their arrangement in the context of the full-length protein. Aro1 forms a flexible dimer allowing relative autonomy of enzymatic function of the individual domains. Our activity and in cellulo data suggest that only four of Aro1's enzymatic domains are functional and essential for viability of C. albicans, whereas the 3-dehydroquinate dehydratase (DHQase) domain is inactive because of active site substitutions. We further demonstrate that in C. albicans, the type II DHQase Dqd1 can compensate for the inactive DHQase domain of Aro1, suggesting an unrecognized essential role for this enzyme in shikimate biosynthesis. In contrast, in Candida glabrata and Candida parapsilosis, which do not encode a Dqd1 homolog, Aro1 DHQase domains are enzymatically active, highlighting diversity across Candida species.

Biochemical, Kinetic, and Computational Structural Characterization of Shikimate Kinase from Methicillin-Resistant Staphylococcus aureus

One of the most widespread pathogens worldwide is methicillin-resistant Staphylococcus aureus, a bacterium that provokes severe life-threatening illnesses both in hospitals and in the community. The principal challenge lies in the resistance of MRSA to current treatments, which encourages the study of different molecular targets that could be used to develop new drugs against this infectious agent. With this goal, a detailed characterization of shikimate kinase from this microorganism (SaSK) is described. The results showed that SaSK has a K_m of 0.153 and 224 µM for shikimate and ATP, respectively, and a global reaction rate of 13.4 µmol/min/mg; it is suggested that SaSK utilizes the Bi-Bi Ping Pong reaction mechanism. Furthermore, the physicochemical data indicated that SaSK is an unstable, hydrophilic, and acidic protein. Finally, structural information showed that SaSK presented folding that is typical of its homologous counterparts and contains the typical domains of this family of proteins. Amino acids that have been shown to be important for SaSK protein function are conserved. Therefore, this study provides fundamental information that may aid in the design of inhibitors that could be used to develop new antibacterial agents.

Unravelling the Secrets of Mycobacterial Cidality through the Lens of Antisense

One of the major impediments in anti-tubercular drug discovery is the lack of a robust grammar that governs the in-vitro to the in-vivo translation of efficacy. Mycobacterium tuberculosis (Mtb) is capable of growing both extracellular as well as intracellular; encountering various hostile conditions like acidic milieu, free radicals, starvation, oxygen deprivation, and immune effector mechanisms. Unique survival strategies of Mtb have prompted researchers to develop in-vitro equivalents to simulate in-vivo physiologies and exploited to find efficacious inhibitors against various phenotypes. Conventionally, the inhibitors are screened on Mtb under the conditions that are unrelated to the in-vivo disease environments. The present study was aimed to (1). Investigate cidality of Mtb targets using a non-chemical inhibitor antisense-RNA (AS-RNA) under in-vivo simulated in-vitro conditions.(2). Confirm the cidality of the targets under in-vivo in experimental tuberculosis. (3). Correlate in-vitro vs. in-vivo cidality data to identify the in-vitro condition that best predicts in-vivo cidality potential of the targets. Using cidality as a metric for efficacy, and AS-RNA as a target-specific inhibitor, we delineated the cidality potential of five target genes under six different physiological conditions (replicating, hypoxia, low pH, nutrient starvation, nitrogen depletion, and nitric oxide).In-vitro cidality confirmed in experimental tuberculosis in BALB/c mice using the AS-RNA allowed us to identify cidal targets in the rank order of rpoB>aroK>ppk>rpoC>ilvB. RpoB was used as the cidality control. In-vitro and in-vivo studies feature aroK (encoding shikimate kinase) as an in-vivo mycobactericidal target suitable for anti-TB drug discovery. In-vitro to in-vivo cidality correlations suggested the low pH (R = 0.9856) in-vitro model as best predictor of in-vivo cidality; however, similar correlation studies in pathologically relevant (Kramnik) mice are warranted. In the acute infection phase for the high fidelity translation, the compound efficacy may also be evaluated in the low pH, in addition to the standard replication condition.

Crystal structure of 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase from the ESKAPE pathogen Acinetobacter baumannii

The enzyme 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase catalyzes the sixth step of the seven-step shikimate pathway. Chorismate, the product of the pathway, is a precursor for the biosynthesis of aromatic amino acids, siderophores and metabolites such as folate, ubiquinone and vitamin K. The shikimate pathway is present in bacteria, fungi, algae, plants and apicomplexan parasites, but is absent in humans. The EPSP synthase enzyme produces 5-enolpyruvylshikimate 3-phosphate and phosphate from phosphoenolpyruvate and shikimate 3-phosphate via a transferase reaction, and is the target of the herbicide glyphosate. The Acinetobacter baumannii gene encoding EPSP synthase, aroA, has previously been demonstrated to be essential during host infection for the growth and survival of this clinically important drug-resistant ESKAPE pathogen. Prephenate dehydrogenase is also encoded by the bifunctional A. baumannii aroA gene, but its activity is dependent upon EPSP synthase since it operates downstream of the shikimate pathway. As part of an effort to evaluate new antimicrobial targets, recombinant A. baumannii EPSP (AbEPSP) synthase, comprising residues Ala301-Gln756 of the aroA gene product, was overexpressed in Escherichia coli, purified and crystallized. The crystal structure, determined to 2.37 Å resolution, is described in the context of a potential antimicrobial target and in comparison to EPSP synthases that are resistant or sensitive to the herbicide glyphosate.