Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a repair enzyme for stalled DNA-topoisomerase 1 (Top1) cleavage complexes and other 3'-end DNA lesions. TDP1 is a perspective target for anticancer therapy based on Top1-poison-mediated DNA damage. Several novel usnic acid derivatives with an enamine moiety have been synthesized and tested as inhibitors of TDP1. The enamines of usnic acid showed IC values in the range of 0.16 to 2.0 μM. These compounds revealed moderate cytotoxicity against human tumor MCF-7 cells. These new compounds enhanced the cytotoxicity of the established Top1 poison camptothecin by an order of magnitude.

Penicillin acylase from Escherichia coli is a unique enzyme that belongs to the recently discovered superfamily of N-terminal nucleophile hydrolases. It catalyzes selective hydrolysis of the side chain amide bond of penicillins and cephalosporins while leaving the labile amide bond in the βlactam ring intact. Despite wide applications of penicillin acylase in the industry of β-lactam antibiotics and production of chiral amino compounds, its catalytic mechanism at atomic resolution has not yet been characterized. The complete cycle of chemical transformations of the most specific substrate of the enzyme, penicillin G, leading to formation of 6-aminopenicillanic and phenylacetic acids was modeled following quantum mechanics−molecular mechanics (QM/MM) calculations of the minimum energy reaction profile. The active site residues and the substrate were included in the QM part, and the rest of the system was treated applying molecular mechanics and classical force field parameters. The 3D structures in the enzyme active site corresponding to the noncovalent enzyme−substrate complex, the covalent acylenzyme intermediate, the noncovalent enzyme−product complex, the tetrahedral intermediates, and the respective transition states have been identified. QM/MM studies have shown that the α-amino group of the N-terminal catalytic βSer1 plays a key role in the catalytic machinery and directly assists its hydroxyl group in a proton relay at major stages of penicillin acylase catalytic mechanism, formation and hydrolysis of the covalent acylenzyme intermediate, which are characterized by close energy barriers. The βSer1 residue together with the oxyanion hole residues βAla69 and βAsn241 as well as βArg263 and βGln23 constitute a buried active site interaction network responsible for stabilization of tetrahedral intermediates, transition states, orientation of substrate and catalytic residues. βArg263 and βGln23 maintain the integrity of the catalytic machinery: βArg263 participates in orientation of the substrate as well as the α-amino group of βSer1 and coordinates the oxyanion hole residue βAsn241 across the whole catalytic cycle, whereas the backbone of βGln23 is responsible for orientation of both the βSer1 and the substrate. These results deliver insight into the earlier unknown ability of Nterminal amino acid to activate its own nucleophilic group directly as well as into organization of the stabilizing interaction network in penicillin acylase's active site and will be used to design more effective enzyme variants for synthesis of new penicillins and cephalosporins.

Poly(ADP-ribose) polymerase 1 (PARP1) is an enzyme involved in DNA repair, chromatin organization and transcription. During transcription initiation, PARP1 interacts with gene promoters where it binds to nucleosomes, replaces linker histone H1 and participates in gene regulation. However, the mechanisms of PARP1-nucleosome interaction remain unknown. Here, using spFRET microscopy, molecular dynamics and biochemical approaches we identified several different PARP1-nucleosome complexes and two types of PARP1 binding to mononucleosomes: at DNA ends and end-independent. Two or three molecules of PARP1 can bind to a nucleosome depending on the presence of linker DNA and can induce reorganization of the entire nucleosome that is independent of catalytic activity of PARP1. Nucleosome reorganization depends upon binding of PARP1 to nucleosomal DNA, likely near the binding site of linker histone H1. The data suggest that PARP1 can induce the formation of an alternative nucleosome state that is likely involved in gene regulation and DNA repair.

The ability of 7-methylguanine, a nucleic acid metabolite, to inhibit poly(ADP-ribose)polymerase-1 (PARP-1) and poly(ADP-ribose)polymerase-2 (PARP-2) has been identified in silico and studied experimentally. The amino group at position 2 and the methyl group at position 7 were shown to be important substituents for the efficient binding of purine derivatives to PARPs. The activity of both tested enzymes, PARP-1 and PARP-2, was suppressed by 7-methylguanine with IC50 values of 150 and 50 μM, respectively. At the PARP inhibitory concentration, 7-methylguanine itself was not cytotoxic, but it was able to accelerate apoptotic death of BRCA1-deficient breast cancer cells induced by cisplatin and doxorubicin, the widely used DNA-damaging chemotherapeutic agents. 7-Methylguanine possesses attractive predictable pharmacokinetics and an adverse-effect profile and may be considered as a new additive to chemotherapeutic treatment.

7-Methylguanine (7-MG), a natural compound that inhibits DNA repair enzyme poly(ADP-ribose) polymerase 1 (PARP-1), can be considered as a potential anticancer drug candidate. Here we describe a study of 7-MG inhibition mechanism using molecular dynamics, fluorescence anisotropy and single-particle Förster resonance energy transfer (spFRET) microscopy approaches to elucidate intermolecular interactions between 7-MG, PARP-1 and nucleosomal DNA. It is shown that 7-MG competes with substrate NAD+ and its binding in the PARP-1 active site is mediated by hydrogen bonds and nonpolar interactions with the Gly863, Ala898, Ser904, and Tyr907 residues. 7-MG promotes formation of the PARP-1–nucleosome complexes and suppresses DNA-dependent PARP-1 automodification. This results in nonproductive trapping of PARP-1 on nucleosomes and likely prevents the removal of genotoxic DNA lesions.