Selective alteration of substrate specificity by replacement of aspartic acid-189 with lysine in the binding pocket of trypsin

To test the role of Asp-189 which is located at the base of the substrate binding pocket in determining the specificity of trypsin toward basic substrates, this residue was replaced with a lysine residue by site-directed mutagenesis. Both rat trypsinogen and Lys-189 trypsinogen were expressed and secreted into the periplasmic space of Escherichia coli. The proteins were purified to homogeneity and activated by porcine enterokinase, and their catalytic activities were determined on natural and synthetic substrates. Lys-189 trypsin displayed no catalytic activity toward arginyl and lysyl substrates. Further, there was no compensatory change in specificity toward acidic substrates; no cleavage of aspartyl or glutamyl bonds was detected. Additional studies of substrate specificity involving gas-phase sequence analyses of digested natural substrates revealed an inherent but low chymotrypsin-like activity of trypsin. This activity was retained but modified by the Asp to Lys change at position 189. In addition to hydrolyzing phenylalanyl and tyrosyl peptide bonds, the mutant enzyme has the unique property of cleaving leucyl bonds. On the basis of computer graphic modeling studies of the Lys-189 side chain, it appears that the positively charged NH2 group is directed outside the substrate binding pocket. The resulting hydrophobic cavity may explain the altered substrate specificity of the mutant enzyme. The relatively low chymotrypsin-like activity of both recombinant enzymes may be due to distorted positioning of the scissile bond with respect to the catalytic triad rather than to the lack of sufficient interaction between the hydrophobic side chains and the substrate binding pocket of the enzyme.     

Regulatory phosphorylation of cyclin-dependent kinase 2: insights from molecular dynamics simulations

The structures of fully active cyclin-dependent kinase-2 (CDK2) complexed with ATP and peptide substrate, CDK2 after the catalytic reaction, and CDK2 inhibited by phosphorylation at Thr14/Tyr15 were studied using molecular dynamics (MD) simulations. The structural details of the CDK2 catalytic site and CDK2 substrate binding box were described. Comparison of MD simulations of inhibited complexes of CDK2 was used to help understand the role of inhibitory phosphorylation at Thr14/Tyr15. Phosphorylation at Thr14/Tyr15 causes ATP misalignment for the phosphate-group transfer, changes in the Mg²⁺ coordination sphere, and changes in the H-bond network formed by CDK2 catalytic residues (Asp127, Lys129, Asn132). The inhibitory phosphorylation causes the G-loop to shift from the ATP binding site, which leads to opening of the CDK2 substrate binding box, thus probably weakening substrate binding. All these effects explain the decrease in kinase activity observed after inhibitory phosphorylation at Thr14/Tyr15 in the G-loop. Interaction of the peptide substrate, and the phosphorylated peptide product, with CDK2 was also studied and compared. These results broaden hypotheses drawn from our previous MD studies as to why a basic residue (Arg/Lys) is preferred at the P₊₂ substrate position. Figure View of the substrate binding site of the fully active cyclin-dependent kinase-2 (CDK2) (pT160-CDK2/cyclin A/ATP). The pThr160 activation site is located in the T-loop (yellow secondary structure). The G-loop, which partly forms the ATP binding site, is shown in blue. The Thr14 and Tyr15 inhibitory phosphorylation sites located in the G-loop are shown in licorice representation     

Stereoselectivity of phosphotriesterase with paraoxon derivatives: a computational study

The bacterial enzyme phosphotriesterase (PTE) exhibits stereoselectivity toward hydrolysis of chiral substrates with a preference for the S_p enantiomer. In this work, docking analysis and two explicit-solvent molecular dynamics (MD) simulations were performed to characterize and differentiate the structural dynamics of PTE bound to the S_p and R_p paraoxon derivative enantiomers (R_p-1 and S_p-1) hydrolyzed with distinct catalytic efficiencies. Comparative analysis of the molecular trajectories for PTE bound to R_p-1 and S_p-1 suggested that substrate binding induced conformational changes in the loops near the active site. After 100 ns of MD simulation, the Zn β²⁺ metal ion formed hexacoordinated- and tetracoordinated geometries in the S_p-1-PTE and R_p-1-PTE ensembles, respectively. Simulation results further showed that the hydrogen bond between Asp301 and His254 occurred with a higher probability after S_p-1 binding to PTE (47.5%) than that after R_p-1 binding (22.2%). These results provide a qualitative and molecular-level explanation for the 10 orders of magnitude increase in the catalytic efficiency of PTE toward the S_p enantiomer of paraoxon.     

Electrostatic interactions play an essential role in DNA repair and cold-adaptation of Uracil DNA glycosylase

Life has adapted to most environments on earth, including low and high temperature niches. The increased catalytic efficiency and thermoliability observed for enzymes from organisms living in constantly cold regions when compared to their mesophilic and thermophilic cousins are poorly understood at the molecular level. Uracil DNA glycosylase (UNG) from cod (cUNG) catalyzes removal of uracil from DNA with an increased k_cat and reduced K_m relative to its warm-active human (hUNG) counterpart. Specific issues related to DNA repair and substrate binding/recognition (K_m) are here investigated by continuum electrostatics calculations, MD simulations and free energy calculations. Continuum electrostatic calculations reveal that cUNG has surface potentials that are more complementary to the DNA potential at and around the catalytic site when compared to hUNG, indicating improved substrate binding. Comparative MD simulations combined with free energy calculations using the molecular mechanics-Poisson Boltzmann surface area (MM-PBSA) method show that large opposing energies are involved when forming the enzyme-substrate complexes. Furthermore, the binding free energies obtained reveal that the Michaelis-Menten complex is more stable for cUNG, primarily due to enhanced electrostatic properties, suggesting that energetic fine-tuning of electrostatics can be utilized for enzymatic temperature adaptation. Energy decomposition pinpoints the residual determinants responsible for this adaptation. Figure Electrostatic isosurfaces of cod uracil DNA glycosylase in complex with double stranded DNA     

In silico and empirical approaches toward understanding the structural adaptation of the alkaline-stable lipase KV1 from Acinetobacter haemolyticus

Interests in Acinetobacter haemolyticus lipases are showing an increasing trend concomitant with growth of the enzyme industry and the widening search for novel enzymes and applications. Here, we present a structural model that reveals the key catalytic residues of lipase KV1 from A. haemolyticus. Homology modeling of the lipase structure was based on the structure of a carboxylesterase from the archaeon Archaeoglobus fulgidus as the template, which has a sequence that is 58% identical to that of lipase KV1. The lipase KV1 model is comprised of a single compact domain consisting of seven parallel and one anti-parallel β-strand surrounded by nine α-helices. Three structurally conserved active-site residues, Ser165, Asp259, and His289, and a tunnel through which substrates access the binding site were identified. Docking of the substrates tributyrin and palmitic acid into the pH 8 modeled lipase KV1 active sites revealed an aromatic platform responsible for the substrate recognition and preference toward tributyrin. The resulting binding modes from the docking simulation correlated well with the experimentally determined hydrolysis pattern, for which pH 8 and tributyrin being the optimum pH and preferred substrate. The results reported herein provide useful insights into future structure-based tailoring of lipase KV1 to modulate its catalytic activity.     

Molecular Modelling Studies on the Catalytic Mechanism of Candida Rugosa Lipase

Quantum chemical and molecular dynamics investigations have been performed on model systems for Candida rugosa lipase (CRL) to study mechanistic and conformational features of the catalytic hydrolysis. Based on X-ray data, a simplified model of the CRL substrate complex was created for the PM3 and ab initio calculations, including the amino acid residues both of the catalytic triad and the oxyanion hole.The energetic and structural properties of significant species along the pathway of the hydrolysis of the model substrate acetic acid methyl ester have been calculated. By modifications of the residues of the oxyanion hole as well as the catalytic triad, the influence of these parts of the active site on the pathway of the reaction was analysed in more detail.Moreover, molecular dynamics simulations have been carried out on CRL adducts with (±)-cis-4-acetamido-cyclopent-2-ene-1-carboxylic esters with different lengths of their alkyl chain and their absolute configuration as substrates. For the MD simulations using the AMBER program, all amino acid residues and water molecules with a cut-off radius less than 1500 pm from the substrate were taken into account. From the analysis of the trajectories and histograms for significant hydrogen bonds in the active site of the enzyme adducts, some hints were obtained for the enantiodifferentiation and the chain dependence of the esters in catalytic hydrolysis by CRL.Electronic Supplementary Material available.     

In silico analyses of substrate interactions with human serum paraoxonase 1

Human paraoxonase (HuPON1) is a serum enzyme that exhibits a broad spectrum of hydrolytic activities, including the hydrolysis of various organophosphates, esters, and recently identified lactone substrates. Despite intensive site-directed mutagenesis and other biological studies, the structural basis for the specificity of substrate interactions of HuPON1 remains elusive. In this study, we apply homology modeling, docking, and molecular dynamic (MD) simulations to probe the binding interactions of HuPON1 with representative substrates. The results suggest that the active site of HuPON1 is characterized by two distinct binding regions: the hydrophobic binding site for arylesters/lactones, and the paraoxon binding site for phosphotriesters. The unique binding modes proposed for each type of substrate reveal a number of key residues governing substrate specificity. The polymorphic residue R/Q192 interacts with the leaving group of paraoxon, suggesting it plays an important role in the proper positioning of this substrate in the active site. MD simulations of the optimal binding complexes show that residue Y71 undergoes an "open-closed" conformational change upon ligand binding, and forms strong interactions with substrates. Further binding free energy calculations and residual decomposition give a more refined molecular view of the energetics and origin of HuPON1/substrate interactions. These studies provide a theoretical model of substrate binding and specificity associated with wild type and mutant forms of HuPON1, which can be applied in the rational design of HuPON1 variants as bioscavengers with enhanced catalytic activity.     

Structure and dynamics of Trichoderma harzianum Cel7B suggest molecular architecture adaptations required for a wide spectrum of activities on plant cell wall polysaccharides

Cellulases from glycoside hydrolase family 7 (GH7) play crucial roles in plant lignocellulose deconstruction by fungi, but structural information available for GH7 fungal endoglucanases is limited when compared to the number of known sequences in the family. Here, we report the X-ray structure of the glycosylated catalytic domain (CD) of Trichoderma harzianum endoglucanase, ThCel7B, solved and refined at 2.9 Å resolution. Additionally, our extensive molecular dynamics simulations of this enzyme in complex with a variety of oligosaccharides provide a better understanding of its promiscuous hydrolytic activities on plant cell wall polysaccharides. The simulations demonstrate the importance of the hydrogen bond between substrate O2 hydroxyl in the subsite −1 and a side chain of catalytic Glu196 which renders ThCel7B capable to catalytically cleave cello and xylooligosaccharides, but not mannooligosaccharides. Moreover, detailed structural analyses and MD simulations revealed an additional binding pocket, suitable for accommodation of oligosaccharide decorations and/or substrates with mixed glycoside bonds that abuts onto the binding cleft close to subsite +2.     

Stereoselectivity of chalcone isomerase with chalcone derivatives: a computational study

Chalcone isomerase (CHI) catalyzes the intramolecular cyclization of chalcones into flavonoids. The activity of CHI is essential for the biosynthesis of flavonoids precursors of floral pigments and phenylpropanoid plant defense compounds. In the present study, we explored the detailed binding structures and binding free energies for two different active site conformations of CHI with s-cis/s-trans conformers of three chalcone compounds by performing molecular dynamics (MD) simulations and binding free energy calculations. The computational results indicate that s-cis/s-trans conformers of chalcone compounds are orientated in the similar binding position in the active site of CHI and stabilized by the different first hydrogen bond network and the same second hydrogen bond network. The first hydrogen bond network results in much lower binding affinity of s-trans conformer of chalcone compound with CHI than that of s-cis conformer. The conformational change of the active site residue T48 from indirectly interacting with the substrate via the second hydrogen bond network to directly forming the hydrogen bond with the substrates cannot affect the binding mode of both conformers of chalcone compounds, but remarkably improves the binding affinity. These results show that CHI has a strong stereoselectivity. The calculated binding free energies for three chalcone compounds with CHI are consistent with the experimental activity data. In addition, several valuable insights are suggested for future rational design and discovery of high-efficiency mutants of CHI.

Understanding the effect of tert-butanol on Candida antarctica lipase B using molecular dynamics simulations

The use of organic solvents as reaction media for enzymatic reactions has many advantages. Several organic solvents have been proposed as reaction media, especially for transesterifications using Candida antarctica lipase B (CalB). Among organic solvents, tert-butanol is associated with an enhanced conversion rate in bio-diesel production. Thus, it is necessary to understand the effect of tert-butanol on CalB to explain the high-catalytic efficiency compared with the reaction in other hydrophilic organic solvents. In this study, the effects of tert-butanol on the structure of CalB were investigated by MD simulations. The overall flexibility was increased in the presence of tert-butanol. The substrate entrance and the binding pocket size of CalB in tert-butanol were maintained as in TIP3P water. The distance between the catalytic residues of CalB in tert-butanol indicated a higher likelihood of forming hydrogen bonds. These structural analyses could be useful for understanding the effect of tert-butanol on lipase transesterification.     

Computational modeling and molecular dynamics simulations of mammalian cytoplasmic tyrosyl-tRNA synthetase and its complexes with substrates

Cytoplasmic tyrosyl-tRNA synthetase (TyrRS) is one of the key enzymes of protein biosynthesis. TyrRSs of pathogenic organisms have gained attention as potential targets for drug development. Identifying structural differences between various TyrRSs will facilitate the development of specific inhibitors for the TyrRSs of pathogenic organisms. However, there is a deficiency in structural data for mammalian cytoplasmic TyrRS in complexes with substrates. In this work, we constructed spatial structure of full-length Bos taurus TyrRS (BtTyrRS) and its complexes with substrates using the set of computational modeling techniques. Special attention was paid to BtTyrRS complexes with substrates [L-tyrosine, K⁺ and ATP:Mg²⁺] and intermediate products [tyrosyl-adenylate (Tyr-AMP), K⁺ and PP_i:Mg²⁺] with the different catalytic loop conformations. In order to analyze their dynamical properties, we performed 100 ns of molecular dynamics (MD) simulations. MD simulations revealed new structural data concerning the tyrosine activation reaction in mammalian TyrRS. Formation of strong interaction between Lys154 and γ-phosphate suggests the additional role of CP1 insertion as an important factor for ATP binding. The presence of a potassium-binding pocket within the active site of mammalian TyrRS compensates the absence of the second lysine in the KMSKS motif. Our data provide new details concerning a role of K⁺ ions at different stages of the first step of the tyrosylation reaction, including the coordination of substrates and involvement in the PP_i releasing. The results of this work suggest that differences between ATP-binding sites of mammalian and bacterial TyrRSs are meaningful and could be exploited in the drug design.     

Engineering a Disulfide Bond in the Lid Hinge Region of Rhizopus chinensis Lipase: Increased Thermostability and Altered Acyl Chain Length Specificity

The key to enzyme function is the maintenance of an appropriate balance between molecular stability and structural flexibility. The lid domain which is very important for “interfacial activation” is the most flexible part in the lipase structure. In this work, rational design was applied to explore the relationship between lid rigidity and lipase activity by introducing a disulfide bond in the hinge region of the lid, in the hope of improving the thermostability of R. chinensis lipase through stabilization of the lid domain without interfering with its catalytic performance. A disulfide bridge between F95C and F214C was introduced into the lipase from R. chinensis in the hinge region of the lid according to the prediction of the “Disulfide by Design” algorithm. The disulfide variant showed substantially improved thermostability with an eleven-fold increase in the t _1/2 value at 60°C and a 7°C increase of T _m compared with the parent enzyme, probably contributed by the stabilization of the geometric structure of the lid region. The additional disulfide bond did not interfere with the catalytic rate (k _cat) and the catalytic efficiency towards the short-chain fatty acid substrate, however, the catalytic efficiency of the disulfide variant towards pNPP decreased by 1.5-fold probably due to the block of the hydrophobic substrate channel by the disulfide bond. Furthermore, in the synthesis of fatty acid methyl esters, the maximum conversion rate by RCLCYS reached 95% which was 9% higher than that by RCL. This is the first report on improving the thermostability of the lipase from R. chinensis by introduction of a disulfide bond in the lid hinge region without compromising the catalytic rate.     

Involvement of Ser94 in RNase HIII from Chlamydophila pneumoniae in the recognition of a single ribonucleotide misincorporated into double-stranded DNA

We recently provided the first report that RNase HIII can cleave a DNA-rN₁-DNA/DNA substrate (rN₁, one ribonucleotide) in vitro. In the present study, mutagenesis analyses and molecular dynamics (MD) simulations were performed on RNase HIII from Chlamydophila pneumoniae AR39 (CpRNase HIII). Our results elucidate the mechanism of ribonucleotide recognition employed by CpRNase HIII, indicating that the G95/K96/G97 motif of CpRNase HIII represents the main surface interacting with single ribonucleotides, in a manner similar to that of the GR(K)G motif of RNase HIIs. However, CpRNase HIII lacks the specific tyrosine required for RNase HII to recognize single ribonucleotides in double-stranded DNA (dsDNA). Interestingly, MD shows that Ser94 of CpRNase HIII forms a stable hydrogen bond with the deoxyribonucleotide at the (5')RNA–DNA(3') junction, moving this nucleotide away from the chimeric ribonucleotide. This movement appears to deform the nucleic acid backbone at the RNA–DNA junction and allows the ribonucleotide to interact with the GKG motif. Based on the inferences drawn from MD simulations, biochemical results indicated that Ser94 was necessary for catalytic activity on the DNA-rN₁-DNA/DNA substrate; mutant S94V could bind this substrate but exhibited no cleavage. Mismatches opposite the single ribonucleotide misincorporated in dsDNA inhibited cleavage by CpRNase HIII to varying degrees but did not interfere with CpRNase/substrate binding. Further MD results implied that mismatches impair the interaction between Ser94 and the deoxyribonucleotide at the RNA–DNA junction. Consequently, recognition of the misincorporated ribonucleotide was disturbed. Our results may help elucidate the distinct substrate-recognition properties of different RNase Hs.     

An in silico mechanistic insight into HDAC8 activation facilitates the discovery of new small-molecule activators

Research interest in the development of histone deacetylase 8 (HDAC8) activators has substantially increased since loss-of-function HDAC8 mutations were found in patients with Cornelia de Lange syndrome (CdLS). A series of N-acetylthioureas (e.g., TM-2-51) have been identified as HDAC8-selective activators, among others; however, their activation mechanisms remain elusive. Herein, we performed molecular dynamics (MD) simulations and fragment-centric topographical mapping (FCTM) to investigate the mechanism of HDAC8 activation. Our results revealed that improper binding of the coumarin group of fluorescent substrates leads to the “flipping out” of catalytic residue Y306, which reduces the enzymatic activity of HDAC8 towards fluorescent substrates. A pocket between the coumarin group of the substrate and thed catalytic residue Y306 was filled with the activator TM-2-51, which not only enhanced binding between HDAC8 and the fluorescent substrate complex but also stabilized Y306 in a catalytically active conformation. Based on this newly proposed substrate-dependent activation mechanism, we performed structure-based virtual screening and successfully identified low-molecular-weight scaffolds as new HDAC8 activators.     

Structures of Human Steroidogenic Cytochrome P450 17A1 with Substrates

The human cytochrome P450 17A1 (CYP17A1) enzyme operates at a key juncture of human steroidogenesis, controlling the levels of mineralocorticoids influencing blood pressure, glucocorticoids involved in immune and stress responses, and androgens and estrogens involved in development and homeostasis of reproductive tissues. Understanding CYP17A1 multifunctional biochemistry is thus integral to treating prostate and breast cancer, subfertility, blood pressure, and other diseases. CYP17A1 structures with all four physiologically relevant steroid substrates suggest answers to four fundamental aspects of CYP17A1 function. First, all substrates bind in a similar overall orientation, rising ∼60° with respect to the heme. Second, both hydroxylase substrates pregnenolone and progesterone hydrogen bond to Asn²⁰² in orientations consistent with production of 17α-hydroxy major metabolites, but functional and structural evidence for an A105L mutation suggests that a minor conformation may yield the minor 16α-hydroxyprogesterone metabolite. Third, substrate specificity of the subsequent 17,20-lyase reaction may be explained by variation in substrate height above the heme. Although 17α-hydroxyprogesterone is only observed farther from the catalytic iron, 17α-hydroxypregnenolone is also observed closer to the heme. In conjunction with spectroscopic evidence, this suggests that only 17α-hydroxypregnenolone approaches and interacts with the proximal oxygen of the catalytic iron-peroxy intermediate, yielding efficient production of dehydroepiandrosterone as the key intermediate in human testosterone and estrogen synthesis. Fourth, differential positioning of 17α-hydroxypregnenolone offers a mechanism whereby allosteric binding of cytochrome b₅ might selectively enhance the lyase reaction. In aggregate, these structures provide a structural basis for understanding multiple key reactions at the heart of human steroidogenesis.     

Regiospecificity of Human Cytochrome P450 1A1-Mediated Oxidations: The Role of Steric Effects

Abstract

Cytochrome P450 1A1 oxidizes a diverse range of substrates, including the procarcinogenic xenobiotic benzo[a]pyrene (B[a]P) and endogenous fatty acid precursors of prostaglandins, such as arachidonic acid (AA) and eicosapentaenoic acid (EA). We have investigated the extent to which enzyme-substrate interactions govern regio- and stereoselectivity of oxidation of these compounds by using docking and molecular dynamics (MD) simulations to examine the likelihood of substrate oxidation at various sites. Due to structural differences between the substrates analyzed, B[a]P and its diols (planar, rigid), and the fatty acids AA and EA (long, flexible), different docking strategies were required. B[a]P, B[a]P-7,8-diols, (+) 7S,85- and (-) 7R,8R-diols, were docked into the active site of a homology model of P450 1A1 using an automated routine. Affinity (Accelrys, San Diego, CA). AA and EA, on the other hand, required a series of restrained MD simulations to obtain a variety of productive binding modes. All complexes were evaluated by MD-based in silico site scoring to predict product profiles based on certain geometric criteria, such as angle and distance of a given substrate atom from the ferryl oxygen. For all substrates studied, the in vitro profiles were generally reflected by the in silico scores, which suggests that steric factors play a key role in determining regiospecificity in P450 1A1-mediated oxidations. We have also shown that molecular dynamics simulations may be very useful in determination of product profiles for structurally diverse substrates of P450 enzymes.     

Elucidating the role of Trp105 in the KPC-2 ��-lactamase

The molecular basis of resistance to β‐lactams and β‐lactam‐β‐lactamase inhibitor combinations in the KPC family of class A enzymes is of extreme importance to the future design of effective β‐lactam therapy. Recent crystal structures of KPC‐2 and other class A β‐lactamases suggest that Ambler position Trp105 may be of importance in binding β‐lactam compounds. Based on this notion, we explored the role of residue Trp105 in KPC‐2 by conducting site‐saturation mutagenesis at this position. Escherichia coli DH10B cells expressing the Trp105Phe, ‐Tyr, ‐Asn, and ‐His KPC‐2 variants possessed minimal inhibitory concentrations (MICs) similar to E. coli cells expressing wild type (WT) KPC‐2. Interestingly, most of the variants showed increased MICs to ampicillin‐clavulanic acid but not to ampicillin‐sulbactam or piperacillin‐tazobactam. To explain the biochemical basis of this behavior, four variants (Trp105Phe, ‐Asn, ‐Leu, and ‐Val) were studied in detail. Consistent with the MIC data, the Trp105Phe β‐lactamase displayed improved catalytic efficiencies, k_cat/K_m, toward piperacillin, cephalothin, and nitrocefin, but slightly decreased k_cat/K_m toward cefotaxime and imipenem when compared to WT β‐lactamase. The Trp105Asn variant exhibited increased K_ms for all substrates. In contrast, the Trp105Leu and ‐Val substituted enzymes demonstrated notably decreased catalytic efficiencies (k_cat/K_m) for all substrates. With respect to clavulanic acid, the K_is and partition ratios were increased for the Trp105Phe, ‐Asn, and ‐Val variants. We conclude that interactions between Trp105 of KPC‐2 and the β‐lactam are essential for hydrolysis of substrates. Taken together, kinetic and molecular modeling studies define the role of Trp105 in β‐lactam and β‐lactamase inhibitor discrimination.     

Engineering the Substrate Specificity of a Thermophilic Penicillin Acylase from Thermus thermophilus

A homologue of the Escherichia coli penicillin acylase is encoded in the genomes of several thermophiles, including in different Thermus thermophilus strains. Although the natural substrate of this enzyme is not known, this acylase shows a marked preference for penicillin K over penicillin G. Three-dimensional models were created in which the catalytic residues and the substrate binding pocket were identified. Through rational redesign, residues were replaced to mimic the aromatic binding site of the E. coli penicillin G acylase. A set of enzyme variants containing between one and four amino acid replacements was generated, with altered catalytic properties in the hydrolyses of penicillins K and G. The introduction of a single phenylalanine residue in position α188, α189, or β24 improved the K_m for penicillin G between 9- and 12-fold, and the catalytic efficiency of these variants for penicillin G was improved up to 6.6-fold. Structural models, as well as docking analyses, can predict the positioning of penicillins G and K for catalysis and can demonstrate how binding in a productive pose is compromised when more than one bulky phenylalanine residue is introduced into the active site.     

Microsolvation effect and hydrogen-bonding pattern of taurine-water TA-(H2O)n (n = 1-3) complexes

The microsolvation of taurine (TA) with one, two or three water molecules was investigated by a density functional theory (DFT) approach. Quantum theory of atoms in molecules (QTAIM) analyses were employed to elucidate the hydrogen bond (H-bond) interaction characteristics in TA-(H₂O)_n (n = 1–3) complexes. The results showed that the intramolecular H-bond formed between the hydroxyl and the N atom of TA are retained in most TA-(H₂O)_n (n = 1–3) complexes, and are strengthened via cooperative effects among multiple H-bonds from n = 1–3. A trend of proton transformation exists from the hydroxyl to the N atom, which finally results in the cleavage of the origin intramolecular H-bond and the formation of a new intramolecular H-bond between the amino and the O atom of TA. Therefore, the most stable TA-(H₂O)₃ complex becomes a zwitterionic complex rather than a neutral type. A many-body interaction analysis showed that the major contributors to the binding energies for complexes are the two-body energies, while three-body energies and relaxation energies make significant contributions to the binding energies for some complexes, whereas the four-body energies are too small to be significant.     

Crystal structure and molecular modeling study of N-carbamoylsarcosine amidase Ta0454 from Thermoplasma acidophilum

A crystal structure of the putative N-carbamoylsarcosine amidase (CSHase) Ta0454 from Thermoplasma acidophilum was solved by single-wavelength anomalous diffraction and refined at a resolution of 2.35 Å. CSHases are involved in the degradation of creatinine. Ta0454 shares a similar fold and a highly conserved C-D-K catalytic triad (Cys123, Asp9, and Lys90) with the structures of three cysteine hydrolases (PDB codes 1NBA, 1IM5, and 2H0R). Molecular dynamics (MD) simulations of Ta0454/N-carbamoylsarcosine and Ta0454/pyrazinamide complexes were performed to determine the structural basis of the substrate binding pattern for each ligand. Based on the MD-simulated trajectories, the MM/PBSA method predicts binding free energies of ?24.5 and ?17.1 kcal/mol for the two systems, respectively. The predicted binding free energies suggest that Ta0454 is selective for N-carbamoylsarcosine over pyrazinamide, and zinc ions play an important role in the favorable substrate bound states.