Kinetic and structural characterization of dihydrofolate reductase from Streptococcus pneumoniae

In vitro and in silico studies for the identification of anti-cancer and antibacterial peptides from camel milk protein hydrolysates

Today, breast cancer and infectious diseases are very worrying that led to a widespread effort by researchers to discover natural remedies with no side effects to fight them. In the present study, we isolated camel milk protein fractions, casein and whey proteins, and hydrolyzed them using pepsin, trypsin, and both enzymes. Screening of peptides with anti-breast cancer and antibacterial activity against pathogens was performed. Peptides derived from whey protein fraction with the use of both enzymes showed very good activity against MCF-7 breast cancer with cell viability of 7.13%. The separate use of trypsin and pepsin to digest whey protein fraction yielded peptides with high antibacterial activity against S. aureus (inhibition zone of 4.17 ± 0.30 and 4.23 ± 0.32 cm, respectively) and E. coli (inhibition zone of 4.03 ± 0.15 and 4.03 ± 0.05 cm, respectively). Notably, in order to identify the effective peptides in camel milk, its protein sequences were retrieved and enzymatically digested in silico. Peptides that showed both anticancer and antibacterial properties and the highest stability in intestinal conditions were selected for the next step. Molecular interaction analysis was performed on specific receptors associated with breast cancer and/or antibacterial activity using molecular docking. The results showed that P3 (WNHIKRYF) and P5 (WSVGH) peptides had low binding energy and inhibition constant so that they specifically occupied active sites of protein targets. Our results introduced two peptide-drug candidates and new natural food additive that can be delivered to further animal and clinical trials.

Temperature-Dependent Kinetic Isotope Effects in R67 Dihydrofolate Reductase from Path-Integral Simulations

Calculation of temperature-dependent kinetic isotope effects (KIE) in enzymes presents a significant theoretical challenge. Additionally, it is not trivial to identify enzymes with available experimental accurate intrinsic KIEs in a range of temperatures. In the current work, we present a theoretical study of KIEs in the primitive R67 dihydrofolate reductase (DHFR) enzyme and compare with experimental work. The advantage of R67 DHFR is its significantly lower kinetic complexity compared to more evolved DHFR isoforms. We employ mass-perturbation-based path-integral simulations in conjunction with umbrella sampling and a hybrid quantum mechanics-molecular mechanics Hamiltonian. We obtain temperature-dependent KIEs in good agreement with experiments and ascribe the temperature-dependent KIEs primarily to zero-point energy effects. The active site in the primitive enzyme is found to be poorly preorganized, which allows excessive water access to the active site and results in loosely bound reacting ligands.

Simultaneous Control of Endogenous and User-Defined Genetic Pathways Using Unique ecDHFR Pharmacological Chaperones

Destabilizing domains (DDs), such as a mutated form of Escherichia coli dihydrofolate reductase (ecDHFR), confer instability and promote protein degradation. However, when combined with small-molecule stabilizers (e.g., the antibiotic trimethoprim), DDs allow positive regulation of fusion protein abundance. Using a combinatorial screening approach, we identified and validated 17 unique 2,4-diaminopyrimidine/triazine-based ecDHFR DD stabilizers, at least 15 of which were ineffective antibiotics against E. coli and S. aureus. Identified stabilizers functioned in vivo to control an ecDHFR DD-firefly luciferase in the mouse eye and/or the liver. Next, stabilizers were leveraged to perform synergistic dual functions in vitro (HeLa cell death sensitization) and in vivo (repression of ocular inflammation) by stabilizing a user-defined ecDHFR DD while also controlling endogenous signaling pathways. Thus, these newly identified pharmacological chaperones allow for simultaneous control of compound-specific endogenous and user-defined genetic pathways, the combination of which may provide synergistic effects in complex biological scenarios.

The crystal structure of a tetrahydrofolate-bound dihydrofolate reductase reveals the origin of slow product release

Dihydrofolate reductase (DHFR) catalyzes the stereospecific reduction of 7,8-dihydrofolate (FH2) to (6s)-5,6,7,8-tetrahydrofolate (FH4) via hydride transfer from NADPH. The consensus Escherichia coli DHFR mechanism involves conformational changes between closed and occluded states occurring during the rate-limiting product release step. Although the Protein Data Bank (PDB) contains over 250 DHFR structures, the FH4 complex structure responsible for rate-limiting product release is unknown. We report to our knowledge the first crystal structure of an E. coli. DHFR:FH4 complex at 1.03 Å resolution showing distinct stabilizing interactions absent in FH2 or related (6R)-5,10-dideaza-FH4 complexes. We discover the time course of decay of the co-purified endogenous FH4 during crystal growth, with conversion from FH4 to FH2 occurring in 2–3 days. We also determine another occluded complex structure of E. coli DHFR with a slow-onset nanomolar inhibitor that contrasts with the methotrexate complex, suggesting a plausible strategy for designing DHFR antibiotics by targeting FH4 product conformations.

Hongnan Cao et al. present the X-ray crystal structure of E. coli dihydrofolate reductase (DHFR) in complex with its reduced substrate, (6s)-5,6,7,8-tetrahydrofolate (FH4). This structure provides the first glimpse of the rate-limiting product release step of the DHFR mechanism and suggests a strategy for designing DHFR-targeting antibiotics.

eModel-BDB: a database of comparative structure models of drug-target interactions from the Binding Database

Background

The structural information on proteins in their ligand-bound conformational state is invaluable for protein function studies and rational drug design. Compared to the number of available sequences, not only is the repertoire of the experimentally determined structures of holo-proteins limited, these structures do not always include pharmacologically relevant compounds at their binding sites. In addition, binding affinity databases provide vast quantities of information on interactions between drug-like molecules and their targets, however, often lacking structural data. On that account, there is a need for computational methods to complement existing repositories by constructing the atomic-level models of drug-protein assemblies that will not be determined experimentally in the near future.

Results

We created eModel-BDB, a database of  200,005 comparative models of drug-bound proteins based on   1,391,403 interaction data obtained from the Binding Database and the PDB library of 31 January 2017. Complex models in eModel-BDB were generated with a collection of the state-of-the-art techniques, including protein meta-threading, template-based structure modeling, refinement and binding site detection, and ligand similarity-based docking. In addition to a rigorous quality control maintained during dataset generation, a subset of weakly homologous models was selected for the retrospective validation against experimental structural data recently deposited to the Protein Data Bank. Validation results indicate that eModel-BDB contains models that are accurate not only at the global protein structure level but also with respect to the atomic details of bound ligands.

Conclusions

Freely available eModel-BDB can be used to support structure-based drug discovery and repositioning, drug target identification, and protein structure determination.

Trade-offs with stability modulate innate and mutationally acquired drug resistance in bacterial dihydrofolate reductase enzymes

Structural stability is a major constraint on the evolution of protein sequences. However, under strong directional selection, mutations that confer novel phenotypes but compromise structural stability of proteins may be permissible. During the evolution of antibiotic resistance, mutations that confer drug resistance often have pleiotropic effects on the structure and function of antibiotic-target proteins, usually essential metabolic enzymes. In the present study, we show that trimethoprim (TMP)-resistant alleles of dihydrofolate reductase from Escherichia coli (EcDHFR) harboring the Trp30Gly, Trp30Arg or Trp30Cys mutations are significantly less stable than the wild-type, making them prone to aggregation and proteolysis. This destabilization is associated with a lower expression level, resulting in a fitness cost and negative epistasis with other TMP-resistant mutations in EcDHFR. Using structure-based mutational analysis, we show that perturbation of critical stabilizing hydrophobic interactions in wild-type EcDHFR enzyme explains the phenotypes of Trp30 mutants. Surprisingly, though crucial for the stability of EcDHFR, significant sequence variation is found at this site among bacterial dihydrofolate reductases (DHFRs). Mutational and computational analyses in EcDHFR and in DHFR enzymes from Staphylococcus aureus and Mycobacterium tuberculosis demonstrate that natural variation at this site and its interacting hydrophobic residues modulates TMP resistance in other bacterial DHFRs as well, and may explain the different susceptibilities of bacterial pathogens to TMP. Our study demonstrates that trade-offs between structural stability and function can influence innate drug resistance as well as the potential for mutationally acquired drug resistance of an enzyme.

Human dihydrofolate reductase and thymidylate synthase form a complex in vitro and co-localize in normal and cancer cells

Enzymes involved in thymidylate biosynthesis, thymidylate synthase (TS), and dihydrofolate reductase (DHFR) are well-known targets in cancer chemotherapy. In this study, we demonstrated for the first time, that human TS and DHFR form a strong complex in vitro and co-localize in human normal and colon cancer cell cytoplasm and nucleus. Treatment of cancer cells with methotrexate or 5-fluorouracil did not affect the distribution of either enzyme within the cells. However, 5-FU, but not MTX, lowered the presence of DHFR-TS complex in the nucleus by 2.5-fold. The results may suggest the sequestering of TS by FdUMP in the cytoplasm and thereby affecting the translocation of DHFR-TS complex to the nucleus. Providing a strong likelihood of DHFR-TS complex formation in vivo, the latter complex is a potential new drug target in cancer therapy. In this paper, known 3D structures of human TS and human DHFR, and some protozoan bifunctional DHFR-TS structures as templates, are used to build an in silico model of human DHFR-TS complex structure, consisting of one TS dimer and two DHFR monomers. This complex structure may serve as an initial 3D drug target model for prospective inhibitors targeting interfaces between the DHFR and TS enzymes.

Structural Insights into Mycobacterium tuberculosis Rv2671 Protein as a Dihydrofolate Reductase Functional Analogue Contributing to para-Aminosalicylic Acid Resistance

Mycobacterium tuberculosis (Mtb) Rv2671 is annotated as a 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione 5'-phosphate (AROPP) reductase (RibD) in the riboflavin biosynthetic pathway. Recently, a strain of Mtb with a mutation in the 5' untranslated region of Rv2671, which resulted in its overexpression, was found to be resistant to dihydrofolate reductase (DHFR) inhibitors including the anti-Mtb drug para-aminosalicylic acid (PAS). In this study, a biochemical analysis of Rv2671 showed that it was able to catalyze the reduction of dihydrofolate (DHF) to tetrahydrofolate (THF), which explained why the overexpression of Rv2671 was sufficient to confer PAS resistance. We solved the structure of Rv2671 in complex with the NADP(+) and tetrahydrofolate (THF), which revealed the structural basis for the DHFR activity. The structures of Rv2671 complexed with two DHFR inhibitors, trimethoprim and trimetrexate, provided additional details of the substrate binding pocket and elucidated the differences between their inhibitory activities. Finally, Rv2671 was unable to catalyze the reduction of AROPP, which indicated that Rv2671 and its closely related orthologues are not involved in riboflavin biosynthesis.

New small-molecule inhibitors of dihydrofolate reductase inhibit Streptococcus mutans

Streptococcus mutans is a major aetiological agent of dental caries. Formation of biofilms is a key virulence factor of S. mutans. Drugs that inhibit S. mutans biofilms may have therapeutic potential. Dihydrofolate reductase (DHFR) plays a critical role in regulating the metabolism of folate. DHFR inhibitors are thus potent drugs and have been explored as anticancer and antimicrobial agents. In this study, a library of analogues based on a DHFR inhibitor, trimetrexate (TMQ), an FDA-approved drug, was screened and three new analogues that selectively inhibited S. mutans were identified. The most potent inhibitor had a 50% inhibitory concentration (IC50) of 454.0±10.2nM for the biofilm and 8.7±1.9nM for DHFR of S. mutans. In contrast, the IC50 of this compound for human DHFR was ca. 1000nM, a >100-fold decrease in its potency, demonstrating the high selectivity of the analogue. An analogue that exhibited the least potency for the S. mutans biofilm also had the lowest activity towards inhibiting S. mutans DHFR, further indicating that inhibition of biofilms is related to reduced DHFR activity. These data, along with docking of the most potent analogue to the modelled DHFR structure, suggested that the TMQ analogues indeed selectively inhibited S. mutans through targeting DHFR. These potent and selective small molecules are thus promising lead compounds to develop new effective therapeutics to prevent and treat dental caries.

Interaction of dihydrofolate reductase and aminoglycoside adenyltransferase enzyme from Klebsiella pneumoniae multidrug resistant strain DF12SA with clindamycin: a molecular modelling and docking study

Klebsiella pneumoniae strain DF12SA (HQ114261) was isolated from diabetic foot wounds. The strain showed resistance against ampicillin, kanamycin, gentamicin, streptomycin, spectinomycin, trimethoprim, tetracycline, meropenem, amikacin, piperacillin/tazobactam, augmentin, co-trimoxazole, carbapenems, penicillins and cefoperazone, and was sensitive to clindamycin. Molecular characterization of the multidrug-resistance phenotype revealed the presence of a class 1 integron containing two genes, a dihydrofolate reductase (DHFR) (PF00186), which confers resistance to trimethoprim; and aminoglycoside adenyltransferase (AadA) (PF01909), which confers resistance to streptomycin and spectinomycin. A class 1 integron in K. pneumoniae containing these two genes was present in eight (18.18%) out of 44 different diabetic foot ulcer (DFU) patients. Hence, there is a need to develop therapeutics that inhibit growth of multidrug resistant K. pneumoniae in DFU patients and still achieve amputation control. Am attempt was made to create a 3D model and find a suitable inhibitor using an in silico study. Rational drug design/testing requires crystal structures for DHFR and AadA. However, the structures of DHFR and AadA from K. pneumoniae are not available. Modelling was performed using Swiss Model Server and Discovery Studio 3.1. The PDBSum server was used to check stereo chemical properties using Ramachandran plot analysis of modeled structures. Clindamycin was found to be suitable inhibitor of DHFR and AadA. A DockingServer based on Autodock & Mopac was used for docking calculations. The amino acid residues Ser(32), Ile(46), Glu(53), Gln(54), Phe(57), Thr(72), Met(76), Val(78), Leu(79), Ser(122), Tyr(128), Ile(151) in case of DHFR and Phe(34), Asp(60), Arg(63), Gln(64), Leu(68), Glu(87), Thr(89), Val(90) for AadA were found to be responsible for positioning clindamycin into the active site. The study identifies amino acid residues crucial to 'DHFR and AadA -drug' and 'DHFR and AadA -inhibitor' interactions that might be useful in the ongoing search for a versatile DHFR and AadA -inhibitor.

Two parallel pathways in the kinetic sequence of the dihydrofolate reductase from Mycobacterium tuberculosis

Dihydrofolate reductase from Mycobacterium tuberculosis (MtDHFR) catalyzes the NAD(P)H-dependent reduction of dihydrofolate, yielding NAD(P)(+) and tetrahydrofolate, the primary one-carbon unit carrier in biology. Tetrahydrofolate needs to be recycled so that reactions involved in dTMP synthesis and purine metabolism can be maintained. Previously, steady-state studies revealed that the chemical step significantly contributes to the steady-state turnover number, but that a step after the chemical step was likely limiting the reaction rate. Here, we report the first pre-steady-state investigation of the kinetic sequence of the MtDHFR aiming to identify kinetic intermediates, and the identity of the rate-limiting steps. This kinetic analysis suggests a kinetic sequence comprising two parallel pathways with a rate-determining product release. Although product release is likely occurring in a random fashion, there is a slight preference for the release of THF first, a kinetic sequence never observed for a wild-type dihydrofolate reductase of any organism studied to date. Temperature studies were conducted to determine the magnitude of the energetic barrier posed by the chemical step, and the pH dependence of the chemical step was studied, demonstrating an acidic shift from the pK(a) observed at the steady state. The rate constants obtained here were combined with the activation energy for the chemical step to compare energy profiles for each kinetic sequence. The two parallel pathways are discussed, as well as their implications for the catalytic cycle of this enzyme.

Antifolate agents: a patent review (2006 - 2010)

INTRODUCTION

For > 50 years, drugs targeting the folate pathway have significantly impacted disease treatment as anticancer, antimicrobial and immunomodulatory agents. The discovery of novel antifolate agents with improved properties and superior activities remains an attractive strategy, both in academia and the pharmaceutical industry.

AREAS COVERED

This review surveys the patent literature from 2006 to 2010 for small molecule inhibitors of enzymatic targets in the folate biosynthetic pathway.

EXPERT OPINION

The pursuit of antifolates as anticancer and antimicrobial agents continues to be an active area of research. New patent disclosures reveal novel antifolate scaffolds, antifolates with improved drug-like properties and new strategies to effectively target cancer cells. The continued use of high resolution structural information has guided the discovery of several compounds. Owing to the need for high levels of potency and selectivity, especially in targeting pathogenic species, the use of high resolution crystal structures remains an important tool to guide the design of novel antifolates. Interestingly, the patents disclosing novel compounds were ones where X-ray crystallography was an integral component of the design process. Finally, a variety of new structures have been reported that may play an important role in the future development of therapeutic antifolates.

Characterization of novel antibiotic resistance genes identified by functional metagenomics on soil samples

The soil microbial community is highly complex and contains a high density of antibiotic-producing bacteria, making it a likely source of diverse antibiotic resistance determinants. We used functional metagenomics to search for antibiotic resistance genes in libraries generated from three different soil samples, containing 3.6 Gb of DNA in total. We identified 11 new antibiotic resistance genes: 3 conferring resistance to ampicillin, 2 to gentamicin, 2 to chloramphenicol and 4 to trimethoprim. One of the clones identified was a new trimethoprim resistance gene encoding a 26.8 kDa protein closely resembling unassigned reductases of the dihydrofolate reductase group. This protein, Tm8-3, conferred trimethoprim resistance in Escherichia coli and Sinorhizobium meliloti (γ- and α-proteobacteria respectively). We demonstrated that this gene encoded an enzyme with dihydrofolate reductase activity, with kinetic constants similar to other type I and II dihydrofolate reductases (K(m) of 8.9 µM for NADPH and 3.7 µM for dihydrofolate and IC(50) of 20 µM for trimethoprim). This is the first description of a new type of reductase conferring resistance to trimethoprim. Our results indicate that soil bacteria display a high level of genetic diversity and are a reservoir of antibiotic resistance genes, supporting the use of this approach for the discovery of novel enzymes with unexpected activities unpredictable from their amino acid sequences.

Kinetic and chemical mechanism of the dihydrofolate reductase from Mycobacterium tuberculosis

Dihydrofolate reductase from Mycobacterium tuberculosis (MtDHFR) catalyzes the NAD(P)-dependent reduction of dihydrofolate, yielding NAD(P)(+) and tetrahydrofolate, the primary one-carbon unit carrier in biology. Tetrahydrofolate needs to be recycled so that reactions involved in dTMP synthesis and purine metabolism are maintained. In this work, we report the kinetic characterization of the MtDHFR. This enzyme has a sequential steady-state random kinetic mechanism, probably with a preferred pathway with NADPH binding first. A pK(a) value for an enzymic acid of approximately 7.0 was identified from the pH dependence of V, and the analysis of the primary kinetic isotope effects revealed that the hydride transfer step is at least partly rate-limiting throughout the pH range analyzed. Additionally, solvent and multiple kinetic isotope effects were determined and analyzed, and equilibrium isotope effects were measured on the equilibrium constant. (D(2)O)V and (D(2)O)V/K([4R-4-(2)H]NADH) were slightly inverse at pH 6.0, and inverse values for (D(2)O)V([4R-4-(2)H]NADH) and (D(2)O)V/K([4R-4-(2)H]NADH) suggested that a pre-equilibrium protonation is occurring before the hydride transfer step, indicating a stepwise mechanism for proton and hydride transfer. The same value was obtained for (D)k(H) at pH 5.5 and 7.5, reaffirming the rate-limiting nature of the hydride transfer step. A chemical mechanism is proposed on the basis of the results obtained here.