Ligand Binding Enhances Millisecond Conformational Exchange in Xylanase B2 from Streptomyces lividans

Mapping secondary substrate-binding sites on the GH11 xylanase from Bacillus subtilis

Xylanases are of significant interest for biomass conversion technologies. Here, we investigated the allosteric regulation of xylan hydrolysis by the Bacillus subtilis GH11 endoxylanase. Molecular dynamics simulations (MDS) in the presence of xylobiose identified binding to the active site and two potential secondary binding sites (SBS) around surface residues Asn54 and Asn151. Arabinoxylan titration experiments with single cysteine mutants N54C and N151C labeled with the thiol-reactive fluorophore acrylodan or the ESR spin-label MTSSL validated the MDS results. Ligand binding at the SBS around Asn54 confirms previous reports, and analysis of the second SBS around N151C discovered in the present study includes residues Val98/Ala192/Ser155/His156. Understanding the regulation of xylanases contributes to efforts for industrial decarbonization and to establishing a sustainable energy matrix.

Altered Nucleotide Insertion Mechanisms of Disease-Associated TERT Variants

Telomere biology disorders (TBDs) are a spectrum of diseases that arise from mutations in genes responsible for maintaining telomere integrity. Human telomerase reverse transcriptase (hTERT) adds nucleotides to chromosome ends and is frequently mutated in individuals with TBDs. Previous studies have provided insight into how relative changes in hTERT activity can lead to pathological outcomes. However, the underlying mechanisms describing how disease-associated variants alter the physicochemical steps of nucleotide insertion remain poorly understood. To address this, we applied single-turnover kinetics and computer simulations to the Tribolium castaneum TERT (tcTERT) model system and characterized the nucleotide insertion mechanisms of six disease-associated variants. Each variant had distinct consequences on tcTERT's nucleotide insertion mechanism, including changes in nucleotide binding affinity, rates of catalysis, or ribonucleotide selectivity. Our computer simulations provide insight into how each variant disrupts active site organization, such as suboptimal positioning of active site residues, destabilization of the DNA 3' terminus, or changes in nucleotide sugar pucker. Collectively, this work provides a holistic characterization of the nucleotide insertion mechanisms for multiple disease-associated TERT variants and identifies additional functions of key active site residues during nucleotide insertion.

Dynamics of Ligand Binding to a Rigid Glycosidase*

The single-domain GH11 glycosidase from Bacillus circulans (BCX) is involved in the degradation of hemicellulose, which is one of the most abundant renewable biomaterials in nature. We demonstrate that BCX in solution undergoes minimal structural changes during turnover. NMR spectroscopy results show that the rigid protein matrix provides a frame for fast substrate binding in multiple conformations, accompanied by slow conversion, which is attributed to an enzyme-induced substrate distortion. A model is proposed in which the rigid enzyme takes advantage of substrate flexibility to induce a conformation that facilitates the acyl formation step of the hydrolysis reaction.

Visualizing Rev1 catalyze protein-template DNA synthesis

During DNA replication, replicative DNA polymerases may encounter DNA lesions, which can stall replication forks. One way to prevent replication fork stalling is through the recruitment of specialized translesion synthesis (TLS) polymerases that have evolved to incorporate nucleotides opposite DNA lesions. Rev1 is a specialized TLS polymerase that bypasses abasic sites, as well as minor-groove and exocyclic guanine adducts. Lesion bypass is accomplished using a unique protein-template mechanism in which the templating base is evicted from the DNA helix and the incoming dCTP hydrogen bonds with an arginine side chain of Rev1. To understand the protein-template mechanism at an atomic level, we employed a combination of time-lapse X-ray crystallography, molecular dynamics simulations, and DNA enzymology on the Saccharomyces cerevisiae Rev1 protein. We find that Rev1 evicts the templating base from the DNA helix prior to binding the incoming nucleotide. Binding the incoming nucleotide changes the conformation of the DNA substrate to orient it for nucleotidyl transfer, although this is not coupled to large structural changes in Rev1 like those observed with other DNA polymerases. Moreover, we found that following nucleotide incorporation, Rev1 converts the pyrophosphate product to two monophosphates, which drives the reaction in the forward direction and prevents pyrophosphorolysis. Following nucleotide incorporation, the hydrogen bonds between the incorporated nucleotide and the arginine side chain are broken, but the templating base remains extrahelical. These postcatalytic changes prevent potentially mutagenic processive synthesis by Rev1 and facilitate dissociation of the DNA product from the enzyme.

Novel xylanase producing Bacillus strain X2: molecular phylogenetic analysis and its application for production of xylooligosaccharides

A Bacillus strain X2 that produced extracellular endo-xylanase (GH 11) (EC: 3.2.1.8) was isolated from the soil of the Northeast India region. This aerobic culture was Gram positive and endospore forming. Chemotaxonomic characterization showed variance with the fatty acid profile of related species in the Bacillus subtilis group. In Bacillus strain X2, distinct occurrence of iso-C14:0 lipids is absent in other related species. The 16S rRNA gene sequence homology showed 99% similarity with Bacillus subtilis subsp. inaquosorum. The phylogenetic analysis by the multilocus sequence analysis (MLSA) of the nucleotide sequence of six concatenated genes (16S rRNA, groEL, gyrA, polC, purH and rpoB) resolved the taxonomic position of the Bacillus strain X2 in the Bacillus subtilis subsp. group. The MLSA showed that it is a member of a clade that includes Bacillus subtilis subsp. stercoris. In in silico DNA-DNA hybridization (DDH), the highest matching score was obtained with Bacillus subtilis subsp. stercoris (87%). The in silico DDH of the genome (G + C 43.7 mol %) shared 48.5%, with Bacillus subtilis subsp. inaquosorum. The MLSA phylogenetic tree and the highest degree of DNA hybridization, indicating that it belongs to the Bacillus subtilis subspecies stercoris.

AP-endonuclease 1 sculpts DNA through an anchoring tyrosine residue on the DNA intercalating loop

Base excision repair (BER) maintains genomic stability through the repair of DNA damage. Within BER, AP-endonuclease 1 (APE1) is a multifunctional enzyme that processes DNA intermediates through its backbone cleavage activity. To accomplish these repair activities, APE1 must recognize and accommodate several diverse DNA substrates. This is hypothesized to occur through a DNA sculpting mechanism where structural adjustments of the DNA substrate are imposed by the protein; however, how APE1 uniquely sculpts each substrate within a single rigid active site remains unclear. Here, we utilize structural and biochemical approaches to probe the DNA sculpting mechanism of APE1, specifically by characterizing a protein loop that intercalates the minor groove of the DNA (termed the intercalating loop). Pre-steady-state kinetics reveal a tyrosine residue within the intercalating loop (Y269) that is critical for AP-endonuclease activity. Using X-ray crystallography and molecular dynamics simulations, we determined the Y269 residue acts to anchor the intercalating loop on abasic DNA. Atomic force microscopy reveals the Y269 residue is required for proper DNA bending by APE1, providing evidence for the importance of this mechanism. We conclude that this previously unappreciated tyrosine residue is key to anchoring the intercalating loop and stabilizing the DNA in the APE1 active site.

Study on effects of co-solvents on the structure of DhaA by molecular dynamics simulation

With the increasing application of enzymes in various research fields, the choices of co-solvents in enzymatic preparations which directly related to the catalytic activity have been attracted attention. Thus, researching on the stabilization or destabilization behaviors of enzymes in different solvents is extremely essential. In this study, the structural changes of DhaA in two typical aprotic co-solvents (acetonitrile and tetrahydrofuran) were firstly investigated by molecular dynamics (MD) simulation. The simulation results revealed the strong van der Waals force between co-solvents and DhaA which could induce the structural change of enzyme. Interestingly, the differences of molecular size and the electrostatic force with enzyme of two co-solvents led to quite different influences on DhaA. As for acetonitrile, solvent molecules could penetrate into the catalytic site of DhaA which promoted by the electrostatic interaction. On the contrary, tetrahydrofuran molecules were mainly distributed around the catalytic site due to the relative weak electrostatic interaction and steric resistance effect. It can be concluded that different co-solvent can affect the key domains, substrate pathway and catalytic pocket of DhaA.Communicated by Ramaswamy H. Sarma.

Nucleotide substrate binding characterization in human pancreatic-type ribonucleases

Human genome contains a group of more than a dozen similar genes with diverse biological functions including antiviral, antibacterial and angiogenesis activities. The characterized gene products of this group show significant sequence similarity and a common structural fold associated with binding and cleavage of ribonucleic acid (RNA) substrates. Therefore, these proteins have been categorized as members of human pancreatic-type ribonucleases (hRNases). hRNases differ in cell/tissue localization and display distinct substrate binding preferences and a wide range of ribonucleolytic catalytic efficiencies. Limited information is available about structural and dynamical properties that influence this diversity among these homologous RNases. Here, we use computer simulations to characterize substrate interactions, electrostatics and dynamical properties of hRNases 1-7 associated with binding to two nucleotide substrates (ACAC and AUAU). Results indicate that even with complete conservation of active-site catalytic triad associated with ribonucleolytic activity, these enzymes show significant differences in substrate interactions. Detailed characterization suggests that in addition to binding site electrostatic and van der Waals interactions, dynamics of distal regions may also play a role in binding. Another key insight is that a small difference in temperature of 300 K (used in experimental studies) and 310 K (physiological temperature) shows significant changes in enzyme-substrate interactions.

Why the Energy Landscape of Barnase Is Hierarchical

We have used NMR and computational methods to characterize the dynamics of the ribonuclease barnase over a wide range of timescales in free and inhibitor-bound states. Using temperature- and denaturant-dependent measurements of chemical shift, we show that barnase undergoes frequent and highly populated hinge bending. Using relaxation dispersion, we characterize a slower and less populated motion with a rate of 750 ± 200 s⁻¹, involving residues around the lip of the active site, which occurs in both free and bound states and therefore suggests conformational selection. Normal mode calculations characterize correlated hinge bending motions on a very rapid timescale. These three measurements are combined with previous measurements and molecular dynamics calculations on barnase to characterize its dynamic landscape on timescales from picoseconds to milliseconds and length scales from 0.1 to 2.5 nm. We show that barnase has two different large-scale fluctuations: one on a timescale of 10⁻⁹−10⁻⁶ s that has no free energy barrier and is a hinge bending that is determined by the architecture of the protein; and one on a timescale of milliseconds (i.e., 750 s⁻¹) that has a significant free energy barrier and starts from a partially hinge-bent conformation. These two motions can be described as hierarchical, in that the more highly populated faster motion provides a platform for the slower (less probable) motion. The implications are discussed. The use of temperature and denaturant is suggested as a simple and general way to characterize motions on the intermediate ns-μs timescale.

Differential Substrate Recognition by Maltose Binding Proteins Influenced by Structure and Dynamics

The genome of the hyperthermophile Thermotoga maritima contains three isoforms of maltose binding protein (MBP) that are high-affinity receptors for di-, tri-, and tetrasaccharides. Two of these proteins (tmMBP1 and tmMBP2) share significant sequence identity, approximately 90%, while the third (tmMBP3) shares less than 40% identity. MBP from Escherichia coli (ecMBP) shares 35% sequence identity with the tmMBPs. This subset of MBP isoforms offers an interesting opportunity to investigate the mechanisms underlying the evolution of substrate specificity and affinity profiles in a genome where redundant MBP genes are present. In this study, the X-ray crystal structures of tmMBP1, tmMBP2, and tmMBP3 are reported in the absence and presence of oligosaccharides. tmMBP1 and tmMBP2 have binding pockets that are larger than that of tmMBP3, enabling them to bind to larger substrates, while tmMBP1 and tmMBP2 also undergo substrate-induced hinge bending motions (∼52°) that are larger than that of tmMBP3 (∼35°). Small-angle X-ray scattering was used to compare protein behavior in solution, and computer simulations provided insights into dynamics of these proteins. Comparing quantitative protein-substrate interactions and dynamical properties of tmMBPs with those of the promiscuous ecMBP and disaccharide selective Thermococcus litoralis MBP provides insights into the features that enable selective binding. Collectively, the results provide insights into how the structure and dynamics of tmMBP homologues enable them to differentiate between a myriad of chemical entities while maintaining their common fold.

Modulating Enzyme Activity by Altering Protein Dynamics with Solvent

Optimal enzyme activity depends on a number of factors, including structure and dynamics. The role of enzyme structure is well recognized; however, the linkage between protein dynamics and enzyme activity has given rise to a contentious debate. We have developed an approach that uses an aqueous mixture of organic solvent to control the functionally relevant enzyme dynamics (without changing the structure), which in turn modulates the enzyme activity. Using this approach, we predicted that the hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase (DHFR) from Escherichia coli in aqueous mixtures of isopropanol (IPA) with water will decrease by ∼3 fold at 20% (v/v) IPA concentration. Stopped-flow kinetic measurements find that the pH-independent k_hydride rate decreases by 2.2 fold. X-ray crystallographic enzyme structures show no noticeable differences, while computational studies indicate that the transition state and electrostatic effects were identical for water and mixed solvent conditions; quasi-elastic neutron scattering studies show that the dynamical enzyme motions are suppressed. Our approach provides a unique avenue to modulating enzyme activity through changes in enzyme dynamics. Further it provides vital insights that show the altered motions of DHFR cause significant changes in the enzyme's ability to access its functionally relevant conformational substates, explaining the decreased k_hydride rate. This approach has important implications for obtaining fundamental insights into the role of rate-limiting dynamics in catalysis and as well as for enzyme engineering.

Mapping Conformational Dynamics to Individual Steps in the TEM-1 β-Lactamase Catalytic Mechanism

Conformational dynamics are increasingly recognized as being essential for enzyme function. However, there is virtually no direct experimental evidence to support the notion that individual dynamic modes are required for specific catalytic processes, apart from the initial step of substrate binding. In this work, we use a unique approach based on millisecond hydrogen-deuterium exchange mass spectrometry to identify dynamic modes linked to individual catalytic processes in the antibiotic resistance enzyme TEM-1 β-lactamase. Using a "good" substrate (ampicillin), a poorly hydrolyzed substrate (cephalexin) and a covalent inhibitor (clavulanate), we are able to isolate dynamic modes that are specifically linked to substrate binding, productive lactam ring hydrolysis and deacylation. These discoveries are ultimately translated into specific targets for allosteric TEM-1 inhibitor development.

Applications of NMR and computational methodologies to study protein dynamics

Overwhelming evidence now illustrates the defining role of atomic-scale protein flexibility in biological events such as allostery, cell signaling, and enzyme catalysis. Over the years, spin relaxation nuclear magnetic resonance (NMR) has provided significant insights on the structural motions occurring on multiple time frames over the course of a protein life span. The present review article aims to illustrate to the broader community how this technique continues to shape many areas of protein science and engineering, in addition to being an indispensable tool for studying atomic-scale motions and functional characterization. Continuing developments in underlying NMR technology alongside software and hardware developments for complementary computational approaches now enable methodologies to routinely provide spatial directionality and structural representations traditionally harder to achieve solely using NMR spectroscopy. In addition to its well-established role in structural elucidation, we present recent examples that illustrate the combined power of selective isotope labeling, relaxation dispersion experiments, chemical shift analyses, and computational approaches for the characterization of conformational sub-states in proteins and enzymes.

Engineering a xylanase from Streptomyce rochei L10904 by mutation to improve its catalytic characteristics

Protein engineering was performed by N-terminal region replacement and site-directed mutagenesis in the cord of a xylanase (Srxyn) from Streptomyce rochei L10904 to improve its catalytic characteristics. Three mutants SrxynF, SrxynM and SrxynFM displayed 2.1-fold, 3.2-fold and 5.3-fold higher specific activities than that of Srxyn, respectively. Moreover, all of the mutants showed greater substrate affinity and k_cat/K_m than the native Srxyn. In addition, the enzymes showed improved hydrolysis characteristics, of which the most noteworthy is the enhanced ability of producing xylobiose (X2) and xylotriose (X3) from polymeric substrates. The engineered xylanases have greater potential for applications in oligosaccharide preparation industry.