A DNA aptamer reveals an allosteric site for inhibition in metallo-β-lactamases

The hydrolysis of β-lactam antibiotics by β-lactamase enzymes is the most prominent antibiotic resistance mechanism for many pathogenic bacteria. Out of this broad class of enzymes, metallo-β-lactamases are of special clinical interest because of their broad substrate specificities. Several in vitro inhibitors for various metallo-β-lactamases have been reported with no clinical efficacy. Previously, we described a 10-nucleotide single stranded DNA aptamer (10-mer) that inhibits Bacillus cereus 5/B/6 metallo-β-lactamase very effectively. Here, we find that the aptamer shows uncompetitive inhibition of Bacillus cereus 5/B/6 metallo-β-lactamase during cefuroxime hydrolysis. To understand the mechanism of inhibition, we report a 2.5 Å resolution X-ray crystal structure and solution-state NMR analysis of the free enzyme. Chemical shift perturbations were observed in the HSQC spectra for several residues upon titrating with increasing concentrations of the 10-mer. In the X-ray crystal structure, these residues are distal to the active site, suggesting an allosteric mechanism for the aptamer inhibition of the enzyme. HADDOCK molecular docking simulations suggest that the 10-mer docks 26 Å from the active site. We then mutated the three lysine residues in the basic binding patch to glutamine and measured the catalytic activity and inhibition by the 10-mer. No significant inhibition of these mutants was observed by the 10-mer as compared to wild type. Interestingly, mutation of Lys50 (Lys78; according to standard MBL numbering system) resulted in reduced enzymatic activity relative to wild type in the absence of inhibitor, further highlighting an allosteric mechanism for inhibition.


Introduction
β-lactam antibiotics are the most widely prescribed class of antimicrobial drugs because of their high effectiveness and relatively low cost [1]. Consequently, the evolution of β-lactam antibiotic resistance in pathogenic bacteria is a major threat to human health. The production of β-lactamase enzymes, which catalyze the hydrolysis of the endocyclic amide bond of the β-lactam ring, is the most common mechanism for resistance to these antibiotics [2]. Based on sequence identity, there are four classes of β-lactamases. Classes A, C, and D are serine a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 10-mer. Our data suggests that the association of 10-mer with 5/B/6 MBL during cefuroxime hydrolysis allosterically modulates the enzyme, culminating in uncompetitive enzymatic inhibition. This result identifies a novel site for future 5/B/6 and BcII MBL inhibitor design.

Bacillus cereus 5/B/6 metallo-β-lactamase expression and purification
For expression and purification of unlabeled enzyme, E. coli BL21 (DE3) codon plus competent cells (Stratagene) were transformed with pET29 plasmid containing the structural gene of B. cereus 5/B/6 MBL. Transformed cells were grown at 37˚C in LB medium supplemented with 0.1 mM ZnSO 4 and 50 μg/ml kanamycin. Enzyme overexpression was induced with 1 mM IPTG added at attenuance at 600 nm of 1.0; the cells were further grown for 12 hours at 20˚C. Cells were harvested by centrifugation, washed, and resuspended in 20 mM MOPS, 1 mM ZnSO 4 , pH 7.0 buffer (20/1 buffer). Then the cells were lysed via four passes through a French pressure cell. Cell lysate was centrifuged using a Fiberlite F21-8x50y rotor (Thermo Scientific) at 20000 rpm (47360 g) for 1 hour at 4˚C. Solid (NH 4 ) 2 SO 4 was added to the supernatant to a final concentration of 200 mM. Insoluble materials were removed by ultracentrifugation using a Beckman Ti 90 rotor at 90000 rpm (694000 g) for 2 hours at 4˚C. Supernatant was loaded onto a 25 ml POROS XS strong cation exchange column (Applied Biosystems) previously equilibrated with 20/1 buffer. This column was washed with 20/1 buffer. After elution of unbound materials, the bound enzyme was eluted with a step gradient to 20 mM MOPS, 1 mM ZnSO 4 , 200 mM (NH 4 ) 2 SO 4 , pH 7. Fractions that contained the enzyme were loaded onto a 7×15.6 cm Bio-gel P-60 column previously equilibrated with 20/1 buffer. The column was washed with 20/1 buffer. Enzyme containing fractions were concentrated by using an Amicon concentrator (Cole-Parmer, USA) equipped with an YM-10 membrane (10000 molecular weight cutoff). After every step of the purification, enzyme activity was calculated with a modified method described by Kim et al. [14] and Davies et al. [30] and total protein was determined by the method of Lowry et al. [31] using bovine serum albumin for the standard. Enzyme purity was ascertained by specific activity and SDS-PAGE and was judged to be >98% pure. Glycerol (20% v/v final concentration) was added to the pure enzyme, which was stored at -20˚C. For NMR experiments, isotopically labeled enzyme was purified in the same way, however, the cells were grown in 2x M9 minimal medium (13.6 g/L Na 2 HPO 4 , 6.0 g/L KH 2 PO 4 , 1.0 g/L NaCl, 0.5 g/L Mg 2 SO 4 �7H 2 O, pH 7.1) [32] supplemented with 12 C-or 13 C-D-glucose (3 g/L) and ( 15 NH 4 ) 2 SO 4 (1 g/L) as the sole carbon and nitrogen sources, respectively, along with vitamins (1 μg/mL each biotin and thiamine) and kanamycin (50 μg/ml). Generally,~40 and~25 milligrams of purified enzyme were produced from per liter LB or minimal media, respectively.

Selection of the 10-mer DNA
The process of the 10-mer selection by SELEX was described in Kim et al. [14]. The 10-mer (5'-CCAAACTTGG-3') was purchased from Oligo Factory (Holliston, MA, USA) and used in all inhibition assays and NMR experiments without further purification.

Enzyme kinetic studies
Using cefuroxime as the substrate, we measured 5/B/6 MBL activity in 50 mM MOPS, 1 mM ZnSO 4 , pH 7 through a spectrophotometric method described by Kim et al. [14]. In this protocol, the decrease in substrate absorbance (Δε 276nm = 4.42 mM -1 cm -1 ) was continuously monitored at 276 nm during hydrolysis using a Shimadzu UV160U spectrophotometer and a quartz cuvette with 0.1 cm pathlength. All assays were performed with a final concentration of 0.3 μM enzyme in a reaction volume of 300 μL. One unit of enzyme activity was defined as the amount of enzyme required for catalyzing the hydrolysis of 1 μmol of substrate in 1 minute at 30˚C. For inhibition assays, diluted enzyme was pre-incubated with or without 10-mer DNA aptamer in the buffer for 15 min at 25˚C. All kinetic measurements were repeated in at least triplicate. A global nonlinear curve fitting was performed to determine the inhibition type using the K mðappÞ þ½S� in GraphPad Prism 6.0 [33], where v is the reaction rate,

Crystallography
Crystallization trials for 5/B/6 MBL were carried out using the structure screen 1 + 2 HT-96 (Molecular Dimensions). The concentration of the enzyme sample used for the screening was 20 mg/ml in 5 mM MOPS and 0.5 mM ZnSO 4 at pH 7.0. Crystals typically appear within a week when incubated at 23˚C. Further optimization was performed using 24-well VDX plates and 1 mL reservoir volume containing 0.1 M Tris at pH 8.5 and 8% w/v PEG 8000. Hanging droplets were comprised of 1 μL of reservoir solution and 1 μL of protein. Final X-ray data were collected at 100 K and a wavelength of 1.283 Å using beam line 7-1 at Stanford Linear Accelerator Center (SLAC). Data were integrated using XDS [34]. Merging and scaling were performed using AIMLESS [35]. Molecular replacement, starting from PDB ID 1BC2 [Fabiane et al 1998], and refinement were done with PHENIX [36]. COOT [37] was used for manual density fitting. The coordinates and structure factors have been deposited in the Protein Data Bank (http://pdb.org) under the PDB ID 6DJA. Table 1 summarizes the data collection and refinement statistics. Ramachandran statistics showed that 97.3% and 2.7% of all residues were in favored and allowed regions, respectively.

NMR spectroscopy and sequential backbone assignments
NMR experiments were performed on an Agilent DD2 600 MHz spectrometer (Santa Clara, CA, USA) equipped with a z-axis pulsed field gradient room temperature HCN probe. Standard 3D gradient-selected, sensitivity enhanced triple resonance backbone assignment experiments (HNCACB/CBCAcoNH, HNCA/HNcoCA and HNCO/HNcaCO) [38,39] were recorded on an~1.4 mM enzyme sample in 20 mM MOPS, 1 mM ZnSO 4 , pH 7 at a set point of 25˚C. Spectra were processed and analyzed with the NMRPipe/NMRDraw software package [40] and CCPN analysis program [41]. The secondary structure of 5/B/6 MBL was predicted by the program TALOS+ [42], using the 1 HN, 15 N, 13 CO, 13 C α , and 13 C β chemical shift resonance assignments, and compared with the X-ray crystal structure using PyMol [43]. Additionally, the 1 HN, 15 N, 13 CO, 13 C α , and 13 C β chemical shift resonance assignments were submitted to the CS-ROSETTA [44] server (https://csrosetta.bmrb.wisc.edu/csrosetta/submit) and 3000 structures were calculated using default parameters. Note, this structure was only used for diagnostic purposes, to confirm that the measured NMR chemical shifts conform to our X-ray crystal structure. Backbone chemical shift assignments have been deposited in the Biological Magnetic Resonance Data Bank (http://www.bmrb.wisc.edu/) under BMRB ID 27564.  [45,46] spectra were recorded on an Agilent DD2 600 MHz spectrometer at a set temperature of 25˚C and were processed and analyzed with NMRPipe and CCPN analysis. Chemical shift perturbations at a given inhibitor concentration (δ NH,i ) were calculated according to
The 200 calculated structures were automatically classified into 8 clusters.

Site-directed mutagenesis
Site-directed lysine to glutamine mutations were generated through a modified QuikChange (Stratagene) protocol. Plasmids encoding these mutants were transformed into E. coli BL21 (DE3) codon plus cells for over-expression. Protein purification and activity and inhibition assays were performed as described above.

Enzyme kinetic studies suggest a novel site for aptamer binding
To understand the mechanism of 10-mer inhibition of 5/B/6 MBL, we quantified Michaelis-Menten enzyme kinetics by monitoring the hydrolysis of the antibiotic cefuroxime (second generation semisynthetic cephalosporin with a furan containing side chain, Fig 1A). For wild type, uninhibited 5/B/6 MBL, K m , V max , and k cat were calculated to be 1.1 ± 0.1 mM, 370 ± 20 μmol�min -1 �mg -1 , and 164 ± 8, respectively, from a non-linear regression plot (black points in Fig 1B). The 10-mer aptamer inhibits 5/B/6 MBL cefuroxime hydrolysis with an IC 50 of 120 ± 5 nM (S1A Fig). In contrast to the noncompetitive inhibition of 5/B/6 MBL by the 10-mer during cephalosporin C (a cephalosporin antibiotic with a diamino adipoyl side chain, for which a K m and V max of 0.39 mM and 1333 μmol�min -1 �mg -1 for the 5/B/6 MBL, respectively [51], Fig 1A) hydrolysis [14], uncompetitive inhibition ( Fig 1B and S1B Fig) was observed for the hydrolysis of cefuroxime with a calculated inhibition constant (K 0 I ) of 63 ± 3 nM. Thus, the inhibition pattern and kinetic parameters of 5/B/6 MBL differ between substrates and suggests that the 10-mer does not bind 5/B/6 MBL competitively. Note, the 10-mer was originally derived from a 30 nucleotide DNA obtained from our SELEX experiment. As part of that experiment, we showed that the inhibitory effect of this DNA aptamer resulted solely from the 10-mer sequence. Deleting the flanking DNA sequences upstream and downstream of the predicted 10-mer hairpin structure resulted in the same inhibition of 5/B/6 MBL activity as the 30-mer DNA, and the isolated flanking sequences did not inhibit on their own [14].
Uncompetitive inhibitors require the formation of the enzyme-substrate complex in order to interact with the enzyme and are characterized by decreases in both K m and V max , as seen here for cefuroxime in Fig 1B and S1B Fig. During uncompetitive inhibition, an enzyme-catalyzed reaction becomes blocked beyond ES formation [52]. On the other hand, noncompetitive inhibitors can either bind to the enzyme alone or to the enzyme-substrate complex and are characterized by an increase in K m and decrease in V max . Therefore, the 10-mer aptamer inhibition pattern exhibited for each substrate indicates that the interaction of the inhibitor with 5/B/6 MBL probably occurs at an allosteric site distal to the active site (i.e. is not competitive with the substrate) [20,53].

Structure of wild type B. cereus 5/B/6 metallo-β-lactamase
To put the mechanism of inhibition into a structural context, X-ray crystallography trials were initiated for the aptamer-free and 10-mer-bound enzyme. The 2.5 Ångström (Å) resolution crystal structure of aptamer-free 5/B/6 MBL with both Zn 2+ ions bound is shown in Fig 2. Crystal structures of several B1 metallo-β-lactamases have been determined and can be found on the MBLED (Metallo-Beta-Lactamase Engineering Database; http://www.mbled.uni-stuttgart.de) [54]. Subclass B1 MBLs display a αβ/βα sandwich fold in their tertiary structures with a consensus sequence of HXHXD(X) i H(X) j C(X) k H (in single letter code for amino acids where X = any amino acid, i = 55-74, j = 18-24, and k = 37-41), with H, D, and C residues coordinating the Zn 2+ ions [15]. Our structure shows a similar fold. Table 1 contains the crystallography data collection and refinement statistics.
Because of difficulties in obtaining diffraction quality crystals of the 10-mer-bound 5/B/6 MBL, solution state NMR studies were also initiated. The 1 H-15 N HSQC of aptamer-free 5/ B/6 MBL is shown in Fig 3; over 87% (199 non-proline residues out of 228 total residues) of the backbone 1 HN, 15 N, 13 CO, 13 C α , and 13 C β resonances were assigned. Main chain assignments were not obtained for Glu28-Lys34, Glu39, Lys50, Ser62, Pro68, Ser69, Ala140, Glu168, Glu169, Pro170, Ser176-Asn179, Asn184, Pro192, Gly193, Pro206, Ser227, Asn233, and Pro261. Most of the unassigned residues are found either in turns or loop regions. The secondary structure propensities were calculated from the NMR data by TALOS+ and compared with those of the X-ray crystal structure. The length and position of the secondary structure elements agreed between the X-ray crystallography and solution state NMR data sets (S2 Fig). The CS-ROSETTA calculated solution NMR structure also shows the typical αβ/βα sandwich fold for metallo-β-lactamases (S3A Fig). Because many of the loop resonances were not assigned and the Zn 2+ ions were not present in the CS-ROSETTA modeling, small differences exist between the X-ray crystal and solution NMR structures. This is shown in S3B Fig where the α-helix and β-sheet regions of the X-ray crystal and NMR structures were aligned in PyMOL to compare the two structures. The C α r.m.s.d. for regions of secondary structure is 2.37 Å indicating that the two structures are indeed similar. Together, these structural data are the starting point for understanding the structure of the 10-mer aptamerinhibited 5/B/6 MBL.  [55]. While the vast majority of residues experience no change in resonance position upon addition of 10-mer (S4 Fig), six residues showed significant CSPs upon titrating with increasing concentrations of 10-mer (0 to 4 molar equivalents): Thr76, Lys78, Phe103, Lys104, Lys107, and Tyr208. Interestingly, all of these residues are on the opposite side of the enzyme from the active site in three structurally adjacent loops (L4, L6, L13). Among these residues were three lysines (78, 104, and 107) on loops L4 and L6, shown in the crystal structure in Fig  2A, that make up a basic patch. The side chain amines of Lys78 and Lys107 are 11.7 Å away from each other providing ample space to occupy the loop region of the 10-mer (~8.7 Å distance between A5 and C6 of the aptamer). Moreover, the distance between the backbone amide groups of Lys104 and Tyr208 is~22 Å, which is within the range of the length of a 10-mer hairpin structure (~20 Å). A spectral overlay highlighting the chemical shift perturbations between the reference and several inhibitor concentrations for these lysine residues is shown in Fig 4A. Since binding of 10-mer to enzyme occurred in the fast exchange regime of the chemical shift timescale, binding affinities were extracted from fits of the CSPs (Fig 4). While the CSPs are small in magnitude, likely reflecting that the interaction between 10-mer and 5/B/6 MBL occurs largely through side chain interactions, we were able to analyze the data Uncovering allostery in a metallo-β-lactamase to determine binding affinities. Typical hyperbolic binding curves were observed for all three lysine residues, shown in Fig 4B, and K D s of 100-233 nM were obtained from fits to a twostate quadratic binding isotherm. Note that this binding affinity is comparable to the K 0 I obtained from analysis of the kinetic inhibition data (63 ± 3 nM).
It was previously hypothesized that the 10-mer exerts its inhibition by binding near the active site thereby perturbing the coordination of Zn 2+ ions [14]. Significantly, no CSPs were observed in the HSQC spectra for the Zn 2+ -coordinating active site residues (His116, His118, Asp120, His196, Cys221 and His263; S4 Fig) indicating that the 10-mer does not strip the catalytic ions. This binding of aptamer to a site away from the active site supports the hypothesis of allosteric inhibition of the 5/B/6 MBL by the 10-mer.

Molecular docking identifies probable interaction of the 10-mer with 5/B/6 MBL at the allosteric site
For a low-resolution visualization of the 10-mer-enzyme complex, we performed molecular docking simulations using HADDOCK [47]. 200 models of the complex were calculated using Uncovering allostery in a metallo-β-lactamase our X-ray crystallography model and a hairpin model of the 10-mer aptamer, which was calculated in 3D-NuS [48]. Constraints were placed between the six 5/B/6 MBL residues that experienced CSPs and all ten residues of the 10-mer. Fig 5 and S5 Fig show the resulting structures of the simulated enzyme-aptamer complexes with different orientations of the 10-mer.
The docking models suggest that loops L4 and L6 predominantly interact with the 10-mer (Fig 5A). In the majority of the representative models, Lys104 (5 of 8  Lastly, we did not observe any direct interactions in the HADDOCK models between the 10-mer and Thr76 and Phe103, so the CSPs for these residues must be due to indirect effects from aptamer binding. Note, that the small backbone amide CSPs observed in Fig 5 and S5 Fig are indeed consistent with these models, which suggest that the binding of 10-mer to 5/B/6 MBL is mediated through side chain interactions. Thus, we hypothesize that the 10-mer interacts with the lysine residues ( Fig  5B) on L4 and L6 and disrupts a network of residues towards the Zn2 ion.

Mutations confirm the presence of an allosteric site on B. cereus 5/B/6 metallo-β-lactamase
To corroborate our binding model, three lysine-to-glutamine mutants were made via sitedirected mutagenesis (K78Q, double mutation K104Q/K107Q, and triple mutation K78Q/ K104Q/K107Q). As these mutations are far from the active site, it was expected that there would be no change in enzymatic activity during cefuroxime hydrolysis. Specific activities of wild type and mutant enzymes are shown in Table 2. Without the inhibitor, the double mutant (K104Q/K107Q) showed no significant change in activity as compared to wild type. On the other hand, the single mutant (K78Q) and triple mutant (K78Q/K104Q/K107Q) enzymes Uncovering allostery in a metallo-β-lactamase showed~30% less activity than wild type enzyme. Analysis of the steady-state kinetics data (Fig 6) indicates that there are statistically significant (p-value = 0.0018) decreases in k cat for K78Q and K78Q/K104Q/K107Q and no change in K m ( Table 2) as compared to wild type enzyme. This decrease in k cat would likely have minimal effect on the activity of mutated enzyme in vivo; nevertheless, this result suggests that an allosteric network extends through the interior of the enzyme from the active site. Finally, inhibition of the three mutant enzymes by 10-mer was also studied. As shown in Table 2, no significant inhibition was observed in any of the mutant enzymes as compared to wild type, confirming our NMR and HADDOCK results that the aptamer must bind to these residues. Thus, disruption of ionic interactions between these lysine residues and the DNA leads to an inability of the 10-mer to bind the enzyme. This is consistent with our structural models where the inhibition by the aptamer occurs through binding to a unique allosteric binding site (Fig 5).

Discussion
Here, we described the structural basis for nanomolar uncompetitive inhibition of B. cereus 5/ B/6 metallo-β-lactamase during cefuroxime hydrolysis by a 10-nucleotide DNA aptamer. We observe that the inhibition pattern of the 10-mer differs substrate-to-substrate (noncompetitive Table 2. Steady-state parameters of 5/B/6 MBL mutants.

Enzyme Cefuroxime Hydrolysis
No 10-mer a + 500 μM 10-mer a K m (mM) k cat (s -1 ) k cat /K m (mM -1 �s -1 ) K m (mM) k cat (s -1 ) k cat /K m (mM -1 �s -1 ) for cephalosporin C [14], uncompetitive for cefuroxime); thus, the kinetic mechanism details for the inhibition by the 10-mer are different for the two substrates. Different reaction mechanisms have been proposed for NDM-1 MBL hydrolysis of carbapenem and cephalosporin βlactams [56,57]. Like FEZ-1 MBL, the activity of the 5/B/6 MBL depends in part on the chemical moiety at the C-7 position of cephalosporin substrates, as the presence of an α-methoxy group at C-7 of cefoxitin has a negative effect on 5/B/6 MBL activity when compared to cephalothin hydrolysis [58]. Other differences could come from the charged groups in cephalosporins, which could interact electrostatically with the enzyme [59]. The difference in kinetic mechanism could then be extended to the inhibition pattern of the 10-mer on cephalosporin antibiotic hydrolysis by 5/B/6 MBL. Our 2.5 Å crystal structure of aptamer-free 5/B/6 MBL overlays favorably with the structure of the closely related BcII enzyme (~93% sequence identity; PDB ID: 1BC2; C α r.m.s.d. = 1.08 Å). Additionally, our structure also overlays extremely well with the crystal structures of NDM-1, VIM-2, and IMP-1 MBLs (PDB IDs: 3RKJ, 4NQ2, and 1DDK) with C α r.m.s.d.'s of 1.07, 0.71, and 0.98 Å, respectively, highlighting the highly conserved nature of these folds. Interestingly, our structure shows that in 5/B/6 MBL Zn2 has a higher B-factor compared to Zn1, indicating that Zn2 exhibits a partial occupancy in the active site pocket unlike Zn1. Coupled to this, the active site residue His263 is found to be in two conformations in the X-ray structure (S6 Fig). This result suggests that His263 plays an important role in directing Zn2 to its location within the active site by acting as a gate locking Zn2 in place once it is bound. In fact, molecular dynamics simulations, directed evolution, and NMR relaxation experiments have suggested that the presence of a glycine residue at position 262, which is present in 5/B/6 MBL, increases the flexibility of His263 and other active site residues, alters the coordination of the Zn 2+ ions, and affects the substrate specificity of the MBL [60][61][62].
Like antibody-antigen binding reactions, aptamers interact with their target through structural recognition [63,64]. Usually, unpaired nucleotides in loop or bulge regions of a ssDNA molecule are involved in recognition of the protein through hydrogen bonding, which imparts the specificity of the aptamer-protein interaction [65]. Therefore, we hypothesize that the 10-mer in this study also interacts with 5/B/6 MBL through the loop region of its hairpin structure. Moreover, charged amino acids are generally found on the surface of proteins, and play many essential roles including the formation of protein-nucleic acid complexes [66]. To compensate for the negatively charged phosphate backbone, the protein-nucleic acid binding regions are primarily positively charged. For 5/B/6 MBL, the change in chemical shifts (and therefore the chemical environment) of the three positively charged lysine residues distal from the active site are indicative of 10-mer interaction at that site ( Fig 4A). The shortest distance between the active site and inhibitor binding site is 25 Å, as measured from His116 to Lys78too far away for the aptamer to interact directly with both sites. However, any allosteric changes to the active site must be subtle, as the amide NMR chemical shifts of active site residues are not perturbed by 10-mer binding. One possibility is that the 10-mer binding could alter conformational changes or local dynamics around the active site required for catalysis [57,67,68]. Our proposed structural models for the enzyme-inhibitor complex (Fig 5 and S5 Fig) lack experimental restraints on the 10-mer structure in solution. Clearly, an experimentally derived structure of the 10-mer and additional restraints between aptamer and 5/B/6 MBL are required to fully understand the nature of this interaction. Nevertheless, instead of a potential therapeutic, the 10-mer should be viewed as a tool-the low-resolution models of the complex provided by the NMR titration and HADDOCK and the mutagenesis data have allowed us to define a novel allosteric site on this enzyme, which could now be subjected to rational drug design.
Interestingly, five of the six residues in 5/B/6 MBL that were perturbed upon the 10-mer binding and were used as restraints in the HADDOCK docking simulations are also conserved in the BcII MBL enzyme-Lys104 is a glutamine in the BcII MBL. This immediately suggests that the 10-mer will also bind to the BcII MBL, since these enzymes share~93% sequence identity. In fact, preliminary studies have shown that the 10-mer does indeed inhibit cephalosporin antibiotic hydrolysis by the BcII MBL (R.W.S. personal communication). The K104Q/K107Q double mutation of 5/B/6 MBL, which contains the analogous BcII MBL Gln76 (103), was not inhibited by the 10-mer aptamer (Table 2). However, this could be due to the additive effect of losing two charge-charge interactions with the 10-mer; whereas, BcII MBL would retain one. A more in-depth mutagenesis study is necessary to tease these effects out, while the effect of the 10-mer on BcII MBL can be directly tested. Sequence alignment of the 340 subclass B1 sequences in the MBLED reveals that the basic residues utilized by the 10-mer for binding to 5/B/6 MBL are each present in~30% of sequences. Thus, the 10-mer will probably not be a general inhibitor of subclass B1 MBLs. Nevertheless, given the high degree of structural similarity between the 5/B/6 MBL, NDM-1, VIM-4, and IMP enzymes, it is tempting to hypothesize that related allosteric sites also exist within these enzymes. In fact, epitope mapping of the VIM-4 inhibitory nanobody, which also utilizes an allosteric mechanism for inhibition, revealed that the nanobody binds VIM-4 MBL on the structurally analogous L6 that contains residues Lys104 and Lys107, which are perturbed in our NMR titration experiments, in 5/B/6 MBL [20]. Thus, there is very strong evidence that the allosteric site described herein is a common motif among several highly transmissible β-lactam neutralizing metallo-enzymes.
Allosteric drugs are used as therapeutics for several human diseases. The most common uses of allosteric modulators are for neurotransmitter receptor proteins in central nervous system disorders. For example, benzodiazepines act as positive modulators to enhance the effect of γ-amino butyric acid (GABA) by interacting with GABA A receptors; whereas, 6,7-dihydropyrazolo[1,5-a]pyrazin-4(5H)-one acts as the negative modulator of metabotropic glutamate receptor-2 (mGluR2) [69]. To treat viral infection, use of allosteric inhibitors is also well established. For example, dasabuvir, an hepatitis C virus NS5B polymerase inhibitor, is used against chronic viral infection [70], and noncompetitive binding non-nucleotide reverse transcriptase inhibitors are a critical components of combination therapy used to treat HIV [71]. The development of similar allosteric inhibitors for bacterial enzymes has received attention very recently [20]. The alarming problem of antibiotic resistance has gained considerable interest and has pushed the discovery of new therapeutic agents to fight against bacterial infection. Because of the consensus active site architecture, finding effective inhibitors for MBLs have been mostly limited to orthosteric investigations. Due to their toxicity to the human body, divalent cation chelating agents specifically for MBLs are impossible to develop [14,16,72,73]. Therefore, targeting allosteric sites for developing new drugs has great potential, as it is likely that specific allosteric modulators can be found for a unique site on the target protein that consequently avoids adverse effects to the human host [69,74]. Prediction of secondary structure elements from the NMR data was derived from backbone NMR chemical shifts using TALOS+ and are shown in orange bars for α-helices and cyan bars for β-sheets. Grey bars indicate unassigned residues. Secondary structure elements of 5/B/6 MBL x-ray crystal structure are indicated in orange rectangles for α-helices and cyan arrows for β-sheets. PDB ID: 6DJA was used to determine secondary structural elements from the crystal structure.  13 CO, 13 Cα, and 13 Cβ chemical shift resonance assignments were submitted to the CS-ROSETTA and 3000 structures were calculated using default parameters. The lowest energy structure is shown here as a representative of all structures. The zinc ions coordinating the active site residues are not shown here. (B) Overlay of CS-ROSETTA derived model of 5/B/6 MBL (olive green) and the X-ray crystal structure (light green), which highlights the similarity of the secondary structure and the overall fold.