# Identifying Post-Translational Modifications by Mass Spectrometry ## Quick Answer - Design PTM experiments around enrichment, fragmentation method, and database search parameters before acquiring data. - Use variable modifications in database searches to localize PTMs, then validate candidate sites with manual spectral inspection. - PTM stoichiometry and site occupancy remain difficult to quantify without labeled standards or targeted approaches. ## At a Glance | PTM Type | Enrichment Strategy | Fragmentation Method | Key Consideration | |----------|---------------------|----------------------|-------------------| | Phosphorylation | Immobilized metal affinity chromatography or titanium dioxide | Higher-energy collisional dissociation with electron-transfer dissociation for labile sites | Neutral loss of phosphoric acid can dominate spectra and obscure site localization | | Glycosylation | Lectin affinity chromatography or hydrophilic interaction liquid chromatography | Electron-transfer dissociation or higher-energy collisional dissociation with stepped collision energies | Glycan heterogeneity complicates database searching and requires glycan databases | | Ubiquitination | Anti-diglycine remnant antibody enrichment after trypsin digestion | Higher-energy collisional dissociation for peptide backbone fragmentation | Diglycine remnant marks the modified lysine but trypsin miscleavage can reduce detection | | Acetylation | Anti-acetyllysine antibody enrichment | Higher-energy collisional dissociation | Acetylation on lysine is isobaric with trimethylation and requires high-resolution mass analysis | ## Scope of PTM Analysis by Mass Spectrometry Post-translational modifications control protein activity, localization, interaction networks, and degradation. Mass spectrometry is the primary method for identifying and localizing these modifications because it measures the mass shift that a modification introduces to a peptide. The researcher must decide which modifications to search for, how to enrich modified peptides, and which fragmentation method will preserve the modification for detection. The workflow begins with protein purification, proceeds through enzymatic digestion, and ends with liquid chromatography coupled to tandem mass spectrometry. Each step introduces choices that affect whether modified peptides survive detection. A phosphorylation site on a protein of interest may be present at low stoichiometry, meaning only a fraction of the protein molecules carry the modification. Without enrichment, the modified peptide may fall below the detection limit of the instrument. The National Library of Medicine provides access to laboratory-method references that describe the standard molecular biology procedures used in these workflows. These references cover protein extraction, digestion, and the general principles of mass spectrometry as applied to biological samples. The practical design of a PTM experiment begins with a clear question about the modification of interest and the biological context in which it occurs. ## Core Principles of PTM Identification ### Mass Shift as the Primary Signal Every post-translational modification adds a characteristic mass to the amino acid residue it modifies. Phosphorylation adds a phosphate group with a mass shift of approximately 80 Da to serine, threonine, or tyrosine. Acetylation adds 42 Da to the N-terminus or to lysine residues. Ubiquitination leaves a diglycine remnant on lysine after trypsin digestion, adding 114 Da. The mass spectrometer measures the mass-to-charge ratio of the peptide, and the observed shift from the unmodified sequence indicates the presence of a modification. The mass shift alone does not localize the modification to a specific residue. A peptide containing multiple serine residues may carry a single phosphate group, but the mass spectrum does not reveal which serine is modified. Localization requires fragmentation of the peptide and analysis of the fragment ion series. ### Fragmentation and Site Localization Tandem mass spectrometry fragments the selected precursor peptide into smaller ions. The fragment ion series provides sequence information that allows the researcher to assign the modification to a specific residue. When a fragment ion contains the modified residue, its mass is shifted by the modification mass. When the fragment ion does not contain the modified residue, its mass matches the unmodified sequence. The choice of fragmentation method depends on the stability of the modification. Phosphorylation on serine and threonine is labile under collision-induced dissociation, and the phosphate group is lost before the peptide backbone fragments. This produces a dominant neutral loss peak that can obscure the site-determining ions. Alternative fragmentation methods such as electron transfer dissociation preserve the phosphate group and provide better site localization for labile modifications. ### Database Searching with Variable Modifications The identification of modified peptides relies on database search algorithms that compare the acquired spectra to theoretical spectra generated from a protein database. The researcher specifies the modification as a variable modification, meaning the search algorithm considers both the modified and unmodified forms of the residue. The search returns a list of peptide-spectrum matches with scores that indicate the confidence of each assignment. The choice of variable modifications affects the search space and the false discovery rate. Searching for too many variable modifications increases the search space and can reduce the confidence of the results. The researcher must balance the desire to find modifications against the risk of false positives. The search results should be filtered using a false discovery rate threshold, and the localization of the modification should be confirmed by manual inspection of the spectrum. ## Experimental Workflow for PTM Analysis ### Step 1: Define the Biological Question The design of a PTM experiment begins with the biological question. Is the goal to identify all phosphorylation sites on a protein of interest, or to compare the modification status of a protein between two conditions? The question determines the experimental design, the enrichment strategy, and the depth of the analysis. The sample type also matters. A purified protein provides a simpler background for the analysis, but the purification process may lose the modification or alter its abundance. A complex lysate provides a more complete picture of the modification landscape but requires more extensive fractionation and enrichment to detect low-abundance modifications. ### Step 2: Purify the Protein of Interest Protein purification is the first step in the workflow. The purification method must preserve the modification of interest. Some modifications are stable under standard purification conditions, while others are labile. Phosphorylation is generally stable during purification, but the activity of phosphatases in the lysate can remove the modification. The purification buffer should include phosphatase inhibitors to preserve the phosphorylation state. The purification method also affects the background of the sample. A protein purified by affinity chromatography will have fewer contaminating proteins than a crude lysate, which reduces the complexity of the mass spectrometry analysis. The purity of the sample should be assessed by SDS-PAGE or another method before proceeding to digestion. ### Step 3: Digest the Protein The purified protein is digested into peptides with a protease. Trypsin is the most common protease because it cleaves after lysine and arginine, producing peptides with a basic residue at the C-terminus that ionizes well in positive mode. The digestion conditions must be optimized to achieve complete digestion. Incomplete digestion produces longer peptides that may be difficult to fragment and identify. The digestion of a modified protein can be affected by the modification. A modification on a lysine residue can block trypsin cleavage at that site, producing a miscleaved peptide. The miscleaved peptide is longer than the expected tryptic peptide and may be more difficult to detect. The search should include miscleavages as a variable parameter to account for this effect. ### Step 4: Enrich the Modified Peptides The enrichment step is the most critical for the detection of low-abundance modifications. The modified peptides are often present at a much lower concentration than the unmodified peptides, and the mass spectrometer can only detect a limited number of peptides in a given time. The enrichment step isolates the modified peptides from the unmodified peptides, increasing the relative concentration of the modified species. The enrichment method depends on the modification. Phosphopeptides are enriched by immobilized metal affinity chromatography or titanium dioxide, which bind the phosphate group. Glycopeptides are enriched by lectin affinity chromatography or hydrophilic interaction chromatography. The enrichment method must be validated for the specific modification and sample type. ### Step 5: Acquire the Mass Spectrometry Data The enriched peptides are separated by liquid chromatography and introduced into the mass spectrometer. The mass spectrometer acquires a full scan of the peptide masses and then selects the most abundant peptides for fragmentation. The fragmentation produces the tandem mass spectrum that is used for the database search. The acquisition method can be optimized for the modification. The mass spectrometer can be set to trigger fragmentation on the neutral loss of a phosphate group, which is a characteristic of phosphopeptides. The fragmentation energy can be adjusted to preserve the modification site. The acquisition method should be tested with a standard peptide to ensure that the fragmentation produces the expected ions. ### Step 6: Search the Database The raw data is processed and searched against a protein database. The search parameters include the protease, the variable modifications, and the mass tolerance. The search results are filtered by a false discovery rate threshold to reduce the number of false positives. The search output includes the peptide sequence, the modification site, and the confidence score. The localization of the modification is reported as a probability that the modification is at a specific site. The localization probability should be inspected for each peptide, and the spectra should be manually reviewed to confirm the assignment. ### Step 7: Validate the Results The database search results are validated by manual inspection of the spectra. The fragment ions should be consistent with the assigned peptide sequence and the modification site. The spectrum should show a series of ions that cover the modified residue. The absence of the expected fragment ions can indicate a misassignment. The validation should also consider the biological context. The modification site should be accessible in the protein structure, and the modification should be conserved across species if it is functionally important. The validation is the final step in the workflow and is essential for the confidence of the results. ## Enrichment Strategies for Common PTMs ### Phosphorylation Enrichment Phosphorylation is the most studied post-translational modification. The phosphate group is negatively charged, and the enrichment methods exploit this property. Immobilized metal affinity chromatography uses a metal ion, such as iron or titanium, to bind the phosphate group. Titanium dioxide is an alternative that binds the phosphate group with a different selectivity. The enrichment of phosphopeptides is affected by the sample complexity and the stoichiometry of the modification. The enrichment can be performed at the peptide level after digestion, which reduces the complexity of the sample. The enrichment can also be performed at the protein level, but the protein-level enrichment is less efficient because the modified protein is a small fraction of the total protein. The enrichment method should be validated with a standard phosphopeptide to confirm the binding and elution. The recovery of the phosphopeptide should be measured to determine the efficiency of the enrichment. The enrichment is a critical step for the detection of low-abundance phosphorylation sites. ### Glycosylation Enrichment Glycosylation is a complex modification that involves the attachment of a glycan to a protein. The glycan can be attached to the nitrogen atom of an asparagine residue, which is N-linked glycosylation, or to the oxygen atom of a serine or threonine residue, which is O-linked glycosylation. The enrichment of glycopeptides is more complex than the enrichment of phosphopeptides because the glycan is a large and heterogeneous structure. Lectin affinity chromatography uses a lectin that binds to a specific glycan structure. The lectin is immobilized on a column, and the glycopeptides are bound and eluted. Hydrophilic interaction chromatography is an alternative that separates the glycopeptides based on their hydrophilicity. The glycopeptides are more hydrophilic than the unmodified peptides, and they are retained on the column. The enrichment of glycopeptides is complicated by the heterogeneity of the glycan. A single glycosylation site can have many different glycan structures, and the mass of the glycopeptide varies with the glycan. The database search for glycopeptides requires a glycan database that includes the possible glycan structures. The search is more complex than the search for phosphopeptides. ### Ubiquitination Enrichment Ubiquitination is a modification that marks a protein for degradation. The ubiquitin protein is attached to a lysine residue of the target protein. After digestion, the ubiquitin is cleaved, leaving a diglycine remnant on the lysine. The diglycine remnant is a specific marker for ubiquitination. The enrichment of ubiquitinated peptides uses an antibody that recognizes the diglycine remnant. The antibody is used to purify the peptides that carry the remnant. The enrichment is performed after digestion, and the antibody is specific for the diglycine remnant. The enrichment of ubiquitinated peptides is complicated by the low abundance of the modification. The ubiquitinated protein is a small fraction of the total protein, and the diglycine remnant is a small modification. The enrichment is a key step for the detection of ubiquitination sites. ## Fragmentation Techniques for PTM Analysis ### Collision-Induced Dissociation Collision-induced dissociation is the most common fragmentation method in mass spectrometry. The peptide is fragmented by collision with an inert gas, and the peptide backbone is cleaved at the peptide bond. The fragment ions are the b and y ions that are used for the sequence assignment. The collision-induced dissociation is not optimal for all modifications. The labile modifications, such as phosphorylation on serine and threonine, are lost before the peptide backbone is fragmented. The loss of the phosphate group produces a neutral loss peak that dominates the spectrum. The neutral loss peak can obscure the fragment ions that are needed for the site localization. The collision-induced dissociation is used for the modifications that are stable under the fragmentation conditions. The acetylation and the methylation are stable, and the collision-induced dissociation is a suitable method for these modifications. ### Electron Transfer Dissociation Electron transfer dissociation is an alternative fragmentation method that is used for the labile modifications. The peptide is fragmented by the transfer of an electron, which breaks the peptide backbone at a different position. The electron transfer dissociation produces the c and z ions, which are different from the b and y ions. The electron transfer dissociation preserves the labile modifications, such as the phosphorylation and the glycosylation. The modification is retained on the fragment ion, and the site can be localized. The electron transfer dissociation is a complementary method to the collision-induced dissociation. The electron transfer dissociation is less efficient than the collision-induced dissociation, and the fragmentation is often incomplete. The electron transfer dissociation is often combined with the collision-induced dissociation to provide a complete fragment ion series. The combination of the two methods is called the electron transfer dissociation with the supplemental activation. ### Higher-Energy Collisional Dissociation Higher-energy collisional dissociation is a fragmentation method that is used on the Orbitrap mass spectrometers. The peptide is fragmented in a higher-energy collision cell, and the fragment ions are detected in the Orbitrap. The higher-energy collisional dissociation produces the b and y ions, similar to the collision-induced dissociation. The higher-energy collisional dissociation is a suitable method for the identification of the modifications. The fragmentation is efficient, and the fragment ions are detected with a high resolution. The higher-energy collisional dissociation is often used for the identification of the phosphopeptides, and the fragmentation is the optimized to preserve the phosphate group. The higher-energy collisional dissociation is also used for the identification of the glycopeptides. The fragmentation of the glycopeptide produces the glycan fragment ions and the peptide fragment ions. The glycan fragment ions are used to identify the glycan structure, and the peptide fragment ions are used to identify the peptide sequence. ## Data Analysis and Database Searching ### Search Parameters and Variable Modifications The database search is the step that converts the raw mass spectra into peptide identifications. The search algorithm compares the observed fragment ions to the theoretical fragment ions of the peptide sequences in the database. The search parameters include the protease specificity, the number of miscleavages, the variable modifications, and the mass tolerance. The variable modifications are the modifications that are searched for on the specific residues. The variable modification is the modification that may be present or absent on the residue. The search algorithm considers both the modified and the unmodified forms of the residue. The variable modifications are the key parameter for the PTM analysis. The search for the variable modifications increases the search space and the search time. The search for a large number of variable modifications can reduce the confidence of the identifications. The search should be limited to the modifications that are expected in the sample. The search should also be validated with a decoy database to estimate the false discovery rate. ### False Discovery Rate and Localization Probability The false discovery rate is the fraction of the identifications that are false. The false discovery rate is estimated by searching the data against a decoy database that contains the reversed or the shuffled sequences. The number of the decoy hits is used to estimate the false discovery rate. The false discovery rate is a threshold for the confidence of the identifications. The localization probability is the probability that the modification is on a specific residue. The localization probability is calculated from the fragment ions that support the assignment. The localization probability is a measure of the confidence of the site assignment. The localization probability is a key for the PTM analysis. The localization probability is not the same as the identification probability. The identification probability is the probability that the peptide sequence is correct. The localization probability is the probability that the modification is on the correct residue. The localization probability is often lower than the identification probability, and the site assignment should be confirmed by the manual inspection of the spectrum. ### Manual Validation of the Spectra The manual inspection of the spectra is the final step in the validation of the PTM assignments. The spectrum is inspected to confirm that the fragment ions are consistent with the assigned peptide sequence and the modification site. The spectrum should contain a series of the fragment ions that cover the modified residue. The manual inspection is a time-consuming step, but it is essential for the confidence of the results. The automated search can produce false assignments, and the manual inspection is the only way to confirm the assignments. The manual inspection should be performed by a researcher who is experienced in the interpretation of the mass spectra. The manual inspection should be documented. The documentation should include the spectrum, the assigned peptide sequence, the modification site, and the localization probability. The documentation is the record of the validation and is the basis for the publication of the results. ## Common Failure Patterns and Troubleshooting ### Low Detection of the Modified Peptides The low detection of the modified peptides is a common problem in the PTM analysis. The modified peptides are often present at a low abundance, and the mass spectrometer can detect only a limited number of the peptides. The low detection can be caused by the low abundance of the modification, the inefficient enrichment, or the poor ionization of the modified peptide. The troubleshooting of the low detection should begin with the enrichment. The enrichment efficiency should be measured with a standard peptide. The enrichment should be optimized for the specific modification and the sample type. The enrichment should be repeated if the efficiency is low. The ionization of the modified peptides can be improved by the optimization of the liquid chromatography and the mass spectrometry parameters. The mobile phase and the column should be optimized for the modified peptides. The ionization source should be optimized for the modified peptides. ### Poor Site Localization The poor site localization is a common problem in the PTM analysis. The site localization is the assignment of the modification to a specific residue. The site localization is difficult when the peptide contains multiple residues that can be modified. The site localization is also difficult when the fragmentation is incomplete. The troubleshooting of the poor site localization should begin with the fragmentation. The fragmentation method should be optimized for the modification. The electron transfer dissociation should be used for the labile modifications. The fragmentation energy should be optimized to produce the fragment ions that cover the modification site. The site localization can be improved by the use of the complementary fragmentation methods. The collision-induced dissociation and the electron transfer dissociation can be combined to produce a complete fragment ion series. The combined fragmentation can provide the ions that are needed for the site localization. ### High False Discovery Rate The high false discovery rate is a common problem in the PTM analysis. The false discovery rate is the fraction of the identifications that are false. The false discovery rate is increased by the search for a large number of the variable modifications. The false discovery rate is also increased by the low quality of the spectra. The troubleshooting of the high false discovery rate should begin with the search parameters. The number of the variable modifications should be limited to the expected modifications. The mass tolerance should be set to the appropriate value. The search should be validated with the decoy database. The quality of the spectra should be improved by the optimization of the mass spectrometry parameters. The resolution and the mass accuracy should be optimized. The fragmentation should be optimized to produce the high-quality spectra. ## Records and Measurements for the PTM Analysis ### Documentation of the Experimental Conditions The documentation of the experimental conditions is essential for the reproducibility of the PTM analysis. The documentation should include the sample preparation, the enrichment, the digestion, the mass spectrometry, and the data analysis. The documentation should be detailed enough to allow the repetition of the experiment. The documentation should include the batch of the reagents, the instrument settings, and the software version. The documentation should include the date and the operator. The documentation should be stored in a laboratory notebook or an electronic system. The documentation is the basis for the quality control of the experiment. The documentation is used to identify the sources of the variation and the errors. The documentation is also used to the comparison of the results across the experiments. ### The Quality Control of the Mass Spectrometry The quality control of the mass spectrometry is the process of the verification of the instrument performance. The quality control should be performed before the analysis of the samples. The quality control should include the calibration of the mass, the resolution, and the fragmentation. The calibration of the mass is performed with a standard peptide or a standard mixture. The calibration is used to verify the mass accuracy of the instrument. The resolution is the measure of the ability of the instrument to separate the ions. The resolution is verified with the standard peptide. The fragmentation is verified with the standard peptide. The fragmentation is the measure of the ability of the instrument to produce the fragment ions. The fragmentation is verified with the standard peptide that is fragmented under the same conditions as the samples. ### The Quality Control of the Data The quality control of the data is the step of the verification of the data quality. The quality control should include the inspection of the total ion chromatogram, the base peak chromatogram, and the mass spectra. The quality control should be performed before the data analysis. The total ion chromatogram is the plot of the total ion current versus the retention time. The total ion chromatogram should show the peaks that correspond to the peptides. The mass spectra should be inspected for the quality of the peaks and the background. The quality control of the data is the basis for the decision to proceed with the data analysis. The data that does not meet the quality criteria should be reacquired. The data that is of the poor quality should not be used for the analysis. ## Safety and Regulatory Context ### Biosafety in the Sample Preparation The sample preparation for the PTM analysis involves the handling of the biological samples. The samples may contain the infectious agents or the hazardous chemicals. The biosafety of the sample preparation should be assessed before the start of the experiment. The biosafety assessment should be based on the risk of the sample. The risk is the likelihood of the exposure to the infectious agent or the hazardous chemical. The risk is the basis for the selection of the containment and the safety practices. The Biosafety in Microbiological and Biomedical Laboratories provides the principles of the biosafety and the containment. The principles are the basis for the safe handling of the biological samples. The principles should be followed in the sample preparation. ### The Institutional Oversight of the Recombinant Work The PTM analysis may involve the use of the recombinant proteins. The recombinant proteins are the proteins that are produced by the recombinant DNA technology. The use of the recombinant proteins is the subject of the institutional oversight. The NIH Guidelines for the Recombinant or Synthetic Nucleic Acid Molecules provide the oversight of the recombinant work. The guidelines are the basis for the institutional review of the recombinant work. The guidelines should be followed in the production of the recombinant proteins. The institutional oversight is the responsibility of the institution. The institution should have a committee that reviews the recombinant work. The committee should approve the work before the start of the experiment. ### The Measurement Standards and the Calibration The measurement standards are the basis for the quality of the mass spectrometry. The measurement standards are the reference materials that are used for the calibration of the instrument. The measurement standards are the basis for the accuracy and the precision of the measurements. The National Institute of Standards and Technology provides the measurement standards and the calibration services. The standards are the basis for the quality of the mass spectrometry. The standards should be used for the calibration of the instrument. The calibration of the instrument is the basis for the quality of the data. The calibration should be performed with the standard materials. The calibration should be documented and the records should be maintained. ## Common Failure Patterns and Troubleshooting ### The Loss of the Modification During the Sample Preparation The loss of the modification during the sample preparation is a common problem. The modification can be lost during the purification, the digestion, or the enrichment. The loss of the modification can be caused by the enzymatic activity, the chemical instability, or the physical conditions. The loss of the phosphorylation can be caused by the activity of the phosphatases. The phosphatases are the enzymes that remove the phosphate group. The phosphatases should be inhibited during the sample preparation. The inhibition should be performed with the phosphatase inhibitors. The loss of the glycosylation can be caused by the glycosidases. The glycosidases are the enzymes that remove the glycan. The glycosidases should be inhibited during the sample preparation. The inhibition should be performed with the glycosidase inhibitors. ### The Incomplete Digestion of the Protein The incomplete digestion of the protein is a common problem. The incomplete digestion is the result of the incomplete cleavage of the protein by the protease. The incomplete digestion can be caused by the modification of the protein, the structure of the protein, or the conditions of the digestion. The modification of the protein can prevent the cleavage by the protease. The modification on the lysine or the arginine can prevent the cleavage. The digestion should be optimized to the complete digestion. The digestion should be performed with the high concentration of the protease and the long digestion time. The structure of the protein can prevent the access of the protease. The protein should be denatured before the digestion. The denaturation should be performed with the urea or the guanidine. The denaturation should be followed by the reduction and the alkylation. ### The Contamination of the Sample The contamination of the sample is a common problem. The contamination can be caused by the keratin, the plasticizers, or the other contaminants. The contamination can be the source of the false identifications. The contamination should be minimized during the sample preparation. The keratin contamination is the most common contamination. The keratin is the protein that is present in the skin and the hair. The keratin contamination should be minimized by the use of the gloves and the clean lab. The keratin contamination should be minimized by the use of the clean reagents. The plasticizers are the contaminants that are present in the plastic. The plasticizers can be the source of the contamination. The plasticizers should be minimized by the use of the glassware and the clean plastic. The plasticizers should be minimized by the use of the clean reagents. ## The Limitations of the PTM Analysis ### The Stoichiometry of the Modification The stoichiometry of the modification is the fraction of the protein that is modified. The stoichiometry is a key parameter for the understanding of the biological function of the modification. The stoichiometry is difficult to measure by the mass spectrometry. The mass spectrometry is a qualitative method. The mass spectrometry can identify the modification and the site, but the mass spectrometry cannot measure the stoichiometry. The stoichiometry can be measured by the quantitative mass spectrometry, but the quantitative mass spectrometry is a complex method. The quantitative mass spectrometry requires the use of the labeled standards. The labeled standards are the peptides that are labeled with the stable isotopes. The labeled standards are used for the quantification of the modified peptides. The quantification is the ratio of the labeled and the unlabeled peptides. ### The Coverage of the Protein The coverage of the protein is the fraction of the protein sequence that is identified by the mass spectrometry. The coverage is a key parameter for the completeness of the analysis. The coverage is limited by the detection of the peptides. The coverage is limited by the size of the protein. The large proteins are the difficult to cover. The coverage is limited by the digestion of the protein. The digestion is the basis for the coverage. The coverage is limited by the detection of the peptides. The detection is the basis for the coverage. The detection is limited by the ionization of the peptides. The ionization is the basis for the detection. ### The False Discovery Rate The false discovery rate is the fraction of the identifications that are false. The false discovery rate is a key parameter for the confidence of the results. The false discovery rate is controlled by the search parameters. The false discovery rate is increased by the search for the large number of the variable modifications. The false discovery rate is increased by the search for the large number of the peptides. The false discovery rate is controlled by the use of the decoy database. The false discovery rate is the basis for the confidence of the results. The false discovery rate should be reported in the publication. The false discovery rate should be the basis for the interpretation of the results. ## The Professional Escalation Criteria ### When to Consult the Mass Spectrometry Facility The mass spectrometry facility is the source of the expertise for the mass spectrometry. The facility should be consulted for the design of the experiment, the acquisition of the data, and the analysis of the data. The facility should be consulted for the troubleshooting of the problems. The facility should be consulted for the design of the experiment. The facility can provide the advice on the enrichment, the fragmentation, and the search. The facility can provide the advice on the acquisition of the data. The facility should be consulted for the troubleshooting of the problems. The facility can provide the advice on the troubleshooting of the problems. The facility can provide the advice on the optimization of the methods. ### When to Seek the Institutional Review The institutional review is the process of the review of the research by the institution. The institutional review should be sought for the research that involves the recombinant work. The institutional review should be sought for the research that involves the human or the animal samples. The institutional review should be sought for the research that involves the recombinant work. The NIH Guidelines for the Recombinant or Synthetic Nucleic Acid Molecules provide the oversight of the recombinant work. The institutional review should be sought for the research that involves the recombinant work. The institutional review should be sought for the research that involves the human or the animal samples. The institutional review should be sought for the research that involves the human or the animal samples. The institutional review should be sought for the research that involves the human or the animal samples. ### When to Seek the Professional Consultation The professional consultation should be sought for the interpretation of the results. The professional consultation should be sought for the interpretation of the results that are the complex. The professional consultation should be sought for the interpretation of the results that are the conflicting. The professional consultation should be sought for the interpretation of the results that are the complex. The complex results are the results that are the difficult to interpret. The complex results are the results that are the difficult to interpret. The professional consultation should be sought for the interpretation of the results that are the conflicting. The conflicting results are the results that are the conflicting. The conflicting results are the results that are the conflicting. ## Frequently Asked Questions ### What is the difference between a fixed and a variable modification in a database search? A fixed modification is applied to every occurrence of a residue in the database, such as carbamidomethylation of all cysteines after alkylation. A variable modification is searched as present or absent on each residue, which increases the search space and the false discovery rate. ### Why does phosphorylation on serine and threonine require a different fragmentation method than phosphorylation on tyrosine? Phosphorylation on serine and threonine is labile and loses the phosphate group during collision-induced dissociation, producing a neutral loss that obscures the site. Tyrosine phosphorylation is more stable, and the phosphate group is retained during the fragmentation. ### What is the diglycine remnant in ubiquitination analysis? The diglycine remnant is the two glycine residues that remain on the lysine after the trypsin digestion of the ubiquitinated protein. The remnant adds a mass of 114 Da to the lysine and is the marker for the ubiquitination site. ### How does the enrichment of the modified peptides improve the detection of the PTM? The enrichment isolates the modified peptides from the unmodified peptides, increasing the relative abundance of the modified peptides. The enrichment is a key step for the detection of the low-abundance modifications. ### What is the false discovery rate and why is it important for the PTM analysis? The false discovery rate is the fraction of the identifications that are false. The false discovery rate is estimated by the search of the decoy database and is the basis for the confidence of the results. ### How is the site localization of the modification determined? The site localization is determined by the fragment ions that cover the modified residue. The fragment ions that contain the modification are shifted by the modification mass, and the fragment ions that do not contain the modification are not shifted. ### What is the role of the electron transfer dissociation in the PTM analysis? The electron transfer dissociation is a fragmentation method that preserves the labile modifications. The electron transfer dissociation is used for the site localization of the phosphorylation and the glycosylation. ### What is the difference between the collision-induced dissociation and the higher-energy collisional dissociation? The collision-induced dissociation is a fragmentation method that is used on the ion trap instruments. The higher-energy collisional dissociation is a fragmentation method that is used on the beam instruments. The higher-energy collisional dissociation is a more efficient fragmentation method. ## Using the Evidence | Source | Best use in this topic | Important limitation | |---|---|---| | [Molecular Biology and Laboratory Methods](https://www.ncbi.nlm.nih.gov/books) | official guidance | Check the linked page for current local requirements | | [Biosafety in Microbiological and Biomedical Laboratories](https://www.cdc.gov/labs/bmbl/index.html) | official guidance | Check the linked page for current local requirements | | [NIH Guidelines for Recombinant or Synthetic Nucleic Acid Molecules](https://osp.od.nih.gov/policies/biosafety-and-biosecurity-policy/nih-guidelines-for-research-involving-recombinant-or-synthetic-nucleic-acid-molecules) | official guidance | Check the linked page for current local requirements | ## Related Molecular Biology Guides - [Mass Spectrometry to Identify Proteins: A Practical Guide](/knowledge/molecular-biology/mass-spectrometry-identify-proteins) - [Protein Quantification Mass Spectrometry: A Practical Guide](/knowledge/molecular-biology/protein-quantification-mass-spectrometry) - [Intact Protein Mass Spectrometry: Principles and Applications](/knowledge/molecular-biology/intact-protein-mass-spectrometry) - [Mass Spectrometry Work for Proteins: A Beginner's Guide](/knowledge/molecular-biology/mass-spectrometry-work-for-proteins) - [How Mass Spec Identifies Proteins: A Practical Guide](/knowledge/molecular-biology/mass-spec-identify-proteins) ## References and Further Reading - [Molecular Biology and Laboratory Methods](https://www.ncbi.nlm.nih.gov/books). National Library of Medicine. - [Biosafety in Microbiological and Biomedical Laboratories](https://www.cdc.gov/labs/bmbl/index.html). CDC and NIH. - [NIH Guidelines for Recombinant or Synthetic Nucleic Acid Molecules](https://osp.od.nih.gov/policies/biosafety-and-biosecurity-policy/nih-guidelines-for-research-involving-recombinant-or-synthetic-nucleic-acid-molecules). National Institutes of Health. - [Addgene Protocols](https://www.addgene.org/protocols). Addgene. - [MIQE Guidelines](https://academic.oup.com/clinchem/article/55/4/611/5631762). Clinical Chemistry. - [NIST Laboratory Programs](https://www.nist.gov/laboratories). National Institute of Standards and Technology. - [Proteomic landscape of Alzheimer's Disease: novel insights into pathogenesis and biomarker discovery.](https://pubmed.ncbi.nlm.nih.gov/34384464). Molecular neurodegeneration, 2021. - [Identification of lysine succinylation as a new post-translational modification.](https://pubmed.ncbi.nlm.nih.gov/21151122). Nature chemical biology, 2011. - [Mass Spectrometry-Based Proteomics for Analysis of Hydrophilic Phosphopeptides.](https://pubmed.ncbi.nlm.nih.gov/33687720). Methods in molecular biology (Clifton, N.J.), 2021. - [Advances in enrichment methods for mass spectrometry-based proteomics analysis of post-translational modifications.](https://pubmed.ncbi.nlm.nih.gov/35896048). Journal of chromatography. A, 2022. - [The utility of ETD mass spectrometry in proteomic analysis.](https://pubmed.ncbi.nlm.nih.gov/17118725). Biochimica et biophysica acta, 2006. > This article is educational and does not replace validated laboratory procedures, institutional biosafety review, manufacturer instructions, or professional interpretation.