Combined thermal and carboxypeptidase Y stability assays for probing the threaded fold of lasso peptides.

Lasso peptides are natural products belonging to the superfamily of ribosomally synthesized and post-translationally modified peptides (RiPPs). The defining characteristic of lasso peptides is their threaded structure, which is reminiscent of a lariat knot. When working with lasso peptides, it is therefore of major importance to understand and evidence their threaded folds. While the full elucidation of their three-dimensional structures via NMR spectroscopy or crystallization remains the gold standard, these methods are time-consuming, require large quantities of highly pure lasso peptides, and therefore might not always be applicable. Instead, the unique properties of lasso peptides in context of their behavior at elevated temperatures and toward carboxypeptidase Y treatment can be leveraged as a tool to investigate and evidence the threaded lasso fold using only minute amounts of compound that does not need to be purified first. This chapter will provide insights into the thermal stability properties of lasso peptides and their behavior when treated with carboxypeptidase Y in comparison to a branched-cyclic peptide with the same amino acid sequence. Furthermore, it will be described in detail how to set up a combined thermal and carboxypeptidase Y stability assay and how to analyze its outcomes.

Human carboxylesterase 1A plays a predominant role in the hydrolytic activation of remdesivir in humans.

Remdesivir, an intravenous nucleotide prodrug, has been approved for treating COVID-19 in hospitalized adults and pediatric patients. Upon administration, remdesivir can be readily hydrolyzed to form its active form GS-441524, while the cleavage of the carboxylic ester into GS-704277 is the first step for remdesivir activation. This study aims to assign the key enzymes responsible for remdesivir hydrolysis in humans, as well as to investigate the kinetics of remdesivir hydrolysis in various enzyme sources. The results showed that remdesivir could be hydrolyzed to form GS-704277 in human plasma and the microsomes from human liver (HLMs), lung (HLuMs) and kidney (HKMs), while the hydrolytic rate of remdesivir in HLMs was the fastest. Chemical inhibition and reaction phenotyping assays suggested that human carboxylesterase 1 (hCES1A) played a predominant role in remdesivir hydrolysis, while cathepsin A (CTSA), acetylcholinesterase (AchE) and butyrylcholinesterase (BchE) contributed to a lesser extent. Enzymatic kinetic analyses demonstrated that remdesivir hydrolysis in hCES1A (SHUTCM) and HLMs showed similar kinetic plots and much closed Km values to each other. Meanwhile, GS-704277 formation rates were strongly correlated with the CES1A activities in HLM samples from different individual donors. Further investigation revealed that simvastatin (a therapeutic agent for adjuvant treating COVID-19) strongly inhibited remdesivir hydrolysis in both recombinant hCES1A and HLMs. Collectively, our findings reveal that hCES1A plays a predominant role in remdesivir hydrolysis in humans, which are very helpful for predicting inter-individual variability in response to remdesivir and for guiding the rational use of this anti-COVID-19 agent in clinical settings.

Structural and kinetic evidence of aging after organophosphate inhibition of human Cathepsin A.

Human Cathepsin A (CatA) is a lysosomal serine carboxypeptidase of the renin-angiotensin system (RAS) and is structurally similar to acetylcholinesterase (AChE). CatA can remove the C-terminal amino acids of endothelin I, angiotensin I, Substance P, oxytocin, and bradykinin, and can deamidate neurokinin A. Proteomic studies identified CatA and its homologue, SCPEP1, as potential targets of organophosphates (OP). CatA could be stably inhibited by low µM to high nM concentrations of racemic sarin (GB), soman (GD), cyclosarin (GF), VX, and VR within minutes to hours at pH 7. Cyclosarin was the most potent with a kinetically measured dissociation constant (KI) of 2 µM followed by VR (KI = 2.8 µM). Bimolecular rate constants for inhibition by cyclosarin and VR were 1.3 × 103 M-1sec-1 and 1.2 × 103 M-1sec-1, respectively, and were approximately 3-orders of magnitude lower than those of human AChE indicating slower reactivity. Notably, both AChE and CatA bound diisopropylfluorophosphate (DFP) comparably and had KIDFP = 13 µM and 11 µM, respectively. At low pH, greater than 85% of the enzyme spontaneously reactivated after OP inhibition, conditions under which OP-adducts of cholinesterases irreversibly age. At pH 6.5 CatA remained stably inhibited by GB and GF and <10% of the enzyme spontaneously reactivated after 200 h. A crystal structure of DFP-inhibited CatA was determined and contained an aged adduct. Similar to AChE, CatA appears to have a "backdoor" for product release. CatA has not been shown previously to age. These results may have implications for: OP-associated inflammation; cardiovascular effects; and the dysregulation of RAS enzymes by OP.

Identification and immunoregulatory role of cathepsin A in the red swamp crayfish, Procambarus clarkii.

Cathepsins are a group of lysosomal hydrolytic enzymes, broadly distributed in animals, and regulate various physiological processes. However, the immune functions of cathepsins are poorly understood in invertebrates. Therefore, to further provide information about the importance of cathepsins in the innate immune system of crustaceans, cathepsin A from Procambarus clarkii (Pc-cathepsin A) was characterized and its distribution in different tissues was determined. The immunological functions of the Pc-cathepsin A were also evaluated. The Pc-cathepsin A showed high sequence homology to cathepsins of other species, as it contained serine and histidine active sites. Quantitative RT-PCR analysis revealed that the expression of Pc-cathepsin A was highest in the gill, gut, and the hepatopancreas, with variable amounts in the muscle, stomach, heart, and hemocytes. The mRNA expression of Pc-cathepsin A was significantly increased in hepatopancreas challenged with lipopolysaccharide (LPS), peptidoglycan (PGN), and polycytidylic acid (poly I:C). The results of an in vivo analysis revealed that Pc-cathepsin A knockdown by double-stranded RNA in P. clarkii modulated the expression of immune-pathway associated genes in hepatopancreas. Collectively, these results suggest that Pc-cathepsin A modulates innate immune responses by affecting the expression of immune-pathway associated genes, thus revealing a regulatory link between Pc-cathepsin A and immune pathways in P. clarkii, and that Pc-cathepsin A plays an essential biological role in the immune defence against microbial pathogens.

Mechanisms of the Ase1/PRC1/MAP65 family in central spindle assembly.

During cytokinesis, the organization of the spindle midzone and chromosome segregation is controlled by the central spindle, a microtubule cytoskeleton containing kinesin motors and non-motor microtubule-associated proteins. The anaphase spindle elongation 1/protein regulator of cytokinesis 1/microtubule associated protein 65 (Ase1/PRC1/MAP65) family of microtubule-bundling proteins are key regulators of central spindle assembly, mediating microtubule crosslinking and spindle elongation in the midzone. Ase1/PRC1/MAP65 serves as a complex regulatory platform for the recruitment of other midzone proteins at the spindle midzone. Herein, we summarize recent advances in understanding of the structural domains and molecular kinetics of the Ase1/PRC1/MAP65 family. We summarize the regulatory network involved in post-translational modifications of Ase1/PRC1 by cyclin-dependent kinase 1 (Cdk1), cell division cycle 14 (Cdc14) and Polo-like kinase 1 (Plk1) and also highlight multiple functions of Ase1/PRC1 in central spindle organization, spindle elongation and cytokinesis during cell division.

Paired Carboxylic Acids in Enzymes and Their Role in Selective Substrate Binding, Catalysis, and Unusually Shifted pKa Values.

Cathepsin A (CatA, EC 3.4.16.5, UniProtKB P10619 ) is a human lysosomal carboxypeptidase. Counterintuitively, crystal structures of CatA and its homologues show a cluster of Glu and Asp residues binding the C-terminal carboxylic acid of the product or inhibitor. Each of these enzymes functions in an acidic environment and contains a highly conserved pair of Glu residues with side chain carboxyl group oxygens that are approximately 2.3-2.6 Šapart. In small molecules, carboxyl groups separated by ∼3 Šcan overcome the repulsive interaction by protonation of one of the two groups. The pKa of one group increases (pKa ∼ 11) and can be as much as ∼6 pH units higher than the paired group. Consequently, at low and neutral pH, one carboxylate can carry a net negative charge while the other can remain protonated and neutral. In CatA, E69 and E149 form a Glu pair that is important to catalysis as evidenced by the 56-fold decrease in kcat/Km in the E69Q/E149Q variant. Here, we have measured the pH dependencies of log(kcat), log(Km), and log(kcat/Km) for wild type CatA and its variants and have compared the measured pKa with calculated values. We propose a substrate-assisted mechanism in which the high pKa of E149 (>8.5) favors the binding of the carboxylate form of the substrate and promotes the abstraction of the proton from H429 of the catalytic triad effectively decreasing its pKa in a low-pH environment. We also identify a similar motif consisting of a pair of histidines in S-formylglutathione hydrolase.

Detection of casein kinase II by aggregation-induced emission.

A novel aggregation-induced emission (AIE) probe comprised of a hydrophilic protein kinase specific peptide and a hydrophobic tetraphenylethene (TPE) unit was synthesized through click reaction. The prepared TPE-peptide probe could be completely degraded by carboxypeptidase Y (CPY) to release hydrophobic TPE part, which aggregated in buffer solution and showed strong TPE emission. In the presence of casein kinase (CKII), the phosphorylation of peptide prevented the complete degradation by CPY producing the nonemissive probe. Thus, the developed probe can be used to detect CKII homogeneously and conveniently. This detection process can be finished within 1.5 h with high sensitivity (0.05 mU/µL) and good selectivity. The developed method can also be used to screen protein kinase inhibitor even in a complex biological system.

Symmetrical Diamidates as a Class of Phosphate Prodrugs to Deliver the 5'-Monophosphate Forms of Anticancer Nucleoside Analogues.

The application of phosphorodiamidate technology to pyrimidine and purine nucleosides with anticancer activity to potentially overcome the resistance mechanisms associated with parent nucleosides is reported. Sixteen symmetrical phosphorodiamidates were prepared from the natural amino acids l-alanine and glycine. All the compounds were evaluated for their cytotoxic activity against a wide panel of solid and leukaemic tumour cell lines. In addition, a carboxypeptidase Y assay was performed on a representative phosphorodiamidate in order to reveal the putative bioactivation pathway for the reported phosphorodiamidate-type prodrugs.

Molecular Cloning, Characterization, and Expression Analysis of Cathepsin A in the Chinese Giant Salamander Andrias davidianus.

Cathepsin A (CTSA) is serine carboxypeptidase, an important protease in the lysosome. In this study, the full complementary DNA (cDNA) sequence of CTSA in Chinese giant salamanders Andrias davidianus was cloned, and its sequence features were analyzed. Tissue expression patterns of CTSA in healthy and Aeromonas hydrophila-infected salamanders were also investigated. The full cDNA sequence of salamander CTSA was 1,620 base pairs in length, encoding 472 amino acids. Salamander CTSA shared high sequence identities with other vertebrates' CTSAs, ranging from 62.7% to 68.9%. In healthy salamanders, CTSA was highly expressed in spleen, followed by brain, intestine, and stomach. After A. hydrophila infection, salamander CTSA was significantly upregulated in lung, heart, muscle, and kidney; was downregulated in liver, spleen, and intestine; and exhibited no significant changes in stomach and skin, indicating that salamander CTSA might play defense roles in multiple tissues during bacterial infection. These results provide a solid basis for further study of the immune function of amphibian CTSA. Received September 18, 2016; accepted June 18, 2017.

Label-Free Homogeneous Electrochemical Sensing Platform for Protein Kinase Assay Based on Carboxypeptidase Y-Assisted Peptide Cleavage and Vertically Ordered Mesoporous Silica Films.

Presented herein is a simple, robust, and label-free homogeneous electrochemical sensing platform constructed for the detection of protein kinase activity and inhibition by integration of carboxypeptidase Y (CPY)-assisted peptide cleavage reaction and vertically ordered mesoporous silica films (MSFs). In this sensing platform, the substrate peptide composed of kinase-specific recognized sequence and multiple positively charged arginine (R) residues was ingeniously designed. In the presence of protein kinase, the substrate peptide was phosphorylated and then immediately resisted CPY cleavage. The phosphorylated peptide could be effectively adsorbed on the negatively charged surface of MSFs modified indium-tin oxide (ITO) electrode (MSFs/ITO) by noncovalent electrostatic attraction. The adsorbed peptide was subsequently used as a hamper to prevent the diffusion of electroactive probe (FcMeOH) to the electrode surface through the vertically aligned nanopores, resulting in a detectable reduction of electrochemical signal. As demonstrated for the feasibility and universality of the sensing platform, both protein kinase A (PKA) and casein kinase II (CK2) were selected as the models, and the detection limits were determined to be 0.083 and 0.095 UmL-1, respectively. This sensing platform had the merits of simplicity, easy manipulation, and improved phosphorylation and cleavage efficiency, which benefited from homogeneous solution reactions without sophisticated modification or immobilization procedures. In addition, given the key role of inhibition and protein kinase activity detection in cell lysates, this proposed sensing platform showed great potential in kinase-related bioanalysis and clinical biomedicine.

N-Glycosylation analysis of yeast Carboxypeptidase Y reveals the ultimate removal of phosphate from glycans at Asn368.

Carboxypeptidase Y from Saccharomyces cerivisiae was characterized for its site specific N-glycosylation through mass spectrometry. The N-glycopeptides were derived using non specific proteases and are analysed directly on liquid chromatography coupled to ion trap mass spectrometer in tandem mode. The evaluation of glycan fragment ions and the Y1 ions (peptide+HexNAc)+n revealed the glycan sequence and the corresponding site of attachment. We observed the microheterogeneity in N-glycans such as Man11-15GlcNAc2 at Asn13, Man8-12GlcNAc2 at Asn87, Man9-14GlcNAc2 at Asn168 and phosphorylated Man12-17GlcNAc2 as well as Man11-16GlcNAc2 at Asn368. The presence of N-glycans with Man<18GlcNAc2 indicated that in vacuoles the steady release of mannose/phospho mannose residues from glycans occurs initially at Asn13 or Asn168 followed by at Asn368. However, glycans at Asn87 which comprises Man8-12 residues as reported earlier remain intact suggesting its inaccessibility for a similar processing. This in turn indicates the interaction of the glycan at Asn87 with the polypeptide chain implicating it in the folding of the protein.

Optimization and application of protein C-terminal labeling by carboxypeptidase Y.

Proteolytic cleavage is one of the post-translational modifications and plays important roles in many biological processes, such as apoptosis and tumor cell metastasis. The identification of the cleavage events can improve our understanding of their biological functions in these processes. Although proteomic approaches using N-terminal labeling have resulted in the discovery of many proteolytic cleavages, this strategy has its own inherent drawbacks. Labeling of protein C-termini is an alternative approach. Here, we optimized the labeling procedure in the profiling protein C-termini by enzymatic labeling (ProC-TEL) and improved the labeling efficiency for the positive isolation of protein C-terminal peptides and mass spectrometric identification. We applied this approach to a complex protein mixture from Escherichia coli and identified many C-terminal peptides and internal cleaved peptides from more than 120 proteins. From the identified cleavages, we found several previously known internal proteolytic cleavage sites and many novel ones which may play roles in regulating normal biological processes. This work provides a potential new way, complementary to the N-terminomics, for the identification of proteolytic cleavages in complex biological systems.

Structural basis for the binding of tryptophan-based motifs by δ-COP.

Coatomer consists of two subcomplexes: the membrane-targeting, ADP ribosylation factor 1 (Arf1):GTP-binding ßγδζ-COP F-subcomplex, which is related to the adaptor protein (AP) clathrin adaptors, and the cargo-binding αß'ε-COP B-subcomplex. We present the structure of the C-terminal µ-homology domain of the yeast δ-COP subunit in complex with the WxW motif from its binding partner, the endoplasmic reticulum-localized Dsl1 tether. The motif binds at a site distinct from that used by the homologous AP µ subunits to bind YxxΦ cargo motifs with its two tryptophan residues sitting in compatible pockets. We also show that the Saccharomyces cerevisiae Arf GTPase-activating protein (GAP) homolog Gcs1p uses a related WxxF motif at its extreme C terminus to bind to δ-COP at the same site in the same way. Mutations designed on the basis of the structure in conjunction with isothermal titration calorimetry confirm the mode of binding and show that mammalian δ-COP binds related tryptophan-based motifs such as that from ArfGAP1 in a similar manner. We conclude that δ-COP subunits bind Wxn(1-6)[WF] motifs within unstructured regions of proteins that influence the lifecycle of COPI-coated vesicles; this conclusion is supported by the observation that, in the context of a sensitizing domain deletion in Dsl1p, mutating the tryptophan-based motif-binding site in yeast causes defects in both growth and carboxypeptidase Y trafficking/processing.

High-level expression and characterization of carboxypeptidase Y from Saccharomyces cerevisiae in Pichia pastoris GS115.

Carboxypeptidase Y is widely used in peptide sequencing and mass spectrometry. PRC1 coding for proteinase C from Saccharomyces cerevisiae was expressed in Pichia pastoris GS115 as procarboxypeptidase Y with a yield of ~605 mg/l in shake-flasks after 168 h induction with 1 % (v/v) methanol. This precursor of carboxypeptidase Y was cleaved by endogenous proteinases of P. pastoris and released into the fermentation broth as active carboxypeptidase Y within 2 weeks at 10 °C, which facilitated the preparation of mature carboxypeptidase Y. The recombinant enzyme was purified. It was optimally active at 30 °C and pH 6.0, with an optimal activity of ~305 U/mg using benzyloxycarbonyl-L-phenylalanyl-L-leucine as substrate. This is the first report about high-level expression and activation of carboxypeptidase Y in P. pastoris.

Synthesis and evaluation of radioiodinated acyloxymethyl ketones as activity-based probes for cathepsin B.

Dipeptidyl (acyloxy)methyl ketones (AOMKs) were functionalized with different iodine-containing prosthetic groups to generate a library of candidate cathepsin B probes. Compound 23a, (S)-20-[(S)-2-{[(benzyloxy)carbonyl]amino}-3-phenylpropanamido]-1-(4-iodophenyl)-1,14,21-trioxo-5,8,11-trioxa-2,15-diazadocosan-22-yl 2,4,6-trimethylbenzoate, was identified as a potential lead through in vitro screening, having a Ki = 181 ± 9 nM and demonstrating the ability to effectively label active cathepsin B in vitro. Its less potent analogue 11a, (S)-3-[(S)-2-{[(benzyloxy)carbonyl]amino}-3-phenylpropanamido]-7-[6-(4-iodobenzamido)hexanamido]-2-oxoheptyl 2,4,6-trimethylbenzoate, was also tested as a comparison. Biodistribution studies of the iodine-125-labeled compounds in MDA-MB-231 mouse xenografts exhibited tumor uptake of 0.58% ± 0.06% injected dose per gram (ID/g) for [(125)I]11a and 1.12% ± 0.08% ID/g for [(125)I]23a at 30 min. The tumor-to-blood ratios reached 1.2 for [(125)I]23a and 1.6 for [(125)I]11a after 23 h. The more hydrophilic [(125)I]23a showed an improved clearance profile with a superior tumor-to-muscle ratio of 7.0 compared to 3.4 for [(125)I]11a at 23 h. Iodinated AOMK ligands are suitable in vitro probes for cathepsin B and hold promise as a platform to develop molecular imaging probes.

Misfolded proteins induce aggregation of the lectin Yos9.

A substantial fraction of nascent proteins delivered into the endoplasmic reticulum (ER) never reach their native conformations. Eukaryotes use a series of complementary pathways to efficiently recognize and dispose of these terminally misfolded proteins. In this process, collectively termed ER-associated degradation (ERAD), misfolded proteins are retrotranslocated to the cytosol, polyubiquitinated, and degraded by the proteasome. Although there has been great progress in identifying ERAD components, how these factors accurately identify substrates remains poorly understood. The targeting of misfolded glycoproteins in the ER lumen for ERAD requires the lectin Yos9, which recognizes the glycan species found on terminally misfolded proteins. In a role that remains poorly characterized, Yos9 also binds the protein component of ERAD substrates. Here, we identified a 45-kDa domain of Yos9, consisting of residues 22-421, that is proteolytically stable, highly structured, and able to fully support ERAD in vivo. In vitro binding studies show that Yos9(22-421) exhibits sequence-specific recognition of linear peptides from the ERAD substrate, carboxypeptidase Y G255R (CPY*), and binds a model unfolded peptide ΔEspP and protein Δ131Δ in solution. Binding of Yos9 to these substrates results in their cooperative aggregation. Although the physiological consequences of this substrate-induced aggregation remain to be seen, it has the potential to play a role in the regulation of ERAD.

Fusion of an intact secretory protein permits a misfolded protein to exit from the endoplasmic reticulum in yeast.

Upon exit from the endoplasmic reticulum (ER), the nascent polypeptides of secretory proteins undergo sorting events. If properly folded, they are directly or indirectly recognized by the coat proteins of budding vesicles for forward transport, while unfolded or misfolded proteins are retained in the ER by a quality control mechanism. To gain insight into the interplay between ER export and ER quality control, we fused a secretory protein invertase to the C-terminus of mutated carboxypeptidase Y (CPY*), a model ER-associated degradation (ERAD) substrate in Saccharomyces cerevisiae. This substrate, designated CPY*-Inv, was largely exported from the ER, although it was fully recognized by the ERAD-related lectin, Yos9, and hence degraded by the ERAD when it remained in the ER. CPY*-Inv relied primarily on the p24 complex, a putative ER export receptor for invertase, for escape from ERAD, suggesting that the ERAD and the ER export of soluble secretory proteins are competitive.

Proteolytic activation of human cathepsin A.

Galactosialidosis is a human lysosomal storage disease caused by deficiency in the multifunctional lysosomal protease cathepsin A (also known as protective protein/cathepsin A, PPCA, catA, HPP, and CTSA; EC 3.4.16.5). Previous structural work on the inactive precursor human cathepsin A (zymogen) led to a two-stage model for activation, where proteolysis of a 1.6-kDa excision peptide is followed by a conformational change in a blocking peptide occluding the active site. Here we present evidence for an alternate model of activation of human cathepsin A, needing only cleavage of a 3.3-kDa excision peptide to yield full enzymatic activity, with no conformational change required. We present x-ray crystallographic, mass spectrometric, amino acid sequencing, enzymatic, and cellular data to support the cleavage-only activation model. The results clarify a longstanding question about the mechanism of cathepsin A activation and point to new avenues for the design of mechanism-based inhibitors of the enzyme.

Serine carboxypeptidases of Triatoma brasiliensis (Hemiptera, Reduviidae): Sequence characterization, expression pattern and activity localization.

Using specific oligonucleotides, 5'- and 3'-RACE and sequencing, two cDNAs encoding serine carboxypeptidases (tbscp-1 and tbscp-2) from the midgut of the blood sucking heteropteran Triatoma brasiliensis were identified. Both cDNAs with an open reading frame of 1389bp, encode serine carboxypeptidase precursors of 463 amino acid residues, which possess a signal peptide cleavage site after Ala19. Analysis of tbscp-1 and tbscp-2 genomic DNA showed an absence of introns in both sequences and the presence of a further intron-free SCP encoding gene (tbscp-2b). By reverse transcription polymerase chain reaction (RT-PCR), tbscp-1 and tbscp-2 transcript abundance was found similarly in fifth instar nymphs at different days after feeding (daf), high in the posterior midgut (small intestine), lower in the anterior midgut (stomach) and fat body and almost undetectable in the salivary glands. In the anterior, middle and posterior regions of the small intestine at 5daf the transcript abundance of both genes was almost identical. Also in adult female and male insects at 5daf both genes showed the strongest signal in the posterior midgut. Molecular modeling suggested that TBSCP-1 has carboxypeptidase D activity; activities against Hippuryl-Phenylalanine and Hippuryl-Arginine were also located at the posterior midgut, both were induced after blood feeding. Treatment of the posterior midgut extracts with the serine protease inhibitor PMSF strongly reduced carboxypeptidase activity. These findings suggest that triatomines might use serine carboxypeptidases, which are usually found in lysosomes, as digestive enzymes in the posterior midgut lumen, from which TBSCP-1 and TBSCP-2 are possible candidates to fulfill this function.

Crystal structure of cathepsin A, a novel target for the treatment of cardiovascular diseases.

The lysosomal serine carboxypeptidase cathepsin A is involved in the breakdown of peptide hormones like endothelin and bradykinin. Recent pharmacological studies with cathepsin A inhibitors in rodents showed a remarkable reduction in cardiac hypertrophy and atrial fibrillation, making cathepsin A a promising target for the treatment of heart failure. Here we describe the crystal structures of activated cathepsin A without inhibitor and with two compounds that mimic the tetrahedral intermediate and the reaction product, respectively. The structure of activated cathepsin A turned out to be very similar to the structure of the inactive precursor. The only difference was the removal of a 40 residue activation domain, partially due to proteolytic removal of the activation peptide, and partially by an order-disorder transition of the peptides flanking the removed activation peptide. The termini of the catalytic core are held together by the Cys253-Cys303 disulfide bond, just before and after the activation domain. One of the compounds we soaked in our crystals reacted covalently with the catalytic Ser150 and formed a tetrahedral intermediate. The other compound got cleaved by the enzyme and a fragment, resembling one of the natural reaction products, was found in the active site. These studies establish cathepsin A as a classical serine proteinase with a well-defined oxyanion hole. The carboxylate group of the cleavage product is bound by a hydrogen-bonding network involving one aspartate and two glutamate side chains. This network can only form if at least half of the carboxylate groups involved are protonated, which explains the acidic pH optimum of the enzyme.