Hanna-Kirsti Schroder Leiros

DNA ligases are foundational molecular-biological tools used for cloning and sequencing workflows, and are essential replicative enzymes for all cellular life forms as well as many viruses and bacteriophage. There is considerable interest in structurally and functionally characterizing novel DNA ligases and profiling their suitability for molecular-biological applications. Here, we report the crystal structure of the ATP-dependent DNA ligase from the Rhizobium phage vB_RleM_P10VF bound to a nicked DNA duplex determined to 2.2 Å resolution. The enzyme crystallized in the DNA-encircling conformation, arrested as a step 2 intermediate in the catalytic cycle with the adenylating cofactor transferred to the 5′-phosphate of the DNA nick. The overall structure of the DNA ligase closely resembles that of the T4 DNA ligase, including an α-helical globular DNA-binding domain. Several secondary-structural elements are abbreviated in the P10VF DNA ligase relative to the T4 DNA ligase enzyme, which may account for its lower specific activity, especially on DNA substrates containing double-stranded breaks.

Antibiotic resistance is a growing global health threat that risks the lives of millions. Among the resistance mechanisms, that mediated by metallo-β-lactamases is of particular concern as these bacterial enzymes dismantle most β-lactam antibiotics, which are our widest applied and cheapest to produce antibiotic agents. So far, no clinically applicable metallo-β-lactamase inhibitors are available. Aiming to adapt to structural variations, we introduce the inhibitor concept: dynamically chiral phosphonic acids. We demonstrate that they are straightforward to synthesize, penetrate bacterial membranes, inhibit the metallo-β-lactamase enzymes NDM-1, VIM-2 and GIM-1, and are non-toxic to human cells. Mimicking the transition state of β-lactam hydrolysis, they target the Zn ions of the metallo-β-lactamase active site. As a unique feature, both of their stereoisomers bind metallo-β-lactamases, which provides them unparalleled adaptability to the structural diversity of these enzymes, and may allow them to hamper bacteria’s ability for resistance development.

The crystal structure of the class D β-lactamase OXA-436 was solved to a resolution of 1.80 Å. Higher catalytic rates were found at higher temperatures for the clinically important antibiotic imipenem, indicating better adaptation of OXA-436 to its mesophilic host than OXA-48, which is believed to originate from an environmental source. Furthermore, based on the most populated conformations during 100 ns molecular-dynamics simulations, it is postulated that the modulation of activity involves conformational shifts of the α3–α4 and β5–β6 loops. While these changes overall do not cause clinically significant shifts in the resistance profile, they show that antibiotic-resistance enzymes exist in a continuum. It is believed that these seemingly neutral differences in the sequence exist on a path leading to significant changes in substrate selectivity.

Very low antibiotic concentrations have been shown to drive the evolution of antimicrobial resistance. While substantial progress has been made to understand the driving role of low concentrations during resistance development for different antimicrobial classes, the importance of β-lactams, the most commonly used antibiotics, is still poorly studied.

Our current understanding of how low antibiotic concentrations shape the evolution of contemporary β-lactamases is limited. Using the wide-spread carbapenemase OXA-48, we tested the long-standing hypothesis that selective compartments with low antibiotic concentrations cause standing genetic diversity that could act as a gateway to develop clinical resistance. Here, we subjected Escherichia coli expressing bla _OXA-48 , on a clinical plasmid, to experimental evolution at sub-minimum inhibitory concentrations (sub-MIC) of ceftazidime. We identified and characterized seven single variants of OXA-48. Susceptibility profiles and dose-response curves showed that they increased resistance only marginally. However, in competition experiments at sub-MIC of ceftazidime, they showed strong selectable fitness benefits. Increased resistance was also reflected in elevated catalytic efficiencies towards ceftazidime. These changes are likely caused by enhanced flexibility of the Ω- and β5-β6 loops. In conclusion, low-level concentrations of β-lactams can drive the evolution of β-lactamases through cryptic phenotypes which may act as stepping-stones towards clinical resistance.

Carbapenemases are the main cause of carbapenem resistance in Gram‐negative bacteria. How β‐lactamases with weak carbapenemase activity, such as the OXA‐10‐type class D β‐lactamases, contribute to anti‐bacterial drug resistance is unclear. OXA‐655 is a T26M and V117L OXA‐10 variant, recently identified from hospital wastewater. Despite exhibiting stronger carbapenemase activity towards ertapenem (ETP) and meropenem (MEM) in Escherichia coli, OXA‐655 exhibits reduced activity towards oxyimino‐substituted β‐lactams like ceftazidime. Here, we have solved crystal structures of OXA‐10 in complex with imipenem (IPM) and ETP, and OXA‐655 in complex with MEM in order to unravel the structure–function relationship and the impact of residue 117 in enzyme catalysis. The new crystal structures show that L117 is situated at a critical position with enhanced Van der Waals interactions to L155 in the omega loop. This restricts the movements of L155 and could explain the reduced ability for OXA‐655 to bind a bulky oxyimino group. The V117L replacement in OXA‐655 makes the active site S67 and the carboxylated K70 more water exposed. This could affect the supply of new deacylation water molecules required for hydrolysis and possibly the carboxylation rate of K70. But most importantly, L117 leaves more space for binding of the hydroxyethyl group in carbapenems. In summary, the crystal structures highlight the importance of residue 117 in OXA‐10 variants for carbapenemase activity. This study also illustrates the impact of a single amino acid substitution on the substrate profile of OXA‐10 and the evolutionary potential of new OXA‐10 variants.

MBLs form a large and heterogeneous group of bacterial enzymes conferring resistance to β-lactam antibiotics, including carbapenems. A large environmental reservoir of MBLs has been identified, which can act as a source for transfer into human pathogens. Therefore, structural investigation of environmental and clinically rare MBLs can give new insights into structure–activity relationships to explore the role of catalytic and second shell residues, which are under selective pressure. To investigate the structure and activity of the environmental subclass B1 MBLs MYO-1, SHD-1 and ECV-1. The respective genes of these MBLs were cloned into vectors and expressed in Escherichia coli. Purified enzymes were characterized with respect to their catalytic efficiency (k_cat/K_m). The enzymatic activities and MICs were determined for a panel of different β-lactams, including penicillins, cephalosporins and carbapenems. Thermostability was measured and structures were solved using X-ray crystallography (MYO-1 and ECV-1) or generated by homology modelling (SHD-1). Expression of the environmental MBLs in E. coli resulted in the characteristic MBL profile, not affecting aztreonam susceptibility and decreasing susceptibility to carbapenems, cephalosporins and penicillins. The purified enzymes showed variable catalytic activity in the order of <5% to ∼70% compared with the clinically widespread NDM-1. The thermostability of ECV-1 and SHD-1 was up to 8°C higher than that of MYO-1 and NDM-1. Using solved structures and molecular modelling, we identified differences in their second shell composition, possibly responsible for their relatively low hydrolytic activity. These results show the importance of environmental species acting as reservoirs for MBL-encoding genes.

Carbapenem-resistant Gram-negative pathogens are a critical public health threat and there is an urgent need for new treatments. Carbapenemases (β-lactamases able to inactivate carbapenems) have been identified in both serine β-lactamase (SBL) and metallo β-lactamase (MBL) families. The recent introduction of SBL carbapenemase-inhibitors has provided alternative therapeutic options. Unfortunately, there are no approved inhibitors of MBL-mediated carbapenem-resistance and treatment options for infections caused by MBL-producing Gram-negatives are limited. Here, we present ZN148, a zinc-chelating MBL-inhibitor capable of restoring the bactericidal effect of meropenem and in vitro clinical susceptibility to carbapenems in >98% of a large international collection of MBL-producing clinical Enterobacterales strains (n=234). Moreover, ZN148 was able to potentiate the effect of meropenem against NDM-1-producing Klebsiella pneumoniae in a murine neutropenic peritonitis model. ZN148 showed no inhibition of the human zinc-containing enzyme glyoxylase II at 500 μM and no acute toxicity was observed in an in vivo mouse model with cumulative dosages up to 128 mg/kg. Biochemical analysis showed a time-dependent inhibition of MBLs by ZN148 and removal of zinc ions from the active site. Addition of exogenous zinc after ZN148 exposure only restored MBL activity by ∼30%, suggesting an irreversible mechanism of inhibition. Mass-spectrometry and molecular modelling indicated potential oxidation of the active site Cys221 residue. Overall, these results demonstrate the therapeutic potential of a ZN148-carbapenem combination against MBL-producing Gram-negative pathogens and that ZN148 is a highly promising MBL inhibitor, capable of operating in a functional space not presently filled by any clinically approved compound.

DNA ligases join adjacent 5′ phosphate (5′P) and 3′ hydroxyl (3′OH) termini of double-stranded DNA via a three-step mechanism requiring a nucleotide cofactor and divalent metal ion. Although considerable structural detail is available for the first two steps, less is known about step 3 where the DNA-backbone is joined or about the cation role at this step. We have captured high-resolution structures of an adenosine triphosphate (ATP)-dependent DNA ligase from Prochlorococcus marinus including a Mn-bound pre-ternary ligase–DNA complex poised for phosphodiester bond formation, and a post-ternary intermediate retaining product DNA and partially occupied AMP in the active site. The pre-ternary structure unambiguously identifies the binding site of the catalytic metal ion and confirms both its role in activating the 3′OH terminus for nucleophilic attack on the 5′P group and stabilizing the pentavalent transition state. The post-ternary structure indicates that DNA distortion and most enzyme-AMP contacts remain after phosphodiester bond formation, implying loss of covalent linkage to the DNA drives release of AMP, rather than active site rearrangement. Additionally, comparisons of this cyanobacterial DNA ligase with homologs from bacteria and bacteriophage pose interesting questions about the structural origin of double-strand break joining activity and the evolution of these ATP-dependent DNA ligase enzymes.