Peptide Mapping: Protease Digestion and Sequence Coverage

An intact mass says the molecule weighs what it should. Peptide mapping is the method that says the residues are in the order they should be, and it is the only routine technique that localises a modification to a position rather than reporting it somewhere in the chain.

The principle

The peptide is cut into fragments by a protease with known specificity. Each fragment is separated chromatographically and its mass measured. Because the enzyme’s cut sites are predictable from the sequence, the set of fragment masses expected from the correct sequence can be calculated in advance.

Comparing observed fragments against that calculated list is the test. Every fragment accounted for means every segment of the chain is what it should be. A fragment at an unexpected mass localises a difference to that segment.

Why the enzyme’s specificity is the whole method

A protease that cut randomly would produce an unreadable mixture. The value comes from cutting at defined residues:

• Trypsin cuts after lysine and arginine, not before proline. It is the standard choice, because those residues are common enough to give fragments of useful length and rare enough that the fragments are not too short.
• Chymotrypsin cuts after the large hydrophobic residues — phenylalanine, tryptophan, tyrosine.
• Glu-C cuts after glutamate, and after aspartate as well under some buffer conditions.
• Asp-N cuts before aspartate.

Different enzymes produce different fragment boundaries, which is why a sequence not fully covered by one digest is often covered by a second with a different enzyme.

Coverage, and why it is rarely complete

Coverage is the proportion of the sequence observed in identified fragments. It is almost never 100%, for reasons built into the method.

Fragments can be too small to retain on the column or to detect reliably — a single residue between two lysines produces a fragment of no analytical use. They can be too large and hydrophobic to elute. They can carry no ionisable group and ionise poorly. And regions with no cut sites at all produce one enormous fragment that behaves badly.

A reported coverage figure is therefore part of the result. Ninety-five percent coverage with the uncovered region named is a strong result. A map with no coverage figure has not said what it did not see.

Missed cleavages

Enzymes do not cut every available site every time. A site adjacent to a proline, a site next to a modified residue, or a site in a structured region may be skipped, producing a fragment spanning two expected fragments.

This is normal and predictable. A map is interpreted against a list that includes the expected fragments and the plausible missed-cleavage products, and an unexpectedly long fragment is usually a missed cleavage rather than a finding.

What it localises that intact mass cannot

This is the reason the method exists. An intact mass sixteen daltons high says an oxygen was added somewhere. A map says which fragment carries it, and therefore which residue or small group of residues.

The same applies to deamidation, to a retained protecting group, to a substitution, and to disulfide connectivity — digesting between the cysteines and observing which fragments remain joined is what identifies the pairing, as described in disulfide bond formation and scrambling.

Going one step further, fragmenting the individual peptides inside the mass spectrometer localises a modification to a single residue. That technique is covered in reading a sequence from fragment ions.

Where the method introduces its own artefacts

• Deamidation during digestion. Trypsin digests are typically run for hours at 37 degrees near pH 8 — conditions that actively promote deamidation. A map can therefore report a deamidation that the method created.
• Disulfide scrambling. The same conditions scramble disulfides, which is why a connectivity map is run at lower pH with a faster enzyme where possible.
• Autolysis. The protease digests itself, contributing its own fragments to the chromatogram. Their masses are known and are subtracted.
• Oxidation during sample handling. Methionine oxidises readily; observing it in a map does not establish it was present in the material.

The control for all four is a blank digest and a comparison against a reference sample treated identically.

Why it is uncommon on a peptide certificate

Cost and proportionality. A map is a separate chromatographic run plus a digestion plus interpretation, and for a fifteen-residue peptide the intact mass already constrains the composition tightly enough that mapping adds little.

It becomes worthwhile as length increases, where intact mass stops being decisive: an 80-residue chain with a single substitution can be within routine mass accuracy of the correct value, and a map resolves what the mass cannot. That threshold is discussed in where peptide analysis becomes protein analysis.

What a map does and does not establish

It establishes that the observed fragments are consistent with the expected sequence, to the extent of the coverage reported. It does not establish stereochemistry — a D-residue produces a fragment of identical mass. It does not quantify purity, because it is run on material already isolated. And it does not see anything in the uncovered fraction.

Read alongside an intact mass and a purity chromatogram, it answers the question neither of those can: not what the molecule weighs, but what order it is in.