Asparagine peptide lyase are one of the seven groups in whichproteases, also termed proteolytic enzymes, peptidases, or proteinases, are classified according to theircatalytic residue. Thecatalytic mechanism of the asparagine peptide lyases involves anasparagine residue acting as nucleophile to perform a nucleophilic elimination reaction, rather thanhydrolysis, to catalyse the breaking of apeptide bond.^[1]

The existence of this seventh catalytic type of proteases, in which the peptide bond cleavage occurs by self-processing instead of hydrolysis, was demonstrated with the discovery of thecrystal structure of the self-cleaving precursor of the Tsh autotransporter fromE. coli.^[2]

These enzymes are synthesized as precursors or propeptides, which cleave themselves by an autoproteolytic reaction.^[2]

The self-cleaving nature of asparagine peptide lyases contradicts the general definition of an enzyme given that the enzymatic activity destroys the enzyme. However, the self-processing is the action of a proteolytic enzyme, notwithstanding the enzyme is not recoverable from the reaction.^[1]

All the proteolytic activity of the asparagine peptide lyases is only self-cleavages, then no further peptidase activity occurs.^[3]

The main residue of theactive site is the asparagine and there are other residues involved in thecatalytic mechanism, which are different between the different families of asparagine peptide lyases.^[2][4][5]

The cleavage mechanism consists in thecyclization of the asparagine, assisted by other active site residues. In certain conditions, the asparagine cyclic structure nucleophilically attacks itsC-terminal peptide bond to the main chain forming a new bond to create a stablesuccinimide, cleaving itself from the main chain and consequently releasing the two halves of the product.^[6][7]

Noinhibitors are known.^[3]

TheMEROPS protease database includes the following ten families of asparagine peptide lyases, which are included in 6 different clans of proteases.^[3]

Proteolytic enzymes are classified into families based on sequence similarity. Each family includes proteolytic enzymes with homologous sequences and common catalytic type. Clans are groups of proteolytic enzymes families with related structures, where catalytic type is not conserved.

*Not yet included inIUBMB recommendations.

The ten different families of asparagine peptide lyases are distributed in three different types:

• Viral coat proteins
• Autotransporter proteins
• Intein-containing proteins

There are five families of viral coat proteins (N1, N2, N8, N7 and N5), two families of autotransporter proteins (N6 and N4) and three families of intein-containing proteins (N9, N10 and N11).

There are five families ofviral coat proteins in which processing occurs at an asparagine residue. These five families are included in three clans: Clan NA (Families N1, N2 and N8), clan NC (Family N7) and clan NE (Family N5).^[8]

Family N1: The known autolytic cleavage is mediated by thenodavirus endopeptidase, from the C-terminus of the coat protein and only occurs within the assembledvirion.^[9]

Family N2: Includes tetraviruses endopeptidases. The known autolytic cleavage is from the C-terminus of the coat protein. The cleavage occurs during the late stages of virion assembly.^[10]

Family N8: The known autolytic cleavage is inpoliovirus VP0 viral capsid protein into VP2 and Vp4 in the provirion.^[11]

Family N7: The known autolytic cleavage is from theN-terminus of the coat protein.^[12]

Family N5: The known autolytic cleavage is from theN-terminus of the coat protein.^[13]

Autotransporter proteins are outer membrane or secreted proteins found in a broad variety ofGram-negative bacteria. These proteins contain three structural motifs: a signal sequence, a passenger domain located at the N-terminal, and a translocator orautotransporter domain located at the C-terminal, forming abeta barrel structure. These structures promote the protein self-transport.Autotransporter proteins are usually related to virulence functions. This fact, their interaction with host cells and the broad occurrence of autotransporter encoding genes, bring up the possibility to represent therapeutic targets for the design of vaccines against Gram-negative pathogens.^[14]

Two of the families in which theMEROPS database classifies asparagine peptide lyases are autotransporter proteins, families N4 and N6.^[3]

Family N4 includes secreted virulence factors, or autotransporters, from enterobacteria. Their only proteolytic activity is releasing the virulence factor from the precursor, enabling it to be secreted.The active site residues in family N4 asparagine peptide lyases are N1100, Y1227, E1249 and R1282.

Family N6 includes autoprocessingendopeptidases involved in type III protein secretion system, in which autoproteolysis is essential for mediating the secretion of proteins. Type III secretion system secretes proteins directly into host cells by an injectisome, a hollow tubular structure that penetrates into the host cell. Secreted proteins can pass through theinjectisome into the host cell cytoplasm.The conserved active site residue in family N6 asparagine peptide lyases is N263.

Anintein is a protein contained within another protein, theextein. Parasitic DNA infects an intein gene, which encodes anendonuclease. The resultingcDNA (complementary DNA) encodes the extein along with the intein. The intein contains a self-cleaving domain, which has the endonuclease nested within it. The intein domain performs two proteolytic cleavages at its ownN-terminus andC-terminus and releases from the extein, separating it in two fragments. This two fragments are then spliced together and the extein remains as a completely functional protein.

The N-terminal residue of the intein domain must be aserine,threonine orcysteine, and it attacks its preceding peptide bond in order to form an ester or a thioester. The first residue of the second portion of the extein must be a serine, threonine or cysteine as well, and this second nucleophile forms a branched intermediary. The C-terminal residue of the intein domain is always an asparagine, which cyclizes to form a succinimide, cleaving its own peptide bond and releasing the intein from the extein. Finally, in the extein theester orthioester bond is rearranged to form a normal peptide bond.^[15]

There are three known families of intein-containing proteins (N9, N10 and N11) all of them included in the PD clan, which contains proteolytic enzymes of different catalytic types. The tertiary structure has been solved for the intein V type proton ATPase catalytic subunit (Saccharomyces cerevisiae), a member of family N9 and for several inteins from family N10.

• Asparagine
• Lyase
• Protein precursor
• Proteolysis
• Nucleophilic substitution
• Protease
  • cysteine-
  • serine-
  • threonine-
  • aspartic-
  • glutamic-
  • metallo-

• Rawlings ND, Barrett AJ, Bateman A.Asparagine peptide lyases: a seventh catalytic type of proteolytic enzymes. 2011 Nov 4;286(44):38321-8.
• Alan J. Barrett, Neil D. Rawlings, J. Fred (2012).Handbook of Proteolytic Enzymes. Third edition.ISBN 9780123822208
• Guoyao Wu (2013)Amino Acids: Biochemistry and Nutrition.ISBN 9781439861899
• Klaudia Brix, Walter Stöcker (Jan 21, 2014).Proteases: Structure and Function.ISBN 9783709108857
• Jin Zhang, Sohum Mehta, Carsten Schultz (2016).Optical Probes in Biology.ISBN 9781466510128

• International Proteolysis Society
• Protease cut sites graphical interface
• Merops - the peptidase databaseArchived 2006-11-14 at theWayback Machine
• The Proteolysis Map
• TopFIND protease database covering cut sites, substrates and protein termini
• List of proteases and their specificities (see also[1]Archived 2011-04-30 at theWayback Machine)
• Proteases at the U.S. National Library of MedicineMedical Subject Headings (MeSH)

• Enzymes
• Proteases

• Webarchive template wayback links