Catalytic Activity of the N-Terminal Domain of Escherichia coli Asparagine Synthetase B Can Be Reengineered by Single-Point Mutation

Cryo-EM and Molecular Dynamics Simulations Reveal Hidden Conformational Dynamics Controlling Ammonia Transport in Human Asparagine Synthetase

How motions in enzymes might be linked to catalytic function is of considerable general interest. Recent advances in X-ray crystallography and cryogenic electron microscopy offer the promise of elucidating functionally relevant motions in proteins that are not easily amenable to study by other biophysical methods. Here we use 3D variability analysis (3DVA) on cryo-EM maps for wild type (WT) human asparagine synthetase (ASNS) and the R142I ASNS variant to identify conformational changes in the Arg-142 side chain, which mediates the formation of a catalytically relevant intramolecular tunnel. Our 3DVA results for WT ASNS are consistent with independent molecular dynamics (MD) simulations on a model generated from the X-ray structure of human ASNS. Moreover, MD simulations of computational models for the ASNS/β-aspartyl-AMP/MgPPi and R142I/β-aspartyl-AMP/MgPPi ternary complexes, suggest that the structural integrity of the tunnel is impaired in the R142I variant when β-aspartyl-AMP is present in the synthetase active site. The kinetic properties of the R142I ASNS variant support the proposed function of Arg-142. These studies illustrate the power of cryo-EM to identify localized motions and dissect the conformational landscape of large proteins. When combined with MD simulations, 3DVA is a powerful approach to understanding how conformational dynamics might regulate function in multi-domain enzymes possessing multiple active sites.

High-resolution crystal structure of human asparagine synthetase enables analysis of inhibitor binding and selectivity

Expression of human asparagine synthetase (ASNS) promotes metastatic progression and tumor cell invasiveness in colorectal and breast cancer, presumably by altering cellular levels of L-asparagine. Human ASNS is therefore emerging as a bona fide drug target for cancer therapy. Here we show that a slow-onset, tight binding inhibitor, which exhibits nanomolar affinity for human ASNS in vitro, exhibits excellent selectivity at 10 μM concentration in HCT-116 cell lysates with almost no off-target binding. The high-resolution (1.85 Å) crystal structure of human ASNS has enabled us to identify a cluster of negatively charged side chains in the synthetase domain that plays a key role in inhibitor binding. Comparing this structure with those of evolutionarily related AMP-forming enzymes provides insights into intermolecular interactions that give rise to the observed binding selectivity. Our findings demonstrate the feasibility of developing second generation human ASNS inhibitors as lead compounds for the discovery of drugs against metastasis.

Wen Zhu et al. report the crystal structure of human asparagine synthetase at a 1.85 Å resolution, enabling computational analysis of inhibitor binding. They also find new insights into the intermolecular interactions contributing to binding specificity of inhibitors.

Enzyme promiscuity in earthworm serine protease: substrate versatility and therapeutic potential

Enzymes are the most versatile molecules in the biological world. These amazing molecules play an integral role in the regulation of various metabolic pathways and physiology subsequently. Promiscuity of an enzyme is the capacity to catalyze additional biochemical reactions besides their native one. Catalytic promiscuity has shown great impact in enzyme engineering for commercial enzyme and therapeutics with natural or engineered catalytic promiscuity. The earthworm serine protease (ESP) is a classic example of enzyme promiscuity and studied for its therapeutic potential over the last few decades. The ESP was reported for several therapeutic properties and fibrinolytic activity has been much explored. ESP, a complex enzyme exists as several isoforms of molecular weight ranging from 14 to 33 kDa. The fibrinolytic capacity of the enzyme has been studied in different species of earthworm and molecular mechanism is quite different from conventional thrombolytics. Cytotoxic and anti-tumor activities of ESP were evaluated using several cancer cell lines. Enzyme had shown tremendous scope in fighting against plant viruses and microbes. ESP is also reported for anti-inflammatory activity and anti-oxidant property. Apart from these, recently, ESP is reported for DNase activity. The daunting challenge for researchers is to understand the molecular mechanism for such diverse properties and possibility of enzyme promiscuity. This review emphasizes molecular mechanism of ESP governing various biochemical reactions. Further, the concept of enzyme promiscuity in ESP towards development of novel enzyme based drugs has been reviewed in this study.

Inactivation of microbial arginine deiminases by L-canavanine

Arginine deiminase (ADI) catalyzes the hydrolytic conversion of L-arginine to ammonia and L-citrulline as part of the energy-producing L-arginine degradation pathway. The chemical mechanism for ADI catalysis involves initial formation and subsequent hydrolysis of a Cys-alkylthiouronium ion intermediate. The structure of the Pseudomonas aeruginosa ADI-(L-arginine) complex guided the design of arginine analogs that might react with the ADIs to form inactive covalent adducts during catalytic turnover. One such candidate is L-canavanine, in which an N-methylene of L-arginine is replaced by an N-O. This substance was shown to be a slow substrate-producing O-ureido-L-homoserine. An in depth kinetic and mass spectrometric analysis of P. aeruginosa ADI inhibition by L-canavanine showed that two competing pathways are followed that branch at the Cys-alkylthiouronium ion intermediate. One pathway leads to direct formation of O-ureido-L-homoserine via a reactive thiouronium intermediate. The other pathway leads to an inactive form of the enzyme, which was shown by chemical model and mass spectrometric studies to be a Cys-alkylisothiourea adduct. This adduct undergoes slow hydrolysis to form O-ureido-L-homoserine and regenerated enzyme. In contrast, kinetic and mass spectrometric investigations demonstrate that the Cys-alkylthiouronium ion intermediate formed in the reaction of L-canavanine with Bacillus cereus ADI partitions between the product forming pathway (O-ureido-L-homoserine and free enzyme) and an inactivation pathway that leads to a stable Cys-alkylthiocarbamate adduct. The ADIs from Escherichia coli, Burkholderia mallei, and Giardia intestinalis were examined in order to demonstrate the generality of the L-canavanine slow substrate inhibition and to distinguish the kinetic behavior that defines the irreversible inhibition observed with the B. cereus ADI from the time controlled inhibition observed with the P. aeruginosa, E. coli, B. mallei, and G. intestinalis ADIs.

Minimalist active-site redesign: teaching old enzymes new tricks

Although nature evolves its catalysts over millions of years, enzyme engineers try to do it a bit faster. Enzyme active sites provide highly optimized microenvironments for the catalysis of biologically useful chemical transformations. Consequently, changes at these centers can have large effects on enzyme activity. The prediction and control of these effects provides a promising way to access new functions. The development of methods and strategies to explore the untapped catalytic potential of natural enzyme scaffolds has been pushed by the increasing demand for industrial biocatalysts. This Review describes the use of minimal modifications at enzyme active sites to expand their catalytic repertoires, including targeted mutagenesis and the addition of new reactive functionalities. Often, a novel activity can be obtained with only a single point mutation. The many successful examples of active-site engineering through minimal mutations give useful insights into enzyme evolution and open new avenues in biocatalyst research.

Incorporation of a single His residue by rational design enables thiol-ester hydrolysis by human glutathione transferase A1-1

A strategy for rational enzyme design is reported and illustrated by the engineering of a protein catalyst for thiol-ester hydrolysis. Five mutants of human glutathione (GSH; gamma-Glu-Cys-Gly) transferase A1-1 were designed in the search for a catalyst and to provide a set of proteins from which the reaction mechanism could be elucidated. The single mutant A216H catalyzed the hydrolysis of the S-benzoyl ester of GSH under turnover conditions with a k(cat)/K(M) of 156 M(-1) x min(-1), and a catalytic proficiency of >10(7) M(-1) when compared with the first-order rate constant of the uncatalyzed reaction. The wild-type enzyme did not hydrolyze the substrate, and thus, the introduction of a single histidine residue transformed the wild-type enzyme into a turnover system for thiol-ester hydrolysis. By kinetic analysis of single, double, and triple mutants, as well as from studies of reaction products, it was established that the enzyme A216H catalyzes the hydrolysis of the thiol-ester substrate by a mechanism that includes an acyl intermediate at the side chain of Y9. Kinetic measurements and the crystal structure of the A216H GSH complex provided compelling evidence that H216 acts as a general-base catalyst. The introduction of a single His residue into human GSH transferase A1-1 created an unprecedented enzymatic function, suggesting a strategy that may be of broad applicability in the design of new enzymes. The protein catalyst has the hallmarks of a native enzyme and is expected to catalyze various hydrolytic, as well as transesterification, reactions.

Revisiting the steady state kinetic mechanism of glutamine-dependent asparagine synthetase from Escherichia coli

Escherichia coli asparagine synthetase B (AS-B) catalyzes the formation of asparagine from aspartate in an ATP-dependent reaction for which glutamine is the in vivo nitrogen source. In an effort to reconcile several different kinetic models that have been proposed for glutamine-dependent asparagine synthetases, we have used numerical methods to investigate the kinetic mechanism of AS-B. Our simulations demonstrate that literature proposals cannot reproduce the glutamine dependence of the glutamate/asparagine stoichiometry observed for AS-B, and we have therefore developed a new kinetic model that describes the behavior of AS-B more completely. The key difference between this new model and the literature proposals is the inclusion of an E.ATP.Asp.Gln quaternary complex that can either proceed to form asparagine or release ammonia through nonproductive glutamine hydrolysis. The implication of this model is that the two active sites in AS-B become coordinated only after formation of a beta-aspartyl-AMP intermediate in the synthetase site of the enzyme. The coupling of glutaminase and synthetase activities in AS is therefore different from that observed in all other well-characterized glutamine-dependent amidotransferases.

Tailoring new enzyme functions by rational redesign

Site-directed mutagenesis is still a very efficient strategy to elaborate improved enzymes. Recently, advances have been made in developing rational strategies aimed at reshaping enzyme specificities and mechanisms, and at engineering biocatalysts through molecular assembling. These knowledge-based studies greatly benefit from the most recent computational analyses of enzyme structures and functions. The combination of rational and combinatorial methods opens up new vistas in the design of stable and efficient enzymes.

Catalytic promiscuity and the evolution of new enzymatic activities

Several contemporary enzymes catalyze alternative reactions distinct from their normal biological reactions. In some cases the alternative reaction is similar to a reaction that is efficiently catalyzed by an evolutionary related enzyme. Alternative activities could have played an important role in the diversification of enzymes by providing a duplicated gene a head start towards being captured by adaptive evolution.

Formation and isolation of a covalent intermediate during the glutaminase reaction of a class II amidotransferase

Incubation of Escherichia coli asparagine synthetase B (AS-B) with [14C]-L-glutamine gives a covalent adduct that can be isolated. Radiolabeled protein is not observed (i) when the wild-type enzyme is incubated with 6-diazo-5-oxo-L-norleucine (DON) prior to reaction with [14C]glutamine or (ii) when the C1A AS-B mutant is incubated with [14C]-L-glutamine. Both of these alterations eliminate the ability of the enzyme to utilize glutamine but do not affect ammonia-dependent asparagine synthesis. Formation of the covalent adduct therefore depends on the presence of the N-terminal active site cysteine, which has been shown to be essential for glutamine-dependent activity in this and other class II amidotransferases. The amount of covalent adduct exhibits saturation behavior with increasing concentrations of L-glutamine. The maximum observed quantity of this intermediate is consistent with its involvement on the main pathway of glutamine hydrolysis. The chemical properties of the isolable covalent adduct are consistent with those anticipated for the gamma-glutamyl thioester that has been proposed as an intermediate in the AS-B-catalyzed conversion of glutamine to glutamate. The covalent adduct is acid-stable but is labile under alkaline conditions. On the basis of the measured rates of formation and breakdown of this intermediate, it is kinetically competent to participate in the normal catalytic mechanism. These studies represent the first description of a thioester intermediate for any class II amidotransferase and represent an important step in gaining further insight into the kinetic and chemical mechanisms of AS-B.

Mechanistic issues in asparagine synthetase catalysis

The enzymatic synthesis of asparagine is an ATP-dependent process that utilizes the nitrogen atom derived from either glutamine or ammonia. Despite a long history of kinetic and mechanistic investigation, there is no universally accepted catalytic mechanism for this seemingly straightforward carboxyl group activating enzyme, especially as regards those steps immediately preceding amide bond formation. This chapter considers four issues dealing with the mechanism: (a) the structural organization of the active site(s) partaking in glutamine utilization and aspartate activation; (b) the relationship of asparagine synthetase to other amidotransferases; (c) the way in which ATP is used to activate the beta-carboxyl group; and (d) the detailed mechanism by which nitrogen is transferred.