When activation of plasma kallikrein occurs as a step in the contact activation system (CAS), it is catalyzed by activated factor XIIa (FXIIa), itself generated by the proteolytic cleavage of the FXII zymogen (Hageman factor, encoded by F12) (Samuel M et al., 1992; Shamanaev A, Ivanov I et al., 2022) upon binding to negatively charged surfaces, e.g., biological products, such as DNA, RNA, phospholipids, collagen, extracellular vesicles or artificial entities like kaolin, lipid nanoparticles, silica, celite, etc.(reviewed by Jaffer IH et al., 2015; Schmaier AH 2016; Rangaswamy C et al., 2021). Surface binding induces a conformational change that exposes the catalytic domain, allowing autoactivation or plasma kallikrein-mediated cleavage of FXII to FXIIa, which consists of a heavy chain (20-372) and a light chain (373-615) held together by disulfide bonds (Samuel M et al. 1992; Shamanaev A, Ivanov I et al. 2022; reviewed by Shamanaev A, Litvak M, Gailani D 2022). The serine protease activity of FXIIa then activates its substrates, including plasma prekallikrein (PK) (Ivanov I et al. 2017; Shamanaev A, Ivanov I et al. 2022). Activated PKa, in turn, reciprocally activates more FXII, establishing a positive, amplifying, feedback loop (de Maat S et al., 2019; reviewed in Long AT et al., 2016; Schmaier AH 2016). While these reciprocal cleavage reactions can occur in solution, they are significantly accelerated when FXII and PK bind to a surface in the presence of Zn²⁺.

High-molecular-weight kininogen (HK, KNG1(19 644)) is a plasma protein that facilitates the interaction and reciprocal activation of prekallikrein and FXII on surfaces (Thompson RE et al., 1979; Renné T et al., 2002). Domain 6 (D6) of HK mediates binding to PK (Thompson RE et al., 1979; Tait JF & Fujikawa K, 1987), while domain 5 binds Zn²⁺ and negatively charged surfaces, such as glycosaminoglycans (GAGs) (DeLa Cadena RA & Colman RW, 1992; Herwald H et al., 2001). In addition to GAGs, HK can bind to cell surface receptors, complement C1q binding protein (C1QBP, also known as globular C1q receptor or gC1qR), urokinase plasminogen activator receptor (uPAR, encoded by the PLAUR gene), and cytokeratin 1 (CK1, encoded by the KRT1 gene) (Hasan A et al, 1998, Colman RW et. al, 1997, Joseph K et al. 1996, 2001; Herwald H et al. 1996; Mahdi F et al., 2002; Kaira BG et al., 2020; Stavrou EX et al., 2018). For instance, HK binding to C1QBP facilitates the assembly of HK, PK, and FXII into higher order ternary complexes, where FXII and PK reciprocally activate each other in a Zn²⁺ dependent manner (Joseph K et al., 1996, 1999, 2001, 2004; Kaira BG et al., 2020).

Physiologically, the kallikrein kinin system is primarily activated on endothelial cell surfaces (Lin Y et al., 1997; Motta G et al., 1998; Joseph K et al., 2001; Mahdi F et al., 2003). Although FXII binds endothelial cells, the binding alone is insufficient for activation. FXII can remain bound to cultured endothelial cells for up to 2 hours without being activated; the presence of HK and PK is required to trigger rapid conversion to FXIIa on endothelial surfaces (Merkulova AA et al., 2023). Other cell types may contribute to KKS activation by exposing negatively charged surfaces such as polyphosphate chains in activated platelets (Verhoef JJF et al., 2017), phosphatidylserine on apoptotic T-lymphoblast cells (Yang A et al., 2017) or expressing relevant receptors (reviewed by Schmaier AH, 2016).

Importantly, PK activation on endothelial cells can also occur via S28 serine protease prolylcarboxypeptidase (PRCP) independently of FXIIa (reviewed by Wang J et al., 2014). PRCP was initially characterized as an endopeptidase because it cleaves C-terminal Pro-X bonds, as in bradykinin, generating des-Arg⁹-bradykinin. It also metabolizes the C-terminal Pro-Val bond of α-melanocyte–stimulating hormone (α-MSH1–13) (Wallingford N et al., 2009). Using classical biochemical approaches, a serine protease that activated PK when bound to HK on cultured endothelial cells was isolated and identified as PRCP (Shariat-Madar Z et al., 2002). Recombinant PRCP exhibits identical properties to the native enzyme isolated from plasma (Shariat-Madar Z et al., 2004). Finally, downregulation of PRCP by siRNA and upregulation by PRCP transfection directly regulate PK activation on endothelial cells (Shariat-Madar Z et al., 2005; Merkulova AA et al., 2023).

Beyond its role as an endopeptidase and PK activator, PRCP has defined physiological activities. PRCP gene-trap mice (Prcp gt/gt) are hypertensive and prothrombotic in injured arteries (Adams GN et al., 2011). PRCP exhibits growth factor properties, stimulating endothelial cell growth and proliferation (Adams GN et al., 2013). Following ischemia-reperfusion injury, Prcp gt/gt mice are protected from neointimal cell growth and proliferation, whereas PRCP promotes vascular repair and neoangiogenesis after vessel wall injury.

Activated FXIIa and PKa coordinate multiple pathways. Plasma kallikrein mediated cleavage of complement component C3 (Irmscher S et al., 2018) and FXIIa mediated cleavage of C1 trigger the complement activation (Ghebrehiwet B et al., 1981). FXIIa also promotes fibrin clot formation by activating FXI on the cell surface (Cheng Q et al., 2010), while plasma kallikrein directly binds and activates FIX (Visser M et al., 2020; Kearney KJ et al., 2021). In addition, FXIIa and PKa contribute to fibrinolysis by converting plasminogen to plasmin and to the renin-angiotensin system by converting the inactive precursor prorenin into active renin (Colman RW 1969; Goldsmith GH Jr et al., 1978; Kluft C et al., 1979; reviewed by Renne T et al., 2012; Maas C & Renne T., 2018; Schmaier AH 2016; Long AT et al., 2016; Shamanaev A, Litvak M et al., 2023; Motta G et al., 2023).

This Reactome module covers the key activation events of the plasma KKS pathway, including: