Mechanismen und Funktionen von bakteriellen AAA+­Entfaltungsmaschinen

Bacterial AAA+ proteins play crucial roles in proteostasis networks and ensure protein homeostasis during stress conditions. They function as ATP-dependent components of proteolytic complexes degrading misfolded proteins or as disaggregases reactivating aggregated proteins. AAA+ proteins generate an ATP-fueled threading force driving substrate unfolding and translocation. Their central functions in protein quality control qualify them as antibacterial drug target.

The Discovery of Antibacterial Natural Compound Based on Peptide Deformylase

Background: In recent years, Staphylococcus aureus have developed resistance to medicines used for the treatment of human infections. Therefore, the search for antibacterial agents of high potency against Staphylococcus aureus is of great concern. Peptide deformylase (PDF), a metalloprotease catalyzing the removal of a formyl group from newly synthesized proteins, has been considered to be an important antibacterial drug target. Objective: To discover novel antibacterial drugs based on Staphylococcus aureus peptide deformylase. Method: PDF-based virtual screening of compounds from Traditional Chinese Medicine Database@Taiwan was performed by Sybyl X2.1 Surflex dock software. Compounds which possess high docking score were used for the following antibacterial experiments to evaluate their antibacterial activities. Kanamycin was also used in the antibacterial experiment as a control substance in the assay. Furthermore, molecular docking studies was applied to elucidate binding interaction between some compounds and PDF. In silico pharmacokinetic and toxicity prediction was explored to explain the reasons why these compounds might stand good chance of providing some pharmaceutical benefits. Results: Gentiopicroside, protosappanin B, dihydromyricetin and cryptochlorogenic acid with high docking score were used for our subsequent antibacterial assays. The Minimum Inhibitory Concentration (MIC) of kanamycin and gentiopicroside were 0.008 mg·mL-1 and 0.431 mg·mL-1, respectively, other three compounds, protosappanin B, dihydromyricetin and cryptochlorogenic acid have close MIC value of 0.50 mg·mL-1. Conclusion: Dihydromyricetin, with the MIC value of 0.50 mg·mL-1 and relatively high drug score of 0.82, may serve as a novel antibacterial lead compound.

Mechanism and catalytic strategy of the prokaryotic-specific GTP cyclohydrolase-IB

Guanosine 5′-triphosphate (GTP) cyclohydrolase-I (GCYH-I) catalyzes the first step in folic acid biosynthesis in bacteria and plants, biopterin biosynthesis in mammals, and the biosynthesis of 7-deazaguanosine-modified tRNA nucleosides in bacteria and archaea. The type IB GCYH (GCYH-IB) is a prokaryotic-specific enzyme found in many pathogens. GCYH-IB is structurally distinct from the canonical type IA GCYH involved in biopterin biosynthesis in humans and animals, and thus is of interest as a potential antibacterial drug target. We report kinetic and inhibition data of Neisseria gonorrhoeae GCYH-IB and two high-resolution crystal structures of the enzyme; one in complex with the reaction intermediate analog and competitive inhibitor 8-oxoguanosine 5′-triphosphate (8-oxo-GTP), and one with a tris(hydroxymethyl)aminomethane molecule bound in the active site and mimicking another reaction intermediate. Comparison with the type IA enzyme bound to 8-oxo-GTP (guanosine 5′-triphosphate) reveals an inverted mode of binding of the inhibitor ribosyl moiety and, together with site-directed mutagenesis data, shows that the two enzymes utilize different strategies for catalysis. Notably, the inhibitor interacts with a conserved active-site Cys149, and this residue is S-nitrosylated in the structures. This is the first structural characterization of a biologically S-nitrosylated bacterial protein. Mutagenesis and biochemical analyses demonstrate that Cys149 is essential for the cyclohydrolase reaction, and S-nitrosylation maintains enzyme activity, suggesting a potential role of the S-nitrosothiol in catalysis.

Two-metal versus one-metal mechanisms of lysine adenylylation by ATP-dependent and NAD+-dependent polynucleotide ligases

Polynucleotide ligases comprise a ubiquitous superfamily of nucleic acid repair enzymes that join 3′-OH and 5′-PO4DNA or RNA ends. Ligases react with ATP or NAD+and a divalent cation cofactor to form a covalent enzyme-(lysine-Nζ)–adenylate intermediate. Here, we report crystal structures of the founding members of the ATP-dependent RNA ligase family (T4 RNA ligase 1; Rnl1) and the NAD+-dependent DNA ligase family (Escherichia coliLigA), captured as their respective Michaelis complexes, which illuminate distinctive catalytic mechanisms of the lysine adenylylation reaction. The 2.2-Å Rnl1•ATP•(Mg2+)2structure highlights a two-metal mechanism, whereby: a ligase-bound “catalytic” Mg2+(H2O)5coordination complex lowers the pKaof the lysine nucleophile and stabilizes the transition state of the ATP α phosphate; a second octahedral Mg2+coordination complex bridges the β and γ phosphates; and protein elements unique to Rnl1 engage the γ phosphate and associated metal complex and orient the pyrophosphate leaving group for in-line catalysis. By contrast, the 1.55-Å LigA•NAD+•Mg2+structure reveals a one-metal mechanism in which a ligase-bound Mg2+(H2O)5complex lowers the lysine pKaand engages the NAD+α phosphate, but the β phosphate and the nicotinamide nucleoside of the nicotinamide mononucleotide (NMN) leaving group are oriented solely via atomic interactions with protein elements that are unique to the LigA clade. The two-metal versus one-metal dichotomy demarcates a branchpoint in ligase evolution and favors LigA as an antibacterial drug target.

A High-Throughput Fluorescence Polarization Assay for Inhibitors of Gyrase B

DNA gyrase, a type II topoisomerase that introduces negative supercoils into DNA, is a validated antibacterial drug target. The holoenzyme is composed of 2 subunits, gyrase A (GyrA) and gyrase B (GyrB), which form a functional A2B2 heterotetramer required for bacterial viability. A novel fluorescence polarization (FP) assay has been developed and optimized to detect inhibitors that bind to the adenosine triphosphate (ATP) binding domain of GyrB. Guided by the crystal structure of the natural product novobiocin bound to GyrB, a novel novobiocin–Texas Red probe (Novo-TRX) was designed and synthesized for use in a high-throughput FP assay. The binding kinetics of the interaction of Novo-TRX with GyrB from Francisella tularensis has been characterized, as well as the effect of common buffer additives on the interaction. The assay was developed into a 21-µL, 384-well assay format and has been validated for use in high-throughput screening against a collection of Food and Drug Administration–approved compounds. The assay performed with an average Z′ factor of 0.80 and was able to identify GyrB inhibitors from a screening library.