Kunitz-type protease inhibitor

Kunitz-type protease inhibitors usually are large polypeptides of more than 120 amino acids and will not be discussed here, based on the focus of the review being peptides of fewer than 100 amino acids in length. Searches of publicly available databases yield a few short sequences, which are fragments from larger proteins that have only been partially sequenced. 

Kobayashi H, Sugino D, Terao T. Urinary trypsin inhibitor, a Kunitz-type protease inhibitor, modulates tumor necrosis factor-stimulated activation and translocation of protein kinase C in U937 cells. Int J Oncol 1998 12 95-105. 

APP transcripts have been identified in which exons 7, 8, and 15 are alternatively spliced. Exon 7 encodes 57 amino acids with homology to the Kunitz-type protease inhibitor (KPI) domain (Kitaguchi et al., 1988 Ponte et al., 1988 Tanzi et al., 1988) and exon 8 (Kitaguchi et al., 1988 Lemaire et al., 1989). The A3 peptide is encoded by parts of both exons 16 and 17 (exon and codon numbering based on the APP770 splice variant) (Lemaire et al., 1989) (Pig. 2). In neurons, the predominant isoform is APP695 (Weidemann et al., 1989), which contains exon 15 but excludes exons... 

Schweitz H, Henrteanx C, Bois P, Moinier D, Romey G, Lazdunski M. Calciclndine, a venom peptide of the Kunitz-type protease inhibitor family, is a potent blocker of high-threshold Ca channels with a high affinity for L-type chaimels in cerebellar granule neurons. Proc Natl Acad Sci USA 1994 91(3) 878-82. 

The Bowman-Birk type protease inhibitors represent a class of low molecular weight, cysteine-rich proteins found in legume seeds (.10). The major Bowman-Birk inhibitor in soybean seeds is a double-headed protein capable of blocking the activity of both trypsin and chymotrypsin. This protein represents approximately 4% of the total protein in soybean seeds (1J ). In contrast to the soybean trypsin inhibitor (Kunitz), the "double-headed inhibitor (referred to as BB) is typical of protease inhibitors present in a large number of legume seeds for example, peanuts (12) chick peas (33)5 kidney beans (3JO adzuki beans (33) lima beans (16). 

Based on their sequence homology, disulfide connectivity, and cysteine location within the sequence and chemistry of the reactive site. Pis can be assigned to distinct families, as classified by Laskowski and Kato. Kunitz-type, Bowman—Birk-type, Potato type I and type II, and squash inhibitors are members of these families shown in Table 3. For inhibitors not falling into these classifications more families have been proposed. Pis can also be classified by their target/mode of action. Plants have been found to express Pis that target serine proteinases, cysteine proteinases, aspartic proteinases, and metallo-proteinases. Serine and cysteine protease inhibitors are the best-studied PIs. ... 

Protease nexin 2 is identical to the secreted form of the amyloid precursor protein containing the Kunitz-type serine protease inhibitor domain (128,129), Protease nexin 2 circulates in blood stored as a platelet a-granule protein, which is secreted upon platelet activation (127). Protease nexin 2 inhibits trypsin- and chymotrypsin-like serine proteases and is also a potent inhibitor of factor Xla (126,127,128). Its location in platelets and its ability to inhibit factor Xla suggests a role in regulating blood coagulation for protease nexin 2. 

SPINT2 Serine protease inhibitor, kunitz type 2... 

Solanum (potato) Kunitz PEPs inhibit the aspartic protease cathepsin D as well as trypsin [125-134] and potato cysteine protease inhibitor (PCPI) inhibits a variety of cysteine proteases [185-188]. The crystal structures of soybean trypsin inhibitor (STI) [362, 368] and of Erythrina trypsin inhibitor (ETI) [350] have been determined. The structure of this type of plant Kunitz serine PIP involves a [3-barrel formed by 6 loop-linked antiparallel [3-strands with a lid formed by 6 further loop-linked antiparallel [3-strands. The scissile bond is located within a loop that extends out from the surface of the [3-barrel [350, 362, 368]. 

Bikunin (Bik), a peptide excreted in the urine, is one of the primary inhibitors of the trypsin family of serine proteases. This peptide plays a key role in inflammation and innate immunity because of its two Kunitz-type binding domains [1, 2], Bik suppresses proteolytic activity in a variety of tissues and can also exert localized anti-inflammatory effect [3-5], Inflammation is an important indicator of infection, cancer, and tissue injury in acute and chronic states. In acute inflammation, fluids and plasma components accumulate in the affected tissues due to vascular dilation. Subsequent activation of platelets and increased presence of immune cells occur during repair. Long-standing inflammation may be present before the disorder is identified. Due to its inhibitory role and potential use as an early marker of inflammation, we will review the synthesis, structure, pathophysiology of Bik as well as the various approaches for its measurement in this chapter. 

MEROPS identifies Kunitz-type inhibitors as families 12 and 13, yet they seem to have developed separately in evolutionary history. Families 12 and 13 are referred to as Kunitz-A and Kunitz-P for their origin from animals and plants, respectively. Aprotinin, also known as bovine pancreatic trypsin inhibitor, was one of the first protease inhibitors identified and isolated by Kraut and coworkers in 1930. The 12 family is considerably more homogenous and thought to inhibit only S1 peptidases. In contrast, the 13 family is split into two phylogenetic groups, 13A and 13B, both of which typically inhibit SI peptidases, yet members of the I3A family can also potentially inhibit the A1 family aspartyl proteases and the Cl family cysteine proteases. The first structure of an 13 inhibitor was the... 

Serpins consist of a conserved core of three P-sheets and eight or nine a-helices that act collectively in the inhibitory mechanism. As with the Kazal- and Kunitz-type inhibitors, the mechanism involves a surface exposed loop that is termed the reactive center loop (RCL). The RCL presents a short stretch of polypeptide sequence bearing the Pl-Pl scissile bond. Like other serine protease inhibitor families, the PI residue dominates the thermodynamics that govern the interaction between protease and inhibitor. Exposure of the PI residue to solvent is typically brokered by 15 amino acids N-terminal to the PI residue and 5 residues on the C-terminal prime side of the scissile bond. Evidence for dramatic conformational change in the inhibitory mechanism was first provided by the crystal structure of the cleaved form of ai-antitrypsin (37). In this structure and unlike the native form, the reactive center loop was not solvent exposed but occurred as an additional P-strand within the core of the structure. 

If thrombin and factor Xa, the major activated blood coagulation factors (Fig. 11.6), escape into healthy blood vessels, blood clots will develop and occlude capillaries throughout the body. Direct inhibition of these activated enzymes in the blood flow utilizes serine protease inhibitors, of which there are two common types a Kunitz inhibitor and a serpin. The former possess a Kunitz domain, a convex antiparallel (1-sheet that exactly fits into the concave active site of a serine protease, directly blocking it (lock and key mechanism). By contrast, serpins undergo complex interactions with other proteins to cause conformational changes that bait and block the catalytic action (Fig. 11.12 shows the bait). Table 11.3 fists the major coagulation inhibitors and cofactors, their targets and mechanisms of action. 

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