Jonas Drechsel

Isonitrile natural products exhibit promising antibacterial activities. However, their mechanism of action (MoA) remains largely unknown. Based on the nanomolar potency of xanthocillin X (Xan) against diverse difficult-to-treat Gram-negative bacteria, including the critical priority pathogen Acinetobacter baumannii, we performed in-depth studies to decipher its MoA. While neither metal binding nor cellular protein targets were detected as relevant for Xan’s antibiotic effects, sequencing of resistant strains revealed a conserved mutation in the heme biosynthesis enzyme porphobilinogen synthase (PbgS). This mutation caused impaired enzymatic efficiency indicative of reduced heme production. This discovery led to the validation of an untapped mechanism, by which direct heme sequestration of Xan prevents its binding into cognate enzyme pockets resulting in uncontrolled cofactor biosynthesis, accumulation of porphyrins, and corresponding stress with deleterious effects for bacterial viability. Thus, Xan represents a promising antibiotic displaying activity even against multidrug resistant strains, while exhibiting low toxicity to human cells.

Monoamine oxidases MAOA and MAOB catalyze important cellular functions such as the deamination of neurotransmitters. Correspondingly, MAO inhibitors are used for the treatment of severe neuropsychiatric disorders such as depression. A commonly prescribed drug against refractory depression is tranylcypromine, however, the side effects are poorly understood. In order to decipher putative off-targets, we synthesized two tranylcypromine probes equipped with either an alkyne moiety or an alkyne-diazirine minimal photocrosslinker for in situ proteome profiling. Surprisingly, LC–MS/MS analysis revealed low enrichment of MAOA and relatively promiscuous labeling of proteins. Photoprobe labeling paired with fluorescent imaging studies revealed lysosomal trapping which could be largely reverted by the addition of lysosomotropic drugs.

Reactive oxygen species (ROS) play an important role in the onset of Parkinson`s disease (PD) and deciphering protective mechanisms is a major goal for therapeutic development. Here, DJ-1 (PARK7) gained major attention when a conserved cysteine residue with a putative role in oxidative stress sensing/protection was linked to PD. Inspired by previous studies with a bacterial homolog of DJ-1 several amino-epoxycylcohexenones were screened for enzyme inhibition and a chemical probe with specificity for the human ortholog was selected for cellular studies. The probe selectively labeled the cysteine oxidation sensor and whole proteome analysis in HeLa, A549 and SHSY5Y cell lines confirmed strong enrichment of reduced DJ-1 as the most prominent target. Increasing levels of oxidative stress diminished this signal demonstrating the utility of our tool compound for selective in situ monitoring of this important biomarker in its reduced state.

Identifying the covalent targets of a natural product-derivative in cancer cells.

The formation of different conformers of dinuclear silver(I) and gold(I) 1,1′-(2-hydroxyethane-1,1-diyl) bridge-functionalized bis(NHC) complexes with various wing-tip substituents (R = methyl, isopropyl and mesityl) has been investigated using multinuclear NMR spectroscopy and SC-XRD as well as DFT calculations. The ratio of anti/syn isomers strongly depends both on wing-tip substituents and the metal. Moreover, the reaction temperature plays a significant role during the transmetallation process for the ratio of gold(I) conformers, which is further affected by purification procedures. All obtained Au(I)–bis(NHC) complexes have been applied in a standard MTT assay performed for screening the antiproliferative activity against human lung and liver cancer cells. Strong evidence for a significant influence of both wing-tip substituents and conformation on the cytotoxic properties of the applied complexes has been found.