26.6 Peptide Sequencing: The Edman Degradation

Peptide Sequencing Techniques

Edman degradation process

Edman degradation is the classic method for identifying amino acids one at a time, starting from the N-terminus of a peptide. Each cycle removes and identifies a single residue, then the process repeats on the shortened peptide.

Here's how one cycle works:

1. Labeling: Phenyl isothiocyanate (PITC) reacts with the free $\alpha$-amino group of the N-terminal residue under mildly basic conditions (pH ~8–9). This forms a phenylthiocarbamyl (PTC) derivative, tagging only the first amino acid.
2. Cleavage: Treatment with anhydrous acid (typically trifluoroacetic acid) selectively breaks the peptide bond between the first and second residues. The tagged amino acid cyclizes and falls off as an anilinothiazolinone (ATZ) derivative, leaving the rest of the peptide intact with a new N-terminus.
3. Identification: The ATZ derivative is extracted and converted (in aqueous acid) to a more stable phenylthiohydantoin (PTH) derivative. This PTH-amino acid is then identified by HPLC or electrophoresis.
4. Repeat: The shortened peptide goes back through step 1, and the next residue is identified.

Limitations to know:

• Each cycle isn't 100% efficient. After 50–60 cycles, incomplete reactions accumulate and the signal becomes too noisy to read. That's why Edman degradation works for peptides but not full-length proteins directly.
• The N-terminus must have a free amino group. If it's been modified (e.g., acetylated), PITC can't react, and the method fails entirely.
• Edman degradation reads from the N-terminus only. It doesn't identify the C-terminal residue on its own (you'd need a separate method, like carboxypeptidase digestion, for that).
• The sample must be a single, pure peptide. A mixture of peptides would give overlapping signals at each cycle.

Partial hydrolysis for protein sequencing

Since Edman degradation can only handle ~50–60 residues, larger proteins need to be broken into smaller, overlapping fragments first. Each fragment is sequenced individually, and then the overlapping regions are used to piece together the full sequence, much like assembling a jigsaw puzzle.

There are two main approaches:

Chemical hydrolysis uses dilute acid (e.g., 6 M HCl) or base to randomly break peptide bonds. This produces a complex mixture of fragments of various lengths. The fragments are separated (usually by chromatography), then sequenced by Edman degradation. Because cleavage is random, you get many overlapping pieces, which helps with reconstruction but makes separation harder.

Enzymatic hydrolysis uses proteases that cut at predictable positions. This gives a cleaner, more reproducible set of fragments. The key advantage is specificity: you know where the enzyme cuts, so you can predict what fragments to expect.

The standard strategy is to digest the protein with two different enzymes in separate experiments. Each enzyme produces a different set of fragments. You sequence both sets, then align the overlapping regions to determine the complete sequence.

Enzyme specificity in peptide cleavage

Two proteases come up constantly in sequencing problems:

Trypsin cleaves on the C-terminal side of lysine (Lys, K) and arginine (Arg, R). Every fragment except the last one will end in K or R.

• Exception: if K or R is followed by proline (Pro, P), cleavage is blocked.
• Example: The sequence AKSDARFG is cut after K and after R: $\text{AK | SDAR | FG}$

Chymotrypsin cleaves on the C-terminal side of large hydrophobic residues: phenylalanine (Phe, F), tyrosine (Tyr, Y), and tryptophan (Trp, W). Cleavage is most efficient when these residues are followed by a small side-chain residue (Ala, Gly, Ser).

• Example: The sequence AKFDSYGW is cut after F, Y, and W: $\text{AKF | DSY | GW}$

Predicting fragments on an exam:

1. Write out the full sequence and circle every cleavage site for the given enzyme.
2. Draw vertical lines after each cleavage residue to mark where the chain breaks.
3. List the resulting fragments. Remember that the C-terminal fragment of the original protein won't end in a cleavage residue (unless by coincidence).
4. Check for exceptions (e.g., Pro following Lys/Arg for trypsin).

Protein Structure and Sequencing Methods

Primary structure refers to the linear sequence of amino acids in a polypeptide, held together by peptide bonds. Determining this sequence is the foundation for understanding a protein's higher-order folding and biological function.

In practice, modern protein sequencing often combines Edman degradation with mass spectrometry (MS). Mass spectrometry can rapidly analyze peptide fragments by measuring their mass-to-charge ratios, and it can detect post-translational modifications (phosphorylation, glycosylation, etc.) that chemical sequencing methods might miss. Tandem MS (MS/MS) can even fragment peptides further and read partial sequences directly, making it a powerful complement to classical methods.