Tuberculosis represents a significant public health crisis. There is an urgent need for novel molecular scaffolds against this pathogen. We screened a small library of marine-derived compounds against shikimate kinase from Mycobacterium tuberculosis ( MtSK), a promising target for antitubercular drug development. Six manzamines previously shown to be active against M. tuberculosis were characterized as MtSK inhibitors: manzamine A (1), 8-hydroxymanzamine A (2), manzamine E (3), manzamine F (4), 6-deoxymanzamine X (5), and 6-cyclohexamidomanzamine A (6). All six showed mixed noncompetitive inhibition of MtSK. The lowest K values were obtained for 6 across all MtSK-substrate complexes. Time-dependent analyses revealed two-step, slow-binding inhibition. The behavior of 1 was typical; initial formation of an enzyme-inhibitor complex (EI) obeyed an apparent K of ∼30 μM with forward ( k) and reverse ( k) rate constants for isomerization to an EI* complex of 0.18 and 0.08 min, respectively. In contrast, 6 showed a lower K for the initial encounter complex (∼1.5 μM), substantially faster isomerization to EI* ( k = 0.91 min), and slower back conversion of EI* to EI ( k = 0.04 min). Thus, the overall inhibition constants, K*, for 1 and 6 were 10 and 0.06 μM, respectively. These findings were consistent with docking predictions of a favorable binding mode and a second, less tightly bound pose for 6 at MtSK. Our results suggest that manzamines, in particular 6, constitute a new scaffold from which drug candidates with novel mechanisms of action could be designed for the treatment of tuberculosis by targeting MtSK.
Ilimaquinone (IQ), a marine sponge metabolite, has been considered as a potential therapeutic agent for various diseases due to its broad range of biological activities. We show that IQ irreversibly inactivates Mycobacterium tuberculosis shikimate kinase (MtSK) through covalent modification of the protein. Inactivation occurred with an apparent second-order rate constant of about 60 M s. Following reaction with IQ, LC-MS analyses of intact MtSK revealed covalent modification of MtSK by IQ, with the concomitant loss of a methoxy group, suggesting a Michael-addition mechanism. Evaluation of tryptic fragments of IQ-derivatized MtSK by MS/MS demonstrated that Ser and Thr residues were most frequently modified with lesser involvement of Lys and Tyr. In or near the MtSK active site, three residues of the P-loop (K15, S16, and T17) as well as S77, T111, and S44 showed evidence of IQ-dependent derivatization. Accordingly, inclusion of ATP in IQ reactions with MtSK partially protected the enzyme from inactivation and limited IQ-based derivatization of K15 and S16. Additionally, molecular docking models for MtSK-IQ were generated for IQ-derivatized S77 and T111. In the latter, ATP was observed to sterically clash with the IQ moiety. Out of three other enzymes evaluated, lactate dehydrogenase was derivatized and inactivated by IQ, but pyruvate kinase and catalase-peroxidase (KatG) were unaffected. Together, these data suggest that IQ is promiscuous (though not entirely indiscriminant) in its reactivity. As such, the potential of IQ as a lead in the development of antitubercular agents directed against MtSK or other targets is questionable.
Single dose high-throughput screening (HTS) followed by dose-response evaluations is a common strategy for the identification of initial hits for further development. Early identification and exclusion of false positives is a cost-saving and essential step in early drug discovery. One of the mechanisms of false positive compounds is the formation of aggregates in assays. This study evaluates the mechanism(s) of inhibition of a set of 14 compounds identified previously as actives in Mycobacterium tuberculosis (Mt) cell culture screening and in vitro actives in Mt shikimate kinase (MtSK) assay. Aggregation of hit compounds was characterized using multiple experimental methods, LC-MS, HNMR, dynamic light scattering (DLS), transmission electron microscopy (TEM), and visual inspection after centrifugation for orthogonal confirmation. Our results suggest that the investigated compounds containing oxadiazole-amide and aminobenzothiazole moieties are false positive hits and non-specific inhibitors of MtSK through aggregate formation.
Catalase-peroxidases (KatGs), the only catalase-active members of their superfamily, all possess a 35-residue interhelical loop called large loop 2 (LL2). It is essential for catalase activity, but little is known about its contribution to KatG function. LL2 shows weak sequence conservation; however, its length is nearly identical across KatGs, and its apex invariably makes contact with the KatG-unique C-terminal domain. We used site-directed and deletion mutagenesis to interrogate the role of LL2 and its interaction with the C-terminal domain in KatG structure and catalysis. Single and double substitutions of the LL2 apex had little impact on the active site heme [by magnetic circular dichroism or electron paramagnetic resonance (EPR)] and activity (catalase or peroxidase). Conversely, deletion of a single amino acid from the LL2 apex reduced catalase activity by 80%. Deletion of two or more apex amino acids or all of LL2 diminished catalase activity by 300-fold. Peroxide-dependent but not electron donor-dependent kcat/KM values for deletion variant peroxidase activity were reduced 20-200-fold, and kon for cyanide binding diminished by 3 orders of magnitude. EPR spectra for deletion variants were all consistent with an increase in the level of pentacoordinate high-spin heme at the expense of hexacoordinate high-spin states. Together, these data suggest a shift in the distribution of active site waters, altering the reactivity of the ferric state, toward, among other things, compound I formation. These results identify the importance of LL2 length conservation for maintaining an intersubunit interaction that is essential for an active site water distribution that facilitates KatG catalytic activity.
Abstract:The growing resistance to current antimalarial drugs is a major concern for global public health. The pressing need for new antimalarials has led to an increase in research focused on the Plasmodium parasites that cause human malaria. Thioredoxin reductase (TrxR), an enzyme needed to maintain redox equilibrium in Plasmodium species, is a promising target for new antimalarials. This review paper provides an overview of the structure and function of TrxR, discusses similarities and differences between the thioredoxin reductases (TrxRs) of different Plasmodium species and the human forms of the enzyme, gives an overview of modeling Plasmodium infections in animals, and suggests the role of Trx functions in antimalarial drug resistance. TrxR of Plasmodium falciparum is a central focus of this paper since it is the only Plasmodium TrxR that has been crystallized and P. falciparum is the species that causes most malaria cases. It is anticipated that the OPEN ACCESSMolecules 2015, 20 11460 information summarized here will give insight and stimulate new directions in which research might be most beneficial.
Tuberculosis is one of the world’s leading cause of mortality from a single bacterial pathogen, with over 10 million reported cases each year. There is an alarming increase in the prevalence of drug‐resistant strains, thus the need for the discovery of novel anti‐tubercular agents. The shikimate pathway is a seven‐step metabolic route that produces aromatic amino acids and other cellular metabolites. It has no mammalian counterpart, making any of the enzymes in this pathway suitable targets for screening of potential anti‐tubercular agents. Mycobacterium tuberculosis shikimate kinase (MtSK) catalyzes the 5th step of this pathway, converting shikimate to shikimate‐3‐phosphate using ATP as a co‐substrate. The overall goal of this project is to express and characterize MtSK and screen for potential anti‐tubercular agents. Transformation of XL‐1 blue competent cells was performed using a pET 21b plasmid with aroK gene inserted at the multiple cloning site. Plasmids were cloned and purified and used to transform BL 21 DE3 competent cells for subsequent protein expression. Small scale expression showed the presence of a band of increasing prominence around 20 kDa. This suggests MtSK was successfully expressed. Expression analysis for larger scale supported data from small scale. Purification, characterization, and MtSK kinetic parameters would be determined prior to enzyme inhibition studies using inhibitors like avarone (below), a marine sponge sesquiterpene quinone and derivatives thereof. Support or Funding Information East Stroudsburg University of Pennsylvania Avarone
Tuberculosis is a respiratory infection with over 10 million reported cases each year. This infection rate is responsible for over two million deaths annually, second only to HIV in fatalities among infectious diseases. Since 1960 tuberculosis rates have steadily declined worldwide as medical technology has advanced to produce more effective antibiotics. However, recent studies have shown that tuberculosis rates have seen a steady increase. This observation suggests that there is an alarming increase in the prevalence of drug‐resistant strains of tuberculosis, thus the need for the discovery of novel anti‐tubercular agents. Targeting potential enzymatic pathways for drug discovery requires the pathway to possess minimal overlap with the host. The shikimate pathway is a seven‐step metabolic route that produces aromatic amino acids and other cellular metabolites. This pathway is typically present in microorganisms and has no mammalian counterpart, making any of the enzymes in this pathway suitable targets for screening of potential anti‐tubercular agents. The target enzyme here, Mycobacterium tuberculosis Shikimate Kinase (MtSK), catalyzes the 5th step of this pathway, converting shikimate to shikimate‐3‐phosphate. The overall goal of this project is to express and characterize MtSK in order to screen for potential anti‐tubercular agents. Initial methods included a bacterial transformation of XL‐1 blue competent E.coli cells. This preliminary transformation was performed using a pET‐21b plasmid with an aroK gene inserted at the multiple cloning site. Successfully transformed XL‐1 blue cells were cloned and a second transformation using BL21 DE3 competent cells as an expression host was performed. Small scale expression showed the presence of a band around 20 kDa. The theoretical mass of the enzyme is 19.6 kDa which suggests MtSK was successfully expressed within the transformed cells. Expression analysis for large‐scale supported data from small scale as a band of 20 kDa once again appeared in the verification SDS‐PAGE gel. Purification of MtSK was carried out through nickel affinity chromatography. Subsequently, kinetic characterization and inhibitors studies will be performed using inhibitors like avarone and hymenidin.
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