Protease Cleavage Specificity and Substrate Design

Whether you are planning a digest for peptide mapping, designing a reporter substrate, or placing a cleavable tag in a construct, the same question decides it: which bond will the enzyme actually cut? Protease specificity is described by a nomenclature that is almost sixty years old and still the one everyone uses.

The nomenclature, in one paragraph

Schechter and Berger proposed in 1967 that a protease active site is not a single point but a groove divided into subsites, each able to hold one amino acid side chain.¹ The convention that followed is the one still used: subsites are numbered S1 to Sn toward the substrate's N-terminus and S1′ to Sn′ toward its C-terminus, the substrate residues occupying them are P1 to Pn and P1′ to Pn′, and the bond that is hydrolysed lies between P1 and P1′.²

The practical consequence is that specificity is rarely about one residue. A protease is usually named for what it accepts at P1, but what sits at P1′ and P2 can decide whether the cut happens at all.

Specificity of the commonly used enzymes

Enzyme	Cleaves	Worth knowing
Trypsin	After Lys or Arg (P1)	Strongly disfavoured when proline follows at P1′, the basis of the classical Keil rule, though exceptions are documented. The workhorse of peptide mapping, because Lys and Arg are frequent enough to give peptides of a convenient length.
Lys-C	After Lys	Gives longer peptides than trypsin and tolerates denaturing conditions well, so it is often used before trypsin in a two-step digest.
Arg-C	After Arg	Cleavage is only moderately affected by what sits at P1′.
Chymotrypsin	After Phe, Tyr or Trp at P1, and to a lesser extent after Leu, Met or His	Several documented vetoes: proline at P1′ nearly always blocks cleavage whatever sits at P1; Trp at P1 is blocked by Met or Pro at P1′; Met at P1 is blocked by Tyr at P1′; His at P1 is blocked by Asp, Met or Trp at P1′.4
Glu-C (V8)	After Glu, and also Asp in phosphate buffer	The buffer changes the specificity, which is easy to forget and hard to debug afterwards.
Asp-N	Before Asp	One of the few that cleaves on the N-terminal side, which shifts where your peptide boundaries fall.
Pepsin	Broad, prefers Phe, Leu, Trp, Tyr	Works at low pH, which suits denatured or disulfide-rich material.
Cyanogen bromide	After Met	A chemical rather than an enzyme. Produces very large fragments because methionine is rare.

Recognition sequences for tag removal

These enzymes are chosen for the opposite reason: not because they cut often, but because they cut in very few other places. How clean that selectivity is in practice still depends on the construct and the conditions, so an unexpected second cut is worth checking for rather than ruling out.

Enzyme	Recognition sequence	Note
Thrombin	LVPR ↓ GS	Leaves Gly-Ser on the product.
Factor Xa	IEGR ↓	Cleaves immediately after the arginine, which in a well-designed construct leaves no extra residues.
TEV protease	ENLYFQ ↓ G or S	Leaves a single glycine or serine. Active at 4 °C, which helps with fragile proteins.
HRV 3C (PreScission)	LEVLFQ ↓ GP	Also works well in the cold.
Enterokinase	DDDDK ↓	Leaves no residual residues, but is more prone to secondary cleavage elsewhere.

Designing a substrate rather than predicting a digest

If you are making a peptide for an enzyme to act on, the design rules follow from the subsite picture:

• Fill more than P1. A substrate spanning P4 to P2′ usually behaves far better than a short one built around the P1 residue alone, because the enzyme's affinity comes from the whole groove.
• Check P1′ deliberately. It is the position that most often silently prevents cleavage, proline being the classic case.
• Add the reporter outside the recognition region. A fluorophore or quencher sitting in S2 or S1′ changes the kinetics of the thing you are trying to measure.
• Include a non-cleavable control. The usual choice is the same peptide with the P1 residue substituted, which controls for background hydrolysis and for the reporter itself. Treat it as a candidate rather than a given: a substitution can reduce or redirect cleavage rather than abolish it, so verify experimentally that it is not cleaved under your assay conditions.
• Keep solubility in mind. Substrate peptides are often short and hydrophobic, and a peptide that precipitates in assay buffer produces a rate that looks like weak activity.

Where to check a specific enzyme

For anything beyond the common enzymes, the MEROPS database is the reference work: a curated collection of peptidases and of observed cleavage positions in real substrates, including which sites are physiologically relevant rather than merely observed in vitro.³ For planning a digest of a known sequence, ExPASy's PeptideCutter applies published specificity rules to a sequence you paste in.

What to send for a quote

• The substrate sequence, or the protein and enzyme if you want us to suggest one
• Which positions must stay native, and where a label may be placed
• The reporter format: fluorogenic, FRET pair, chromogenic, or unlabelled
• Whether a non-cleavable control peptide is required
• Purity level and quantity, and the assay buffer you intend to use

Quotation

Please email your project details to sales@lifetein.com or use our online quotation form. We can help turn a cleavage site into a practical substrate, including the controls that make the assay interpretable.

Further reading

• Synthetic peptides in peptide mapping and proteomics
• Stable isotope labeled peptides
• Fluorescent dye labeling
• Is a spacer required for fluorescent labels?
• Peptide stability and degradation
• Peptide library synthesis

References

1. Schechter I, Berger A. On the size of the active site in proteases. I. Papain. Biochemical and Biophysical Research Communications 1967;27(2):157–162.
2. Nomenclature Committee of the IUBMB. Enzyme Nomenclature, EC 3.4: enzymes acting on peptide bonds (peptidases) — subsite and substrate residue numbering. iubmb.qmul.ac.uk
3. Rawlings ND. A large and accurate collection of peptidase cleavages in the MEROPS database. Database 2009;2009:bap015. doi:10.1093/database/bap015
4. ExPASy PeptideCutter, cleavage specificities of selected enzymes and chemicals, with rules compiled from Keil B, Specificity of Proteolysis (Springer, 1992) and checked against Barrett et al. (1998). web.expasy.org

Sequences supplied by customers are treated as confidential and are never disclosed or reused.

All LifeTein peptides are supplied for laboratory research use only. They are not for human or veterinary use, and not for diagnostic or therapeutic use.