Amide hydrolases participate in amino acid recycling, utilization of nitrogen-containing nutrients, mitochondrial carbon and nitrogen metabolism, urea degradation, and termination of lipid signaling by catalyzing the cleavage of amide bonds. Although different members share the common reaction characteristic of amide hydrolysis, they differ substantially in catalytic centers, substrate-backbone recognition, subcellular localization, and physiological functions. Therefore, they should not be regarded as a single enzyme family with uniform structure or function.

Keywords: amide hydrolases; aminoacylase; asparaginase; glutaminase; urease; fatty acid amide hydrolase

1 Reaction Basis of Amide Hydrolases

1.1 Chemical Stability of Amide Bonds

The lone pair of electrons on the nitrogen atom of an amide bond can conjugate with the carbonyl group, giving the C—N bond partial double-bond character and relatively high stability. Therefore, most amides undergo spontaneous hydrolysis slowly in aqueous environments near physiological conditions. Amide hydrolases enable rapid cleavage of amide bonds under mild conditions by positioning the substrate carbonyl group, increasing the electrophilicity of the carbonyl carbon, activating water molecules or nucleophilic groups within the enzyme itself, and stabilizing reaction intermediates. Different enzymes can ultimately generate free amino acids, carboxylic acids, NH₃/NH₄⁺, or corresponding amine products.

1.2 Essential Differences Among Different Amide Hydrolysis Reactions

Aminoacylases mainly remove N-acyl groups from the α-amino groups of amino acids; asparaginases and glutaminases hydrolyze amino acid side-chain amides; ureases act on urea; FAAH mainly hydrolyzes fatty acid amides formed between fatty acids and ethanolamine or related amines. Therefore, “amide hydrolase” mainly describes a reaction chemistry type and does not imply that different members share a common protein fold, catalytic residues, or cofactor requirements.

Table 1 Reaction Characteristics of Representative Amide Hydrolases

1.3 Structural Basis of Substrate Selectivity

Amide hydrolases recognize the complete substrate rather than an isolated amide group. Acyl-chain length, side-chain hydrophobicity and charge, steric hindrance, stereochemical configuration, and the position of the amide bond within the molecule can all affect substrate binding to the active site and formation of the catalytic conformation. When evaluating substrate preference, “can be hydrolyzed” should be distinguished from “is an efficient substrate”: endpoint conversion at a single concentration can only demonstrate that a reaction occurs, whereas Km, kcat, and kcat/Km are more suitable for describing substrate utilization and catalytic efficiency.

2 Aminoacylase

2.1 Deacylation of N-Acyl Amino Acids

Aminoacylases mainly catalyze the deacylation of N-acyl amino acids, releasing the amino group blocked by the acyl group and generating the corresponding free amino acid and carboxylic acid. Their recognition targets include both the N-acyl group and the amino acid backbone, so changes in the amino acid side chain, acyl structure, or substrate chirality may alter catalytic efficiency. This property allows Aminoacylase to participate in N-acyl amino acid metabolism and also gives it value in substrate-spectrum analysis, stereoselectivity studies, and enzymatic resolution.

2.2 Catalytic Mechanism and Metabolic Positioning of ACY1

(1) Metal-Dependent Catalysis

ACY1 is a typical aminoacylase whose active site depends on metal ions to maintain an effective catalytic conformation. The metal center can participate in substrate carbonyl positioning and polarization and promote water activation, allowing nucleophilic water to attack the amide carbonyl and complete deacylation. Therefore, metal-ion status, chelating agents, and buffer composition may all affect ACY1 activity in vitro, and these conditions should be kept consistent when comparing enzyme activity.

(2) Substrate-Backbone Recognition

ACY1 does not exhibit the same catalytic efficiency toward different N-acyl amino acids. N-Acetylglycine can be used to evaluate utilization of substrates with small side chains; N-Acetylvaline, N-Acetylisoleucine, and N-Acetylleucine can be used to compare branched-chain size and steric hindrance; N-Acetylphenylalanine and N-Acetyltryptophan are suitable for evaluating the effects of aromatic side chains on the binding pocket. Different substrates should be compared kinetically under identical enzyme amounts and reaction conditions to determine true selectivity.

(3) N-Acyl Amino Acid Homeostasis

ACY1 allows some N-acyl amino acids to re-enter the free amino acid pool, so changes in its activity first affect clearance of specific acylated metabolites. Reduced ACY1 function can lead to accumulation of corresponding N-acyl amino acids, but free amino acid concentrations are simultaneously regulated by uptake, synthesis, protein degradation, and other metabolic pathways. Therefore, ACY1 protein abundance, actual enzyme activity, and N-acyl metabolite levels represent different levels and should not be used interchangeably.

2.3 ASPA and N-Acetylaspartate Metabolism

Aspartoacylase (ASPA) mainly catalyzes N-Acetyl-L-Aspartate (NAA) to generate L-Aspartate and Acetate. NAA is highly abundant in the central nervous system, and its synthesis and ASPA-mediated degradation show clear cellular division of labor. Severe impairment of ASPA function can cause abnormal NAA accumulation and is associated with the metabolic pathology of Canavan disease. When studying this system, NAA concentration reflects the net result of synthesis, transport, and degradation and should therefore be interpreted together with ASPA protein levels and actual hydrolytic capacity.

2.4 Substrate Range and Metabolic Significance of ACY3

ACY3 and ACY1 show some functional overlap, but their substrate spectra are not identical. In addition to certain N-acetyl amino acids, ACY3 can also participate in the deacetylation of some N-acetyl-S-substituted cysteine compounds. Therefore, its functions involve endogenous amino acid derivatives as well as the biotransformation of some exogenous compounds. For the latter, deacetylated products may have different reactivities and toxicological properties, so ACY3-mediated reactions cannot be uniformly classified as “detoxification”; the specific effect depends on the subsequent chemical and metabolic behavior of the generated products.

2.5 D-Aminoacylase and Chiral Selectivity

D-Aminoacylase preferentially acts on N-acyl-D-amino acids, and its substrate-binding site can recognize the spatial orientation of the chiral center. This property can be used to preferentially deacylate one configuration in racemic N-acyl amino acids, enabling enzymatic kinetic resolution. The core evaluation of such systems includes not only total conversion but also the enantiomeric composition of substrates and products, enantiomeric excess, and catalytic efficiency toward different configurations.

3 Asparaginase

3.1 Deamidation of the L-Asparagine Side Chain

Asparaginase catalyzes hydrolysis of the side-chain —CONH₂ group of L-Asparagine into a carboxyl group, generating L-Aspartate and releasing NH₃/NH₄⁺. This reaction differs from the removal of an N-acyl group from the amino group by Aminoacylase and belongs to amino acid side-chain deamidation. Because the reaction simultaneously changes Asparagine concentration, Aspartate production, and amide-nitrogen release, Asparaginase can directly affect amino acid supply and cellular nitrogen-metabolism status.

3.2 Catalytic Center and Reaction Process

Typical bacterial L-Asparaginases use nucleophilic residues in the active site to attack the carbonyl group of the Asparagine side chain, followed by intermediate formation, hydrolysis, and regeneration of the enzyme active site. Substrate binding and chemical catalysis are distinct steps, so active-site mutations may mainly affect substrate recognition or primarily reduce catalytic turnover. Comparing only the residual activity of mutants is insufficient to explain the mechanism; Km and kcat should be evaluated to determine whether the effect occurs at the binding or catalytic stage.

3.3 Differences Among Enzyme Sources and Substrate Selectivity

(1) Substrate Affinity and Specific Activity

Asparaginases from different sources may differ substantially in Km, specific activity, optimal pH, thermal stability, and oligomeric structure. High-affinity enzymes can maintain effective catalysis at relatively low Asparagine concentrations, whereas low-affinity enzymes require higher substrate concentrations to approach the maximum reaction rate. Therefore, different enzyme preparations should be compared according to actual activity and kinetic parameters rather than assuming that identical mass concentrations represent equivalent catalytic doses.

(2) Glutaminase Side Activity

Some Asparaginases can also hydrolyze L-Glutamine. In purified-enzyme studies, this property reflects differences in substrate selectivity, whereas in cellular systems it may simultaneously perturb glutamine carbon and nitrogen metabolism. Because Glutamine participates in nucleotide synthesis, amino acid metabolism, TCA-cycle replenishment, and redox homeostasis, the Glutaminase side activity of Asparaginase may substantially alter the final biological effect. Therefore, the ratio between the two activities should be evaluated according to the experimental objective.

3.4 Catalytic Characteristics of ASRGL1

ASRGL1 has Asparaginase- and β-aspartyl peptidase-related activities, and its catalytic maturation differs from that of common bacterial L-Asparaginases. ASRGL1 must undergo self-processing to form a catalytically competent N-terminal nucleophile. Therefore, total protein level and the proportion of mature active enzyme may not be consistent. When studying ASRGL1 mutations, self-processing, or catalytic maturation, the precursor form, mature protein, and actual hydrolytic activity should be evaluated separately.

3.5 Asparagine Depletion and Cellular Metabolism

In addition to participating in protein synthesis, Asparagine is associated with cellular amino acid exchange and nutrient adaptation. After exogenous Asparaginase reduces Asparagine, whether cells can maintain metabolic homeostasis depends on endogenous Asparagine-synthesis capacity, Aspartate supply, Glutamine nitrogen supply, and amino acid transport. Therefore, different effects of the same Asparaginase dose in different cells may result from differences in cellular metabolic-compensation capacity rather than differences in the catalytic efficiency of the enzyme itself.

4 Glutaminase

4.1 Metabolic Entry Point of Glutamine Catabolism

Glutaminase catalyzes L-Glutamine to form L-Glutamate and NH₃/NH₄⁺ and is an important initiating reaction in glutamine catabolism. Glutamate can subsequently form α-Ketoglutarate through transamination or the glutamate dehydrogenase pathway and enter the TCA cycle, and it can also be used for Glutathione synthesis and other amino acid metabolic pathways. Therefore, Glutaminase connects glutamine utilization with mitochondrial carbon metabolism, nitrogen transfer, and redox homeostasis.

4.2 Functional Differences Between GLS1 and GLS2

(1) GLS1 and Its Splice Isoforms

GLS1 can generate splice isoforms such as KGA and GAC. They share the major catalytic structure but differ in regulatory regions and expression patterns. The proportions of these isoforms differ among tissues and cellular models. Therefore, Glutaminase protein detection should clarify the antibody-recognition region and expected bands; otherwise, total GLS signal is insufficient to distinguish the contribution of specific isoforms to glutamine catabolism.

(2) GLS2

GLS2 can also catalyze the conversion of Glutamine to Glutamate, but its tissue distribution, transcriptional regulation, and physiological context differ from those of GLS1. In some cells, GLS2 can constitute an important source of Glutaminase activity. Therefore, measurement of GLS1 alone is insufficient to fully evaluate the Glutaminase system when studying glutamine dependence.

4.3 Mitochondrial Localization and TCA-Cycle Replenishment

GLS-related glutaminases mainly function in the mitochondrial environment, directly linking Glutamine hydrolysis to mitochondrial metabolism. Glutamate must still be further converted into α-Ketoglutarate before it can truly enter the TCA cycle. Therefore, increased Glutamine uptake or consumption cannot directly demonstrate enhanced entry of glutamine-derived carbon into the TCA cycle. Some Glutamine can also be used for the synthesis of nucleotides, amino sugars, and other nitrogen-containing compounds, and its specific fate needs to be confirmed through metabolite and isotope-distribution analyses.

4.4 Regulation of Enzyme Activity by Oligomeric State

GLS1 catalytic capacity is regulated by conformation and oligomeric state. When the protein transitions from a low-activity state to a higher-activity oligomeric state, the actual catalytic rate can increase substantially even when protein abundance remains largely unchanged. This means that Western blot reflects only enzyme protein abundance and cannot replace enzyme-activity assessment. Conversely, increased GLS1 expression does not necessarily cause Glutamine→Glutamate flux to increase proportionally.

4.5 Pharmacological Inhibition and Metabolic Dependence

GLS1 inhibitory tools such as CB-839 (Telaglenastat) and IPN60090 can be used to evaluate the dependence of cellular phenotypes on Glutaminase activity. Mechanistic validation should first confirm that the Glutamine/Glutamate relationship or downstream metabolic flux derived from glutamine changes as expected, followed by analysis of proliferation, energy metabolism, or redox phenotypes. Observation of terminal cellular effects alone is insufficient to distinguish GLS inhibition from off-target pharmacological effects.

4.6 Stable Isotope Tracing

¹³C₅-Glutamine can be used to trace transfer of the glutamine carbon skeleton into Glutamate, α-Ketoglutarate, and TCA-cycle intermediates; ¹³C₅,¹⁵N₂-Glutamine additionally provides information on nitrogen flux. The amide nitrogen and α-amino nitrogen of Glutamine participate in different types of metabolic reactions. Therefore, ¹⁵N results need to be interpreted according to the specific labeling position and downstream isotopologue distribution. The core value of stable isotope tracing is to distinguish static metabolite concentrations from actual material flux.

5 Urease

5.1 Urea Hydrolysis Mediated by a Dinuclear Nickel Center

Urease is a typical metal-dependent amide hydrolase whose active site contains two cooperatively acting Ni²⁺ ions. The dinuclear nickel center participates in urea binding, carbonyl polarization, and water activation, allowing urea to first generate NH₃ and carbamate, which subsequently decomposes into another molecule of NH₃ and CO₂/HCO₃⁻. One molecule of urea can therefore release two nitrogen-containing ammonia molecules, allowing Urease to substantially affect local nitrogen status and the acid-base environment.

5.2 Metal Loading and Enzyme Maturation

After Urease polypeptide synthesis is completed, the metal active center must still be properly formed. Ni²⁺ loading and related maturation processes directly determine final specific activity. In recombinant expression systems, insufficient metal supply or incomplete maturation can result in high protein expression but relatively low catalytic capacity. Therefore, when evaluating Urease preparations, enzyme activity units and specific activity such as U/mg protein better reflect the actual effective enzyme amount than protein concentration alone.

5.3 Biological Significance of Urea Hydrolysis

Urease is widely present in plants, fungi, and many microorganisms, and its function is closely associated with nitrogen-source utilization. Some microorganisms rapidly hydrolyze urea to generate NH₃ and increase local pH, thereby altering their survival microenvironment. In environmental and agricultural systems, Urease directly affects the conversion efficiency of urea nitrogen. Therefore, Urease research involves not only enzyme kinetics but also processes such as microenvironmental pH, nitrogen-release rate, and metal dependence.

5.4 Feedback of the Reaction System on Enzyme Activity

After urea hydrolysis generates NH₃/NH₄⁺, the acid-base balance of the reaction system continuously changes. If buffer capacity is insufficient, changes in system pH can in turn affect Urease activity, causing endpoint results to be influenced by both enzyme catalysis and environmental feedback. Kinetic measurements should preferentially analyze the initial rate and maintain stable buffer and metal conditions to prevent substrate depletion, product accumulation, and pH drift from masking intrinsic catalytic differences of the enzyme.

6 Fatty Acid Amide Hydrolase (FAAH)

6.1 Fatty Acid Amide Hydrolysis in the Membrane Environment

FAAH mainly acts on fatty acid amides with long hydrophobic chains, such as Anandamide, whose physicochemical properties differ markedly from those of water-soluble amino acid amides. The structure of FAAH accommodates entry of membrane-lipid substrates and positions the lipid amide bond at the catalytic center through a hydrophobic channel. Therefore, substrate partitioning between membrane and aqueous phases, the membrane environment, and lipid composition can all affect its actual catalytic efficiency.

6.2 Ser-Ser-Lys Catalytic System

FAAH belongs to the amidase signature hydrolase family, and its catalytic core uses a Ser-Ser-Lys network rather than the Ser-His-Asp triad of classical serine proteases. The catalytic Ser nucleophilically attacks the substrate carbonyl to form an acyl-enzyme intermediate, which is subsequently hydrolyzed to release the fatty acid and regenerate the free enzyme. This mechanism is fundamentally different from the metal-dependent hydrolysis of ACY1 and the dinuclear nickel catalysis of Urease and also determines the basis for the actions of different types of inhibitors.

6.3 FAAH and Termination of Lipid Signaling

N-acylethanolamine molecules such as Anandamide have clear lipid-signaling functions, and their homeostasis is jointly determined by synthesis, transport, receptor action, and degradation. Increased FAAH activity can accelerate clearance of some fatty acid amides, whereas inhibition of FAAH generally makes the corresponding substrates more likely to accumulate. Because substrate levels are also affected by the synthetic pathway, an increase in Anandamide alone is insufficient to demonstrate reduced FAAH activity and should be attributed together with enzyme-activity or selective-inhibition results.

6.4 Functional Boundary Between FAAH and NAAA

NAAA also participates in the degradation of N-acylethanolamines, but its catalytic mechanism, cellular localization, and preferred reaction environment differ from those of FAAH, and it is closely associated with metabolism of substrates such as Palmitoylethanolamide (PEA). Therefore, changes in fatty acid ethanolamides such as PEA may arise from FAAH, NAAA, synthetic pathways, or transport processes. Different hydrolytic pathways need to be distinguished according to substrate spectrum and enzyme specificity.

6.5 Pharmacological Inhibition of FAAH

URB597 can be used in FAAH-related pharmacological research. Evaluation should first confirm accumulation of the corresponding fatty acid amide substrates or reduction of hydrolysis products, followed by analysis of downstream signaling and cellular phenotypes. Broad-spectrum serine hydrolase inhibitors such as PMSF can only be used as relatively broad mechanistic controls and are not specific for FAAH, so they cannot independently serve as evidence that FAAH participates in the reaction.

7 Amide Hydrolase-Related Enzymes, Substrates, and Research Reagents

Table 2 Products Related to Amide Hydrolases

Amide hydrolases use different catalytic structures to solve the same chemical problem of amide-bond hydrolysis and are respectively embedded in amino acid, nitrogen, mitochondrial, and lipid metabolic networks.

For more related articles, please see below:

[1] Urease Reaction Systems, Urea Conversion, and Ammonia Nitrogen Quantitative Analysis

[2] β-N-Acetylglucosaminidase: Substrate-Specific Hydrolytic Enzymology, Assays, and Research and Industrial Applications