Synthesis and characterization of an N-acylsulfonamide inhibitor of human asparagine synthetase

Targeting endothelial cell metabolism in cancerous microenvironment: a new approach for anti-angiogenic therapy

Anti-angiogenic therapy is a practical approach to managing diseases with increased angiogenesis, such as cancer, maculopathies, and retinopathies. Considering the fundamental gaps in the knowledge of the vital pathways involved in angiogenesis and its inhibition and the insufficient efficiency of existing angiogenesis inhibitors, there is an increasing focus on the emergence of new therapeutic strategies aimed at inhibiting pathological angiogenesis. Angiogenesis is forming a new vascular network from existing vessels; endothelial cells (ECs), vascular lining cells, are the main actors of angiogenesis in physiological or pathological conditions. Switching from a quiescent state to a highly migratory and proliferative state during new vessel formation called "angiogenic switch" is driven by a "metabolic switch" in ECs, angiogenic growth factors, and other signals. As the characteristics of ECs change by altering the surrounding environment, they appear to have a different metabolism in a tumor microenvironment (TME). Therefore, pathological angiogenesis can be inhibited by targeting metabolic pathways. In the current review, we aim to discuss the EC metabolic pathways under normal and TME conditions to verify the suitability of targeting them with novel therapies.

Facile access to 4'-(N-acylsulfonamide) modified nucleosides and evaluation of their inhibitory activity against SARS-CoV-2 RNA cap N7-guanine-methyltransferase nsp14

N-Acylsulfonamides possess an additional carbonyl function compared to their sulfonamide analogues. Due to their unique physico-chemical properties, interest in molecules containing the N-acylsulfonamide moiety and especially nucleoside derivatives is growing in the field of medicinal chemistry. The recent renewal of interest in antiviral drugs derived from nucleosides containing a sulfonamide function has led us to evaluate the therapeutic potential of N-acylsulfonamide analogues. While these compounds are usually obtained by a difficult acylation of sulfonamides, we report here the easy and efficient synthesis of 20 4'-(N-acylsulfonamide) adenosine derivatives via the sulfo-click reaction. The target compounds were obtained from thioacid and sulfonyl azide synthons in excellent yields and were evaluated as potential inhibitors of the SARS-CoV-2 RNA cap N7-guanine-methyltransferase nsp14.

Asparagine Synthetase in Cancer: Beyond Acute Lymphoblastic Leukemia

Asparagine Synthetase (ASNS) catalyzes the synthesis of the non-essential amino acid asparagine (Asn) from aspartate (Asp) and glutamine (Gln). ASNS expression is highly regulated at the transcriptional level, being induced by both the Amino Acid Response (AAR) and the Unfolded Protein Response (UPR) pathways. Lack of ASNS protein expression is a hallmark of Acute Lymphoblastic Leukemia (ALL) blasts, which, therefore, are auxotrophic for Asn. This peculiarity is the rationale for the use of bacterial L-Asparaginase (ASNase) for ALL therapy, the first example of anti-cancer treatment targeting a tumor-specific metabolic feature. Other hematological and solid cancers express low levels of ASNS and, therefore, should also be Asn auxotrophs and ASNase sensitive. Conversely, in the last few years, several reports indicate that in some cancer types ASNS is overexpressed, promoting cell proliferation, chemoresistance, and a metastatic behavior. However, enhanced ASNS activity may constitute a metabolic vulnerability in selected cancer models, suggesting a variable and tumor-specific role of the enzyme in cancer. Recent evidence indicates that, beyond its canonical role in protein synthesis, Asn may have additional regulatory functions. These observations prompt a re-appreciation of ASNS activity in the biology of normal and cancer tissues, with particular attention to the fueling of Asn exchange between cancer cells and the tumor microenvironment.

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.

Fibrate-based N-acylsulphonamides targeting carbonic anhydrases: synthesis, biochemical evaluation, and docking studies

A large library of fibrate-based N-acylsulphonamides was designed, synthesised, and fully characterised in order to propose them as zinc binders for the inhibition of human carbonic anhydrase (hCA) enzymatic activity. Synthesised compounds were tested against four hCAs (I, II, IX, and XII) revealing a promising submicromolar inhibitory activity characterised by an isozyme selectivity pattern. Structural modifications explored within this scaffold are: presence of an aryl ring on the sulphonamide, p-substitution of this aryl ring, benzothiazole or benzophenone as core nuclei, and an n-propyl chain or a geminal dimethyl at Cα carbon. Biological results fitted well with molecular modelling analyses, revealing a putative direct interaction with the zinc ion in the active site of hCA I, II and IX. These findings supported the exploration of less investigated secondary sulphonamides as potential hCA inhibitors.

The Diverse Functions of Non-Essential Amino Acids in Cancer

Far beyond simply being 11 of the 20 amino acids needed for protein synthesis, non-essential amino acids play numerous important roles in tumor metabolism. These diverse functions include providing precursors for the biosynthesis of macromolecules, controlling redox status and antioxidant systems, and serving as substrates for post-translational and epigenetic modifications. This functional diversity has sparked great interest in targeting non-essential amino acid metabolism for cancer therapy and has motivated the development of several therapies that are either already used in the clinic or are currently in clinical trials. In this review, we will discuss the important roles that each of the 11 non-essential amino acids play in cancer, how their metabolic pathways are linked, and how researchers are working to overcome the unique challenges of targeting non-essential amino acid metabolism for cancer therapy.

Targeting adenylate-forming enzymes with designed sulfonyladenosine inhibitors

Adenylate-forming enzymes are a mechanistic superfamily that are involved in diverse biochemical pathways. They catalyze ATP-dependent activation of carboxylic acid substrates as reactive acyl adenylate (acyl-AMP) intermediates and subsequent coupling to various nucleophiles to generate ester, thioester, and amide products. Inspired by natural products, acyl sulfonyladenosines (acyl-AMS) that mimic the tightly bound acyl-AMP reaction intermediates have been developed as potent inhibitors of adenylate-forming enzymes. This simple yet powerful inhibitor design platform has provided a wide range of biological probes as well as several therapeutic lead compounds. Herein, we provide an overview of the nine structural classes of adenylate-forming enzymes and examples of acyl-AMS inhibitors that have been developed for each.

Structure-Based Design, Synthesis, and Biological Evaluation of Non-Acyl Sulfamate Inhibitors of the Adenylate-Forming Enzyme MenE

N-Acyl sulfamoyladenosines (acyl-AMS) have been used extensively to inhibit adenylate-forming enzymes that are involved in a wide range of biological processes. These acyl-AMS inhibitors are nonhydrolyzable mimics of the cognate acyl adenylate intermediates that are bound tightly by adenylate-forming enzymes. However, the anionic acyl sulfamate moiety presents a pharmacological liability that may be detrimental to cell permeability and pharmacokinetic profiles. We have previously developed the acyl sulfamate OSB-AMS (1) as a potent inhibitor of the adenylate-forming enzyme MenE, an o-succinylbenzoate-CoA (OSB-CoA) synthetase that is required for bacterial menaquinone biosynthesis. Herein, we report the use of computational docking to develop novel, non-acyl sulfamate inhibitors of MenE. A m-phenyl ether-linked analogue (5) was found to be the most potent inhibitor (IC₅₀ = 8 μM; K_d = 244 nM), and its X-ray co-crystal structure was determined to characterize its binding mode in comparison to the computational prediction. This work provides a framework for the development of potent non-acyl sulfamate inhibitors of other adenylate-forming enzymes in the future.

Copper-promoted dehydrogenative cross-coupling reaction of dialkyl phosphites with sulfoximines

Synthesis of sulfoximine derived phosphoramidates was achieved in good to excellent yields from NH-sulfoximines and dialkyl phosphites.

Angiogenesis revisited from a metabolic perspective: role and therapeutic implications of endothelial cell metabolism

Endothelial cell (EC) metabolism has lately emerged as a novel and promising therapeutic target to block vascular dysregulation associated with diseases like cancer and blinding eye disease. Glycolysis, fatty acid oxidation (FAO) and, more recently, glutamine/asparagine metabolism emerged as key regulators of EC metabolism, able to impact angiogenesis in health and disease. ECs are highly glycolytic as they require ATP and biomass for vessel sprouting. Notably, a regulator of the glycolytic pathway, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3, controls vessel sprouting during the angiogenic switch and its inhibition in tumour ECs leads to vessel normalization, thereby reducing metastasis and ameliorating chemotherapy. Moreover, FAO promotes EC proliferation through DNA synthesis, and plays an essential role in lymphangiogenesis via epigenetic regulation of histone acetylation. Pathological angiogenesis was decreased upon blockade of carnitine palmitoyltransferase 1, a regulator of FAO in ECs. More recently, metabolism of glutamine, in conjunction with asparagine, was reported to maintain EC sprouting through TCA anaplerosis, redox homeostasis, mTOR activation and endoplasmic stress control. Inactivation or blockade of glutaminase 1, which hydrolyses glutamine into ammonia and glutamate, impairs angiogenesis in health and disease, while silencing of asparagine synthetase reduces vessel sprouting in vitro. In this review, we summarize recent insights into EC metabolism and discuss therapeutic implications of targeting EC metabolism.

N-acylsulfonamides: Synthetic routes and biological potential in medicinal chemistry

Sulfonamide is a common structural motif in naturally occurring and synthetic medicinal compounds. The rising interest in sulfonamides and N-acyl derivatives is attested by the large number of drugs and lead compounds identified in last years, explored in different fields of medicinal chemistry and showing biological activity. Many acylsulfonamide derivatives were designed and synthesized as isosteres of carboxylic acids, being the characteristics of these functional groups very close. Starting from chemical routes to N-acylsulfonamides, this review explores compounds of pharmaceutical interest, developed as enzymatic inhibitors or targeting receptors.

Designed Small-Molecule Inhibitors of the Anthranilyl-CoA Synthetase PqsA Block Quinolone Biosynthesis in Pseudomonas aeruginosa

The Gram-negative bacterial pathogen Pseudomonas aeruginosa uses three interconnected intercellular signaling systems regulated by the transcription factors LasR, RhlR, and MvfR (PqsR), which mediate bacterial cell–cell communication via small-molecule natural products and control the production of a variety of virulence factors. The MvfR system is activated by and controls the biosynthesis of the quinolone quorum sensing factors HHQ and PQS. A key step in the biosynthesis of these quinolones is catalyzed by the anthranilyl-CoA synthetase PqsA. To develop inhibitors of PqsA as novel potential antivirulence antibiotics, we report herein the design and synthesis of sulfonyladeonsine-based mimics of the anthranilyl-AMP reaction intermediate that is bound tightly by PqsA. Biochemical, microbiological, and pharmacological studies identified two potent PqsA inhibitors, anthranilyl-AMS (1) and anthranilyl-AMSN (2), that decreased HHQ and PQS production in P. aeruginosa strain PA14. However, these compounds did not inhibit production of the virulence factor pyocyanin. Moreover, they exhibited limited bacterial penetration in compound accumulation studies. This work provides the most potent PqsA inhibitors reported to date and sets the stage for future efforts to develop analogues with improved cellular activity to investigate further the complex relationships between quinolone biosynthesis and virulence factor production in P. aeruginosa and the therapeutic potential of targeting PqsA.

A sulfoximine-based inhibitor of human asparagine synthetase kills L-asparaginase-resistant leukemia cells

An adenylated sulfoximine transition-state analogue 1, which inhibits human asparagine synthetase (hASNS) with nanomolar potency, has been reported to suppress the proliferation of an l-asparagine amidohydrolase (ASNase)-resistant MOLT-4 leukemia cell line (MOLT-4R) when l-asparagine is depleted in the medium. We now report the synthesis and biological activity of two new sulfoximine analogues of 1 that have been studied as part of systematic efforts to identify compounds with improved cell permeability and/or metabolic stability. One of these new analogues, an amino sulfoximine 5 having no net charge at cellular pH, is a better hASNS inhibitor (K(I)(∗)=8 nM) than 1 and suppresses proliferation of MOLT-4R cells at 10-fold lower concentration (IC(50)=0.1mM). More importantly, and in contrast to the lead compound 1, the presence of sulfoximine 5 at concentrations above 0.25 mM causes the death of MOLT-4R cells even when ASNase is absent in the culture medium. The amino sulfoximine 5 exhibits different dose-response behavior when incubated with an ASNase-sensitive MOLT-4 cell line (MOLT-4S), supporting the hypothesis that sulfoximine 5 exerts its effect by inhibiting hASNS in the cell. Our work provides further evidence for the idea that hASNS represents a chemotherapeutic target for the treatment of leukemia, and perhaps other cancers, including those of the prostate.

Stable analogues of OSB-AMP: potent inhibitors of MenE, the o-succinylbenzoate-CoA synthetase from bacterial menaquinone biosynthesis

MenE, the o-succinylbenzoate (OSB)-CoA synthetase from bacterial menaquinone biosynthesis, is a promising new antibacterial target. Sulfonyladenosine analogues of the cognate reaction intermediate, OSB-AMP, have been developed as inhibitors of the MenE enzymes from Mycobacterium tuberculosis (mtMenE), Staphylococcus aureus (saMenE) and Escherichia coli (ecMenE). Both a free carboxylate and a ketone moiety on the OSB side chain are required for potent inhibitory activity. OSB-AMS (4) is a competitive inhibitor of mtMenE with respect to ATP (K(i) =5.4±0.1 nM) and a noncompetitive inhibitor with respect to OSB (K(i) =11.2±0.9 nM). These data are consistent with a Bi Uni Uni Bi Ping-Pong kinetic mechanism for these enzymes. In addition, OSB-AMS inhibits saMenE with K(i)(app) =22±8 nM and ecMenE with K(i)(OSB) =128±5 nM. Putative active-site residues, Arg222, which may interact with the OSB aromatic carboxylate, and Ser302, which may bind the OSB ketone oxygen, have been identified through computational docking of OSB-AMP with the unliganded crystal structure of saMenE. A pH-dependent interconversion of the free keto acid and lactol forms of the inhibitors is also described, along with implications for inhibitor design.

A conserved glutamate controls the commitment to acyl-adenylate formation in asparagine synthetase

Inhibitor docking studies have implicated a conserved glutamate residue (Glu-348) as a general base in the synthetase active site of the enzyme asparagine synthetase B from Escherichia coli (AS-B). We now report steady-state kinetic, isotope transfer, and positional isotope exchange experiments for a series of site-directed AS-B mutants in which Glu-348 is substituted by conservative amino acid replacements. We find that formation of the β-aspartyl-AMP intermediate, and therefore the eventual production of asparagine, is dependent on the presence of a carboxylate side chain at this position in the synthetase active site. In addition, Glu-348 may also play a role in mediating the conformational changes needed to (i) coordinate, albeit weakly, the glutaminase and synthetase activities of the enzyme and (ii) establish the structural integrity of the intramolecular tunnel along which ammonia is translocated. The importance of Glu-348 in mediating acyl-adenylate formation contrasts with the functional role of the cognate residues in β-lactam synthetase (BLS) and carbapenem synthetase (CPS) (Tyr-348 and Tyr-345, respectively), which both likely evolved from asparagine synthetase. Given the similarity of the chemistry catalyzed by AS-B, BLS, and CPS, our work highlights the difficulty of predicting the functional outcome of single site mutations on enzymes that catalyze almost identical chemical transformations.

N-(Phenyl-sulfon-yl)-l-asparagine

In the title compound, C₁₀H₁₂N₂O₅S, one of the sulfonyl O atoms is hydrogen bonded to the amido N atom of an adjacent mol­ecule. There is also a weak hydrogen-bonding inter­action between the other sulfonyl O atom and the secondary amino N atom. In addition, the amido O atom is also hydrogen bonded to a carboxyl O atom. These hydrogen-bonding inter­actions give rise to a layer structure parallel to the bc plane.

An inhibitor of human asparagine synthetase suppresses proliferation of an L-asparaginase-resistant leukemia cell line

Drug resistance in lymphoblastic and myeloblastic leukemia cells is poorly understood, with several lines of evidence suggesting that resistance can be correlated with upregulation of human asparagine synthetase (hASNS) expression, although this hypothesis is controversial. New tools are needed to investigate this clinically important question, including potent hASNS inhibitors. In vitro experiments show an adenylated sulfoximine to be a slow-onset, tight-binding inhibitor of hASNS with nanomolar affinity. This binding affinity represents a 10-fold improvement over that reported for the only other well-characterized hASNS inhibitor. The adenylated sulfoximine has a cytostatic effect on L-asparaginase-resistant MOLT-4 cells cultured in the presence of L-asparaginase, an enzyme that depletes L-asparagine in the growth medium. These observations represent direct evidence that potent hASNS inhibitors may prove to be effective agents for the clinical treatment of acute lymphoblastic leukemia.

Synthesis and biological properties of new phosmidosine analogs having an N-acylsulfamate linkage

A new phosmidosine analog 10, in which the proline and 8-oxoadenosine moieties were linked by an N-acyl sulfamate linkage, was successfully synthesized by the sulfamoylation of an 8-oxoadenosine derivative 5 followed by coupling with an L-proline derivative 8. An L-alanine-substituted derivative 13 and its derivative 14 without the alanyl residue were also synthesized. The morphological reversion activity of these synthetic compounds in v-src(ts) NRK cells and their antitumor activity in L1210 and KB cells were studied. As the result, neither L-proline- nor L-alanine-substituted phosmidosine analogs 10 and 13 showed any antitumor activity. Contrary to these results, the derivative 14 lacking the amino acid residue showed potent antitumor activities against cancer cells.

Asparagine synthetase as a causal, predictive biomarker for L-asparaginase activity in ovarian cancer cells

L-Asparaginase (l-ASP), a bacterial enzyme used since the 1970s to treat acute lymphoblastic leukemia, selectively starves cells that cannot synthesize sufficient asparagine for their own needs. Molecular profiling of the NCI-60 cancer cell lines using five different microarray platforms showed strong negative correlations of asparagine synthetase (ASNS) expression and DNA copy number with sensitivity to l-ASP in the leukemia and ovarian cancer cell subsets. To assess whether the ovarian relationship is causal, we used RNA interference to silence ASNS in three ovarian lines and observed 4- to 5-fold potentiation of sensitivity to l-ASP with two of the lines. For OVCAR-8, the line that expresses the least ASNS, the potentiation was >500-fold. Significantly, that potentiation was >700-fold in the multidrug-resistant derivative OVCAR-8/ADR, showing that the causal relationship between ASNS expression and l-ASP activity survives development of classical multidrug resistance. Tissue microarrays confirmed low ASNS expression in a subset of clinical ovarian cancers as well as other tumor types. Overall, this pharmacogenomic/pharmacoproteomic study suggests the use of l-ASP for treatment of a subset of ovarian cancers (and perhaps other tumor types), with ASNS as a biomarker for patient selection.

The design and synthesis of inhibitors of pantothenate synthetase

Pantothenate synthetase catalyses the ATP-dependent condensation of D-pantoate and beta-alanine to form pantothenate. Ten analogues of the reaction intermediate pantoyl adenylate, in which the phosphodiester is replaced by either an ester or sulfamoyl group, were designed as potential inhibitors of the enzyme. The esters were all modest competitive inhibitors, the sulfamoyls were more potent, consistent with their closer structural similarity to the pantoyl adenylate intermediate.

Asparagine synthetase chemotherapy

Modern clinical treatments of childhood acute lymphoblastic leukemia (ALL) employ enzyme-based methods for depletion of blood asparagine in combination with standard chemotherapeutic agents. Significant side effects can arise in these protocols and, in many cases, patients develop drug-resistant forms of the disease that may be correlated with up-regulation of the enzyme glutamine-dependent asparagine synthetase (ASNS). Though the precise molecular mechanisms that result in the appearance of drug resistance are the subject of active study, potent ASNS inhibitors may have clinical utility in treating asparaginase-resistant forms of childhood ALL. This review provides an overview of recent developments in our understanding of (a) the structure and catalytic mechanism of ASNS, and (b) the role that ASNS may play in the onset of drug-resistant childhood ALL. In addition, the first successful, mechanism-based efforts to prepare and characterize nanomolar ASNS inhibitors are discussed, together with the implications of these studies for future efforts to develop useful drugs.

Efficient expression, purification, and characterization of C-terminally tagged, recombinant human asparagine synthetase

Several lines of evidence suggest that up-regulation of asparagine synthetase (AS) in human T-cells results in metabolic changes that underpin the appearance of asparaginase-resistant forms of acute lymphoblastic leukemia (ALL). Inhibitors of human AS therefore have potential as agents for treating leukemia and tools for investigating the cellular basis of AS expression and drug-resistance. A critical problem in developing and characterizing potent inhibitors has been a lack of routine access to sufficient quantities of purified, reproducibly active human AS. We now report an efficient protocol for preparing multi-milligram quantities of C-terminally tagged, wild type human AS in a baculovirus-based expression system. The recombinant enzyme is correctly processed and exhibits high catalytic activity. Not only do these studies offer the possibility for investigating the kinetic behavior of biochemically interesting mammalian AS mutants, but such ready access to large amounts of enzyme also represents a major step in the development and characterization of inhibitors that might have clinical utility in treating asparaginase-resistant ALL.

Synthesis of an N-acyl sulfamate analog of luciferyl-AMP: a stable and potent inhibitor of firefly luciferase

In the first of two half-reactions resulting in the emission of visible light, firefly luciferase forms luciferyl-adenylate from its natural substrates beetle luciferin and Mg-ATP. The acyl-adenylate is subsequently oxidized producing the light emitter oxyluciferin in an electronically excited state. In vitro, under mild conditions of temperature and pH, the acyl-adenylate intermediate is readily hydrolyzed and susceptible to oxidation. We report here the multi-step synthesis and physical and enzymatic characterization of an N-acyl sulfamate analog of luciferyl-adenylate, 5'-O-[(N-dehydroluciferyl)-sulfamoyl]-adenosine (compound 5). This represents the first example of a stable and potent (Ki = 340 nM) reversible inhibitor of firefly luciferase activity based on the structure of the natural acyl-adenylate intermediate. Additionally, we present the results of limited proteolysis studies that demonstrate that the binding of the novel acyl-adenylate analog protects luciferase from proteolysis. The findings presented here are interpreted in the context of the hypothesis that luciferase and the other enzymes in a large superfamily of adenylate-forming proteins adopt two conformations to catalyze two different partial reactions. We anticipate that the novel N-acyl sulfamate analog will be a valuable reagent in future studies designed to elucidate the role of conformational changes in firefly luciferase catalyzed bioluminescence.

Enzyme inhibitors as chemical tools to study enzyme catalysis: rational design, synthesis, and applications

Carefully designed molecules that are intimately related to the reaction mechanism of enzymes are often highly selective and potent inhibitors that serve as extremely useful chemical probes for understanding the reaction mechanism and structure of enzymes. This article describes the design, synthesis, and applications of specific inhibitors of two mechanistically distinct groups of enzymes, ATP-dependent amide ligases and Ser- and Thr-hydrolases. Our strategy is based on the premise that stable analogues of the transition state (transition-state analogues) are highly potent inhibitors that serve as good mechanistic probes, and that a key structure of a good inhibitor of one enzyme is also utilized for the inhibitors of other enzymes that share the same chemistry in their catalyzed reactions, irrespective of the degree of structural similarity and evolutionary link between the enzymes. According to these principles, we designed and synthesized a series of phosphinate- and sulfoximine-based transition-state analogue inhibitors of glutathione synthetase, gamma-glutamylcysteine synthetase and asparagine synthetase. For the second group of enzymes, we synthesized a gamma-monofluorophosphono glutamate analogue for mechanism-based affinity labeling of gamma-glutamyltranspeptidase and fluorescent phosphonic acid esters for the active-site titration of lipase. These inhibitors were used successfully as ligands for detailed kinetic analyses, X-ray crystallography, and mass analysis of the enzymes to identify the key amino acid residues responsible for catalysis and substrate recognition in the transition state.