Activity-based protein profiling, usually called ABPP, is a laboratory technique that uses small chemical probes to find and measure active enzymes inside living cells, tissues, or whole organisms. Unlike methods that simply count how many copies of a protein are present, ABPP tells researchers which proteins are actually doing something at a given moment. The technology has been in use for roughly 25 years and has become one of the central tools in drug discovery and disease research, largely because it can reveal which enzymes are switched on or off in a particular disease state and whether a candidate drug actually hits the intended target.1PubMed Central. Activity-based protein profiling: A graphical review
Why Protein Activity Matters More Than Protein Abundance
Cells contain thousands of different proteins, and many of the most important ones are enzymes that carry out the chemical reactions keeping cells alive, growing, or communicating with their neighbors. Traditional approaches to studying proteins often rely on measuring how much of each protein is present, a field broadly known as proteomics. But knowing that a cell contains a lot of a certain enzyme does not tell you whether that enzyme is actually working. Enzymes can be silenced by natural inhibitors, locked in an inactive shape, or missing a chemical partner they need to function. Two cells could have identical amounts of the same enzyme while one is highly active and the other is dormant.
ABPP was developed to close this gap. Its probes are designed to react only with the active form of an enzyme, so the readout is a direct snapshot of function rather than just inventory. That distinction matters enormously in drug research: if you want to block an enzyme that drives a disease, you first need to know when and where that enzyme is active, not just present.
How the Probes Work
An ABPP probe is a small synthetic molecule built around three functional parts. The first is a reactive group, sometimes called a “warhead,” that forms a chemical bond with the active site of the target enzyme. Because it reacts with the active site specifically, it naturally ignores inactive copies of the same protein. The second part is a binding group that gives the probe some selectivity for a particular family of enzymes. The third is a tag, typically a fluorescent dye or a molecule like biotin, that lets researchers detect or pull out whatever the probe has latched onto.
The earliest and best-known example is a probe called FP-biotin, a biotinylated fluorophosphonate that targets a large family of enzymes called serine hydrolases. When mixed with crude tissue extracts, FP-biotin reacts with numerous serine hydrolases in an activity-dependent manner, meaning inactive forms of those enzymes are left unlabeled. The reaction can be tracked over time, giving researchers a way to monitor changes in both enzyme function and expression simultaneously.2PubMed. Activity-based protein profiling: the serine hydrolases That original fluorophosphonate chemistry, developed by Benjamin Cravatt and colleagues, became the template for what is now a large and growing family of probes.3PubMed. Discovery and Evaluation of New Activity-Based Probes for Serine Hydrolases
Since then, probe design has expanded far beyond serine hydrolases. Probes now exist for cysteine proteases, kinases, ATPases, glycosidases, and other enzyme families. One study designed an ATP-based probe with reactive acyl phosphate groups that capture proteins using ATP as a binding scaffold, allowing researchers to profile kinases and nucleotide-binding proteins in tissues from obese mice.4PLoS ONE. Activity-Based Protein Profiling Reveals Mitochondrial Oxidative Enzyme Impairment and Restoration in Diet-Induced Obese Mice Metalloproteases, a large and structurally diverse enzyme family, were long considered difficult to target. Researchers addressed this by creating a library of hydroxamate-based probes with complementary selectivity and applying them as a cocktail, collectively identifying more than 20 metalloproteases spanning nearly all major branches of the family.5PubMed Central. Proteomic profiling of metalloprotease activities with cocktails of active-site probes The cocktail strategy has since become a standard approach for enzyme classes that are too diverse for any single probe to cover.
What Happens After the Probe Binds
Once probes have labeled their targets in a sample, the labeled proteins need to be identified and, ideally, quantified. Most modern ABPP workflows feed the labeled sample into a mass spectrometer, which separates and identifies proteins by their molecular weight and fragmentation patterns. This process can catalog hundreds of active enzymes in a single experiment. Accurate quantification is crucial because researchers often want to compare two conditions, say a healthy tissue versus a tumor, and identify enzymes whose activity differs between them.6PubMed. CIMAGE2.0: An Expanded Tool for Quantitative Analysis of Activity-Based Protein Profiling (ABPP) Data
Specialized software has been built to handle the large datasets ABPP generates. CIMAGE, for instance, was designed specifically for ABPP data. It uses a relaxed peak-extraction algorithm paired with stringent quality checks to quantify how much of each probe-labeled protein is present. The tool has been upgraded to handle multiplex quantification and to interface with different database search engines, reflecting how the field’s data demands have grown over time.6PubMed. CIMAGE2.0: An Expanded Tool for Quantitative Analysis of Activity-Based Protein Profiling (ABPP) Data
Competitive ABPP and Drug Discovery
One of the most powerful variations of the technique is competitive ABPP. Instead of simply asking “which enzymes are active?”, it asks “does this drug candidate actually bind to and block its intended enzyme?” The principle is straightforward: a researcher treats a sample with the candidate drug first, then adds the activity-based probe. If the drug has occupied the enzyme’s active site, the probe can no longer bind there, so that particular enzyme shows up as less labeled compared to a control sample treated only with the probe. The drop in labeling is a direct readout of how well the drug occupies the target.7PubMed Central. Strategies for Competitive Activity-Based Protein Profiling in Small Molecule Inhibitor Discovery and Characterization
What makes competitive ABPP especially valuable is that it works across the entire proteome at once. You learn not only whether the drug hits its intended target but also what else it binds to, revealing off-target effects before they become problems in a clinical trial. Researchers used this approach to screen a class of compounds called aza-β-lactams against cell and tissue samples, leading to the discovery of lead inhibitors for several serine hydrolases, including a previously uncharacterized enzyme called ABHD10.8PubMed Central. Competitive activity-based protein profiling identifies aza-β-lactams as a versatile chemotype for serine hydrolase inhibition
The technique has also proven especially useful for developing covalent drugs, which form a permanent bond with their target. Covalent drugs were once considered risky because a molecule that bonds irreversibly to the wrong protein could cause serious side effects. ABPP flipped the narrative by giving researchers a way to map every protein a covalent molecule touches. In one notable example, competitive ABPP was used to trace the targets of Withaferin A, a natural compound with anticancer activity, in breast cancer cells. The work identified a specific amino acid on a regulatory protein called PPP2R1A as the compound’s functional target. That finding guided the development of a highly selective synthetic inhibitor that significantly reduced breast cancer growth in animal models.9PubMed Central. The Current Toolbox for Covalent Inhibitors: From Hit Identification to Drug Discovery – Section: ABPP Approaches
Mapping Enzyme Activity in Cancer
Cancer cells rewire their metabolism to fuel rapid growth, and ABPP has been particularly effective at catching those metabolic shifts. Researchers used ABPP to compare serine hydrolase activity in cells driven by different oncogenes and found that two enzymes, PAFAH1B2 and PAFAH1B3, were consistently ramped up regardless of which oncogene was doing the driving. Blocking both enzymes pharmacologically impaired cancer cell behavior across breast, ovarian, melanoma, and prostate cancer lines.10PubMed Central. Activity-Based Protein Profiling of Oncogene-Driven Changes in Metabolism Reveals Broad Dysregulation of PAFAH1B2 and 1B3 in Cancer That kind of cross-cancer consistency is exactly what drug developers look for when choosing a new target, and ABPP provided the functional evidence that traditional protein-counting methods could have missed.
Beyond target discovery, ABPP-based probes are being tested as tools during cancer surgery itself. A fluorescent probe targeting cysteine cathepsins, enzymes often overactive in tumors, was injected intravenously in a mouse model of breast cancer. Tumors lit up under near-infrared imaging, suggesting that this approach could help surgeons see tumor margins more clearly during operations and remove cancerous tissue more completely.11PubMed Central. Fluorescent image-guided surgery in breast cancer by intravenous application of a quenched fluorescence activity-based probe for cysteine cathepsins in a syngeneic mouse model This translational leap, from laboratory profiling to real-time surgical imaging, illustrates how versatile the underlying chemistry has become.
Infectious Disease and Bacterial Targets
ABPP is not limited to human enzymes. Bacteria rely on their own arsenals of enzymes for growth, metabolism, and attacking a host, and activity-based probes can identify which bacterial enzymes are essential to virulence. Methods have been developed using natural-product-inspired probes to study pathogens including Staphylococcus aureus and Listeria monocytogenes, identifying enzymes critical to each pathogen’s ability to cause disease.12PubMed. Activity-Based Protein Profiling in Bacteria
A recent study applied a cocktail of probes targeting different glycosidase families to methicillin-resistant S. aureus (MRSA), a major antibiotic-resistant threat. The probes picked up three enzymes, one of which, a phosphotrehalase called TreC, turned out to contribute to biofilm formation and virulence in an animal infection model. Neither standard growth assays nor conventional proteomics would have flagged TreC as important, because the bacterium grows normally without it in rich laboratory media; only its disease-causing behavior is impaired.13PubMed Central. Activity-Based Protein Profiling Identifies an α-Amylase Family Protein Contributing to the Virulence of Methicillin-Resistant Staphylococcus aureus Findings like this are valuable because they point toward drug targets that would weaken a pathogen’s ability to cause harm without necessarily killing it outright, a strategy that may generate less resistance pressure than traditional antibiotics.
Applications in Plant Science and Agriculture
Over the past decade, ABPP has gained traction in plant biology, where it monitors the activities of hundreds of plant proteins using probes that react with active sites in a mechanism-dependent manner.14PubMed. The Increasing Impact of Activity-Based Protein Profiling in Plant Science One striking application involves studying how plants defend themselves against bacterial pathogens. When tobacco plants were infected with the bacterium Pseudomonas syringae, researchers used a probe cocktail to simultaneously track 171 active hydrolases. They found that the activity of 82 of these enzymes increased during infection while 60 were suppressed. Among the suppressed enzymes was one called NbPR3, which, when overexpressed in plants, decreased bacterial growth in a manner dependent on its enzymatic activity, revealing a previously unknown role in antibacterial immunity.15PubMed Central. Activity-based proteomics uncovers suppressed hydrolases and a neo-functionalised antibacterial enzyme at the plant-pathogen interface
A companion study using hydroxamate-based metalloprotease probes in the same plant-pathogen system uncovered PRp27, a secreted protein whose labeling increased during infection. Overexpressing PRp27 boosted the plant’s resistance to bacterial attack.16PubMed Central. Broad-range metalloprotease profiling in plants uncovers immunity provided by defence-related metalloenzyme These discoveries matter for agriculture because understanding which enzymes plants activate or lose during infection could guide breeding programs or crop-protection strategies aimed at strengthening natural defenses rather than relying solely on chemical pesticides.
Industrial Biotechnology and Biofuels
ABPP’s ability to identify functional enzymes has also attracted interest from the biofuels industry. Converting plant biomass into fuel requires enzymes, particularly cellulases and lipases, that break down tough plant materials into fermentable sugars or fatty acids. ABPP can profile which microbial enzymes are actually active under industrial fermentation conditions, enabling the rapid discovery of new biocatalysts. Researchers have highlighted applications to cellulosic bioethanol production, biodiesel processing, and phototrophic hydrocarbon synthesis, all areas where finding more efficient enzymes could lower costs and improve yields.17PubMed Central. Advancing understanding of microbial bioenergy conversion processes by activity-based protein profiling
Lipid-processing enzymes are another active area. Because ABPP probes can be designed to target individual enzymes or entire enzyme families, the technique is well suited for profiling lipid hydrolases, enzymes that break down fats, in complex microbial communities where many species contribute enzymes simultaneously.18Israel Journal of Chemistry. Research Advances Through Activity‐Based Lipid Hydrolase Profiling
Ligandability Mapping and the Expanding Chemical Toolbox
A newer frontier in ABPP goes beyond asking which enzymes are active and asks a more radical question: which proteins across the entire proteome have binding pockets that a small molecule could latch onto? This approach, sometimes called residue-directed ABPP, uses broad-spectrum reactive probes, often small electrophilic fragments, that label reactive amino acid residues across thousands of proteins. Covalent scout fragments such as KB02, KB03, and KB05 have been screened this way to generate global maps of ligandable sites in diverse biological systems.19Cell Chemical Biology. What is activity-based protein profiling? – Section: Strategies for small molecule library design The result is essentially a roadmap of the druggable proteome, highlighting proteins that were previously considered “undruggable” simply because no one had found the right pocket to exploit.
This expansion from enzyme-family-specific probes to proteome-wide fragment screens represents a philosophical shift in how ABPP is used. The original question, “which enzymes are active,” is still important, but the newer question, “which proteins can be chemically manipulated at all,” opens up target space that traditional drug discovery approaches would never have explored. Recent advances in probe design continue to broaden the range of reactive amino acid residues that can be profiled, pushing the boundaries of what counts as a druggable target.20PubMed Central. Targeting the Reactive Proteome: Recent Advances in Activity-Based Protein Profiling and Probe Design
Common Misconceptions About ABPP
One frequent misunderstanding is that ABPP and traditional proteomics are interchangeable. Standard mass-spectrometry-based proteomics tells you what proteins are in a sample and how abundant they are. ABPP tells you which of those proteins are functionally active. The two approaches answer different questions, and in many cases their results diverge: an enzyme can be abundant but inactive, or scarce but highly active. ABPP’s selectivity for active forms is precisely what makes it valuable for drug discovery, where function matters more than presence.
Another misconception is that ABPP works only for a narrow set of enzyme types. While serine hydrolases were the first family profiled and remain the most extensively studied, the technique now covers cysteine proteases, metalloproteases, kinases, ATPases, glycosidases, and other classes. The cocktail strategy and fragment-based approaches have further eroded the idea that large or structurally diverse enzyme families are out of reach. Practically any enzyme family with a chemically accessible active site is a candidate for ABPP, and the list of probes available grows each year.
Finally, people sometimes assume ABPP is purely an academic exercise. The fluorescent surgical imaging work and the covalent drug discovery pipelines described above show otherwise. The technology is moving steadily toward clinical and industrial use, though most applications are still in early or preclinical stages. A bibliometric analysis of the field found that the published literature spans from 1999 to at least 2022, with nearly two thousand articles analyzed, reflecting sustained and growing interest rather than a passing trend.21PubMed Central. Mapping the Evolution of Activity-Based Protein Profiling: A Bibliometric Review