Kunitz domain

The structure is a disulfide rich alpha+beta fold. Bovine pancreatic trypsin inhibitor is an extensively studied model structure. Certain family members are similar to the tick anticoagulant peptide (TAP, {{UniProt|P17726}}). This is a highly selective inhibitor of factor Xa in the blood coagulation pathways. TAP molecules are highly dipolar,{{cite journal |vauthors=St Charles R, Padmanabhan K, Arni RV, Padmanabhan KP, Tulinsky A | title = Structure of tick anticoagulant peptide at 1.6 A resolution complexed with bovine pancreatic trypsin inhibitor | journal = Protein Sci. | volume = 9 | issue = 2 | pages = 265–72 |date=February 2000 | pmid = 10716178 | pmc = 2144540 | doi = 10.1110/ps.9.2.265 }} and are arranged to form a twisted two-stranded antiparallel beta sheet followed by an alpha helix.{{cite journal |vauthors=Antuch W, Güntert P, Billeter M, Hawthorne T, Grossenbacher H, Wüthrich K | title = NMR solution structure of the recombinant tick anticoagulant protein (rTAP), a factor Xa inhibitor from the tick Ornithodoros moubata | journal = FEBS Lett. | volume = 352 | issue = 2 | pages = 251–7 |date=September 1994 | pmid = 7925983 | doi = 10.1016/0014-5793(94)00941-4| s2cid = 2280234 | doi-access = free }}

The majority of the sequences having this domain belong to the MEROPS inhibitor family I2, clan IB; the Kunitz/bovine pancreatic trypsin inhibitor family, they inhibit proteases of the S1 family{{cite journal |vauthors=Rawlings ND, Barrett AJ, Tolle DP |title=Evolutionary families of peptidase inhibitors |journal=Biochem. J. |volume=378 |issue= Pt 3|pages=705–16 |year=2004 |pmid=14705960 |pmc=1224039 |doi=10.1042/BJ20031825}} and are restricted to the metazoa with a single exception: Amsacta moorei entomopoxvirus, a species of poxvirus. They are short (about 50 to 60 amino acid residues) alpha/beta proteins with few secondary structures. The fold is constrained by three disulfide bonds. The type example for this family is BPTI{{cite journal |doi=10.1016/0022-2836(91)90115-M |vauthors=Wlodawer A, Housset D, Kim KS, Fuchs J, Woodward C |title=Crystal structure of a Y35G mutant of bovine pancreatic trypsin inhibitor |journal=J. Mol. Biol. |volume=220 |issue=3 |pages=757–770 |year=1991 |pmid=1714504}} (or basic protease inhibitor), but the family includes numerous other members,{{cite journal |doi=10.1016/0968-0004(90)90282-G |author=Salier JP |title=Inter-alpha-trypsin inhibitor: emergence of a family within the Kunitz-type protease inhibitor superfamily |journal=Trends Biochem. Sci. |volume=15 |issue=11 |pages=435–439 |year=1990 |pmid=1703675}}{{cite journal |doi=10.1007/BF00160466 |vauthors=Takahashi K, Ikeo K, Gojobori T |title=Evolutionary origin of a Kunitz-type trypsin inhibitor domain inserted in the amyloid beta precursor protein of Alzheimer's disease |journal=J. Mol. Evol. |volume=34 |issue=6 |pages=536–543 |year=1992 |pmid=1593645|s2cid=26698630 }}{{cite journal |doi=10.1073/pnas.91.8.3353 |vauthors=Sprecher CA, Foster DC, Kisiel W, Mathewes S |title=Molecular cloning, expression, and partial characterization of a second human tissue-factor-pathway inhibitor |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=91 |issue=8 |pages=3353–3357 |year=1994 |pmid=8159751 |pmc=43575|bibcode=1994PNAS...91.3353S |doi-access=free }}{{cite journal |doi=10.1002/pro.5560010210 |vauthors=Biemann K, Papayannopoulos IA |title=Amino acid sequence of a protease inhibitor isolated from Sarcophaga bullata determined by mass spectrometry |journal=Protein Sci. |volume=1 |issue=2 |pages=278–288 |year=1992 |pmid=1304909 |pmc=2142190}} such as snake venom basic protease; mammalian inter-alpha-trypsin inhibitors; trypstatin, a rat mast cell inhibitor of trypsin; a domain found in an alternatively spliced form of Alzheimer's amyloid beta-protein; domains at the C-termini of the alpha-1 and alpha-3 chains of type VI and type VII collagens; tissue factor pathway inhibitor precursor; and Kunitz STI protease inhibitor contained in legume seeds.

Kunitz domains are stable as standalone peptides, able to recognise specific protein structures, and also work as competitive protease inhibitors in their free form. These properties have led to attempts at developing biopharmaceutical drugs from Kunitz domains. Candidate domains are selected from molecular libraries containing over 10 million variants with the aid of display techniques like phage display,{{cite journal|last1=Lehmann|first1=A|title=Ecallantide (DX-88), a plasma kallikrein inhibitor for the treatment of hereditary angioedema and the prevention of blood loss in on-pump cardiothoracic surgery|journal=Expert Opinion on Biological Therapy|volume=8|issue=8|pages=1187–99|year=2008|pmid=18613770|doi=10.1517/14712598.8.8.1187|s2cid=72623604}} and can be produced in large scale by genetically engineered organisms.

The first of these drugs to be marketed was the kallikrein inhibitor ecallantide, used for the treatment of hereditary angioedema. It was approved in the United States in 2009.{{cite web |author=Dyax Corp. |year=2009 |title=Full prescibing information Kalbitor |url=http://www.kalbitor.com/pdf/KalbitorFullPrescribingInformation.pdf |access-date=2010-05-02}} Another example is depelestat, an inhibitor of neutrophil elastase that has undergone Phase II clinical trials for the treatment of acute respiratory distress syndrome in 2006/2007{{ClinicalTrialsGov|NCT00455767|Safety and Efficacy Study of Depelestat in Acute Respiratory Distress Syndrome (ARDS) Patients}} and has also been described as a potential inhalable cystic fibrosis treatment.{{cite journal|last1=Attucci|first1=S|last2=Gauthier|first2=A|last3=Korkmaz|first3=B|last4=Delépine|first4=P|last5=Martino|first5=MF|last6=Saudubray|first6=F|last7=Diot|first7=P|last8=Gauthier|first8=F|title=EPI-hNE4, a proteolysis-resistant inhibitor of human neutrophil elastase and potential anti-inflammatory drug for treating cystic fibrosis|journal=The Journal of Pharmacology and Experimental Therapeutics|volume=318|issue=2|pages=803–9|year=2006|pmid=16627747|doi=10.1124/jpet.106.103440|s2cid=1771342}}

Human proteins containing this domain include:

• AMBP, APLP2, APP
• COL6A3, COL7A1, COL28A1
• PAPLN,
• SPINLW1, SPINT1, SPINT2, SPINT3, SPINT4
• TFPI, TFPI2
• WFDC6, WFDC8, WFIKKN1, WFIKKN2

Several plant protease inhibitors of the Kunitz family, the Kunitz-STI protein family, include a beta trefoil fold.{{cite journal |vauthors=Azarkan M, Martinez-Rodriguez S, Buts L, Baeyens-Volant D, Garcia-Pino A | title = The plasticity of the β-trefoil fold constitutes an evolutionary platform for protease inhibition | journal = The Journal of Biological Chemistry | volume = 286 | issue = 51 | pages = 43726–34 |date=Dec 2011 | pmid = 22027836| doi = 10.1074/jbc.M111.291310 | pmc=3243510| doi-access = free }}

{{Reflist|2}}

{{Engineered antibodies}}

{{InterPro content|IPR002223}}

Category:Protease inhibitors

Category:Protein domains

Category:Antibody mimetics