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Edman Degradation| Protein Sequencing

Edman degradation is a widely used method for determining the amino acid sequence of proteins. Named after its inventor, Pehr Edman, this technique involves the sequential removal of amino acids from the N-terminus of a protein.

By cleaving peptide bonds and analyzing the released amino acids, scientists can obtain valuable information about the structure and composition of proteins.

Edman degradation has been instrumental in studying various aspects of protein biology, including peptide sequencing, identifying cleavage sites, and characterizing modified amino acids.

Theory and Principle of Edman Sequencing

The reaction between Phenylisothiocyanate (PITC) and the N-terminal Amino Acid

In Edman sequencing, the process begins by reacting the protein sample with phenylisothiocyanate (PITC). This reaction specifically targets the N-terminal amino acid of the protein.

Formation of Phenylthiocarbamoyl (PTC) Derivative

The reaction between PITC and the N-terminal amino acid results in the formation of a phenylthiocarbamoyl (PTC) derivative. This derivative is stable and can be easily detected and quantified.

Detection and Quantification through Chromatography or Mass Spectrometry

Once the PTC derivative is formed, it can be analyzed to determine the identity of the N-terminal amino acid. This analysis is typically done using chromatography or mass spectrometry techniques.

Chromatography involves separating different components based on their chemical properties, such as charge or size. By comparing the retention time of the released amino acid with known standards, its identity can be determined.

Mass spectrometry measures the mass-to-charge ratio of molecules to identify them accurately. The released amino acid is ionized and fragmented, producing a unique mass spectrum that corresponds to its identity.

Advantages of Edman Sequencing

  • Provides information about the primary structure of proteins.

  • Can determine terminal sequences, which are essential for understanding protein function.

  • Allows for precise identification of individual amino acids at specific positions within a protein sequence.

Limitations of Edman Sequencing

  • Limited by sample size: Requires a relatively large amount of pure protein for accurate sequencing.

  • Limited to shorter sequences: The repetitive nature of Edman degradation limits its application to sequences up to 40-60 residues.

  • Isocratic elution: Some hydrophobic amino acids may not elute well in isocratic elution conditions, leading to potential inaccuracies.

Advantages of Edman Degradation for Protein Sequencing

Edman degradation offers several advantages. Let’s take a closer look at these advantages:

High Accuracy in Determining the Order of Amino Acids

One of the key benefits of Edman degradation is its high accuracy in sequencing proteins. This method allows scientists to precisely identify and determine the sequence of amino acids in a protein molecule.

By selectively removing one amino acid at a time, Edman degradation provides reliable results and helps researchers understand the structure and function of proteins.

Handling Small Amounts of Protein Samples

Edman degradation is particularly useful when working with limited amounts of protein samples.

This method can effectively analyze small quantities, making it suitable for studying proteins that are available only in limited quantities or those obtained from rare sources.

Compatibility with Various Types of Protein Modifications

Proteins often undergo post-translational modifications, such as phosphorylation or glycosylation, which can affect their structure and function. The advantage of Edman degradation is its compatibility with different types of protein modifications.

It allows scientists to accurately determine the sequence even when modifications are present, providing valuable insights into how these modifications impact protein behavior.

Limitations of Edman Degradation

Limited to proteins with less than 50 amino acids

Edman degradation, while a valuable method for protein sequencing, has its limitations. One major constraint is that it is only suitable for proteins with fewer than 50 amino acids.

This is because the technique involves repetitive cycles of chemical reactions, which can become time-consuming and impractical for larger proteins.

Sensitivity decreases as the sequence length increases

As the length of the protein sequence increases, the sensitivity of Edman degradation decreases. The accuracy and efficiency of identifying each amino acid in the sequence diminishes, making it more challenging to obtain precise results.

Therefore, this technique may not be ideal for longer protein sequences.

Difficulties with modified or unusual amino acids

Another challenge faced when using Edman degradation is encountering modified or unusual amino acids within a protein sequence.

Since this method relies on specific chemical reactions to identify each amino acid, any modifications or variations can interfere with the process and lead to inaccurate results.

Despite these limitations and challenges, Edman degradation remains a valuable tool for protein sequencing, especially for smaller proteins. It provides researchers with crucial information about the order of amino acids in a protein chain.

However, alternative methods such as mass spectrometry have emerged as powerful alternatives that can overcome some of these limitations by offering higher sensitivity and broader applicability.

Retention Times and Modified PTHAA Peaks

During the analysis of proteins using the Edman degradation method, retention times play a crucial role in identifying specific amino acids. These retention times refer to the elution characteristics of amino acids as they pass through a chromatographic column.

The modified phenylthiohydantoin-amino acid (PTHAA) peaks observed during Edman degradation analysis provide valuable insights into protein structure and function.

These modified peaks indicate the presence of post-translational modifications or unusual residues within the protein sequence.

Understanding the significance of retention times and modified PTHAA peaks is essential for accurate protein sequencing. Here are some key points to consider:

Retention Times:

  • Retention times are determined by factors such as the hydrophobicity, size, and charge of each amino acid.

  • Each amino acid has a unique retention time, allowing for identification during analysis.

  • Comparing experimental retention times with known standards helps confirm amino acid sequences.

Modified PTHAA Peaks:

  • Modified PTHAA peaks occur when certain amino acids undergo chemical modifications during the Edman degradation process.

  • Post-translational modifications can include phosphorylation, glycosylation, acetylation, or methylation.

  • Unusual residues may also be present due to genetic variations or mutations.

These retention times and modified PTHAA peaks serve as critical markers in deciphering protein structure and function. By analyzing peak areas, researchers can determine the relative abundance of different amino acids within a protein sequence.

Coupled Analysis and Related Resources

To enhance the effectiveness of Edman degradation studies, researchers have turned to coupling liquid chromatography with mass spectrometry. This powerful combination offers improved sensitivity and resolution in the analysis process.

Coupling Liquid Chromatography with Mass Spectrometry

Coupling liquid chromatography with mass spectrometry has revolutionized the field of protein sequencing. By using this technique, scientists can separate individual amino acids and analyze them more accurately.

The liquid chromatography step helps in the separation of complex mixtures, while mass spectrometry aids in identifying and quantifying individual components.

Online Databases: A Wealth of Resources

Online databases like UniProt provide researchers with comprehensive resources for cross-referencing sequencing data obtained from Edman degradation studies.

These databases contain a vast collection of protein sequences, allowing scientists to compare their results with existing information. Researchers can identify specific proteins and gain insights into their functions and properties.

Advanced Software Tools for Data Interpretation

Advanced software tools play a crucial role in analyzing the data generated from Edman degradation studies. These tools assist in data interpretation, alignment, and identification of amino acid derivatives.

They provide valuable assistance by automating processes that were once time-consuming and error-prone.

In addition to coupling liquid chromatography with mass spectrometry and utilizing online databases, researchers also rely on various other resources to enhance their Edman degradation studies:

  • Biomolecular resource facilities offer access to state-of-the-art equipment and expertise.

  • Chemical reagents are used for synthesis purposes during the study.

  • Avian enzymes are employed as catalysts due to their high specificity towards amide bond cleavage.

  • Laboratories equipped with specialized instruments enable precise measurements.

By leveraging these resources effectively, researchers can achieve higher yields, better accuracy, and more reliable results in their Edman degradation studies.


In conclusion, Edman degradation is a powerful technique used in biochemistry to sequence proteins.

It helps us understand protein structures and functions by removing and identifying amino acids from the beginning of a peptide chain. This technique has led to more research on protein interactions, modifications, and diseases.

To use Edman degradation effectively, researchers need to know its limits and challenges. This method can only analyze certain proteins due to their size and complexity.

Some amino acids can make the process more difficult. However, scientists have found ways to improve Edman degradation with techniques like the coupled analysis.

Using Edman degradation in protein sequencing can help advance scientific knowledge in different areas. Whether you’re studying protein folding, investigating disease mutations, or creating new treatments, knowing the structure of proteins is important.

By using this technique well and staying updated with resources and advancements, you can make important contributions to scientific discoveries.


How accurate is Edman degradation for protein sequencing?

Edman degradation is highly accurate for determining the sequence of amino acids in a peptide or protein. It can reliably identify individual amino acids from the N-terminus one at a time with minimal errors.

Can Edman degradation be used for large proteins?

Edman degradation is most suitable for small to medium-sized proteins due to technical limitations associated with larger molecules. However, coupled analysis techniques have been developed to extend its application to larger proteins.

What are some alternatives to Edman degradation for protein sequencing?

Some alternatives to Edman degradation include mass spectrometry-based methods such as tandem mass spectrometry (MS/MS) and de novo sequencing. These techniques offer advantages such as higher sensitivity, faster analysis times, and the ability to analyze larger proteins.

Can Edman degradation detect post-translational modifications?

Edman degradation can detect certain post-translational modifications that occur at or near the N-terminus of a protein. However, it may not be suitable for analyzing modifications located further down the peptide chain.

Is Edman degradation a widely used technique in protein research?

Yes, Edman degradation has been widely used in protein research for several decades. It has played a crucial role in determining the primary structure of numerous proteins and advancing our understanding of their functions and interactions.