01Distinguish a predicted digest from an observed map

Proteases have different cleavage preferences. New England Biolabs describes trypsin as cleaving on the C-terminal side of lysine and arginine, while other enzymes recognise different positions or residues. The chosen enzyme therefore determines the expected fragment pattern.New England Biolabs — Protease recognition sites (opens in a new tab)

For an original simplified example, consider AGKSLRVEKAAG. A model that cleaves after each internal K or R predicts AGK, SLR, VEK and AAG. This is an in-silico expectation for the stated rule, not a claim that a real digest will produce exactly four measurable peaks.

An uncleaved expected site produces a longer piece, commonly described as a missed-cleavage product. A real analytical method must accommodate relevant incomplete cleavage and evaluate whether the observed products support the proposed sequence.

02Count the residues actually supported by assignments

If AGK, SLR and VEK are confidently assigned in the example while AAG is not, the assigned fragments cover nine of twelve residues: 75%. Observing AGK a second time in another charge state does not add three new sequence positions.

Coverage describes the union of supported sequence regions, not the total number of peaks or spectra. Overlapping fragments can strengthen evidence for a region without increasing its unique residue count.

Nor does a nominal 100% coverage statement automatically determine every stereocentre, disulfide pairing or low-level variant. The information content depends on how fragments were assigned and which molecular differences the method can distinguish.

For a very short peptide, digestion may create fragments with too little distinctive information. A mapping strategy should serve the actual identity question rather than being added merely because the instrument can analyse digests.

03Use a different cleavage pattern to investigate a gap

Swaney, Wenger and Coon compared multiple proteases in yeast proteomics. Different digests improved sequence coverage and protein identification beyond the results from a single protease. Their study demonstrates the value of complementary cleavage patterns in the system examined.Swaney, Wenger and Coon — Value of using multiple proteases (opens in a new tab)

For the twelve-residue example, an independently identified fragment spanning positions 8–12 could add evidence for the missing end. Its overlap with positions already assigned also provides a connection between the new information and the existing map.

This is a conceptual coverage example, not a suggested enzyme recipe. The appropriate strategy depends on the sequence, modifications, sample and measurement method.

Repeating the same digest can assess reproducibility, but it may repeatedly leave the same region unobserved. Reproducibility and complementary sequence information are useful for different reasons.

04Investigate changes introduced by the mapping workflow

Hara and colleagues studied artefacts in mapping recombinant human insulin-like growth factor-I. They reported unwanted products under previously described conditions and evaluated a combination of proteases that produced a suitable map without those apparent changes.Hara, Rosenfeld and Lu — Preventing peptide-map artefacts (opens in a new tab)

An unexpected fragment therefore requires investigation before it is assigned to an impurity in the original material. Preparation effects, enzyme-derived material, incomplete cleavage and a genuine sequence difference are distinct possibilities.

A useful report retains the expected digest, observed assignments, coverage map and unresolved features. It explains which differences are supported and which remain uncertain. That makes the map an interpretable body of evidence rather than a decorative chromatogram.