Coagulation and the Serine Protease Cascade

The serine protease cascade — clotting and beyond

Serine proteases constitute one of the most functionally diverse enzyme families in mammalian biology. Beyond classical blood coagulation, the serine protease cascade architecture extends into innate immunity via the lectin complement pathway, inflammation through the kallikrein-kinin system, extracellular matrix remodeling, epithelial barrier maintenance, and cell-surface signaling. Each reaction typically proceeds through zymogen activation, where proteolytic cleavage converts an inactive precursor into an active enzyme, enabling signal amplification and tight temporal control.

Triple Point Biologics has supplied rabbit polyclonal antibodies for serine protease research since 1994, with validated reactivity for Western blot and immunohistochemistry. The catalog spans membrane-anchored, secreted, and intracellular family members, as well as their cognate serpin inhibitors, enabling paired detection of protease and inhibitor in the same experimental system.

Von Willebrand Factor (vWF) and ADAMTS13 — the matched substrate/protease pair

Von Willebrand factor is a multimeric glycoprotein synthesized in endothelial cells and megakaryocytes that mediates platelet adhesion to sites of vascular injury. Following secretion, ultra-large vWF multimers are cleaved by ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin motifs, member 13) at the Tyr1605-Met1606 bond within the vWF A2 domain. Shear stress unfolds the A2 domain and exposes the scissile bond, coupling ADAMTS13 activity to hemodynamic conditions. Deficiency or inhibition of ADAMTS13 results in persistence of ultra-large vWF multimers, spontaneous platelet aggregation, and thrombotic thrombocytopenic purpura.

Our Anti-vWF Antibody recognizes both full-length and cleaved forms in plasma and tissue, while the Anti-ADAMTS-13 Antibody detects the metalloproteinase in endothelial lysates and conditioned media. These reagents are commonly used in tandem to assess vWF processing in models of thrombotic microangiopathy and preeclampsia.

What is vWF and why is ADAMTS13 important for its regulation?

Von Willebrand factor (vWF) is a large multimeric glycoprotein that mediates platelet adhesion at sites of endothelial injury. After secretion, ultra-large vWF multimers circulate briefly before being cleaved by ADAMTS13, a plasma metalloproteinase. ADAMTS13 cleaves vWF specifically in the A2 domain under high shear stress, reducing multimer size and preventing spontaneous platelet aggregation. Loss of ADAMTS13 activity—through genetic deficiency or autoantibodies—causes accumulation of ultra-large vWF multimers and leads to thrombotic thrombocytopenic purpura (TTP), characterized by widespread microvascular thrombosis.

Tissue kallikrein 1 (KLK1) and the kinin-generating system

The kallikrein-kinin system regulates vascular tone, inflammation, and pain signaling. Tissue kallikrein 1 (KLK1) cleaves low-molecular-weight kininogen to release lysyl-bradykinin (kallidin), which is converted to bradykinin by aminopeptidases. Bradykinin binds B2 receptors on endothelial cells, stimulating nitric oxide and prostacyclin release, and causing vasodilation and increased vascular permeability. Plasma kallikrein, a distinct enzyme encoded by KLKB1, acts on high-molecular-weight kininogen within the contact activation (intrinsic coagulation) pathway.

In addition to KLK1, the human genome encodes 14 additional kallikrein-related peptidases (KLK2–KLK15), many of which are expressed in hormone-responsive tissues and implicated in cancer. KLK2 and KLK3 (prostate-specific antigen, PSA) are androgen-regulated serine proteases in seminal fluid. KLK5, KLK7, and KLK14 form a proteolytic cascade in the stratum corneum that regulates desquamation. Triple Point Biologics offers antibodies to all 15 kallikreins: KLK1, KLK2, KLK3, KLK4, KLK5, KLK6, KLK7, KLK8, KLK9, KLK10, KLK11, KLK12, KLK13, KLK14, and KLK15, enabling comparative profiling across tissues and disease states.

What is the kallikrein-kinin system?

The kallikrein-kinin system is a proteolytic cascade that generates bradykinin and related kinins, which are potent vasoactive peptides. Tissue kallikrein (KLK1) cleaves kininogen to release kallidin, which is converted to bradykinin. Plasma kallikrein, part of the contact activation pathway, also liberates bradykinin from high-molecular-weight kininogen. Bradykinin binds to B2 receptors on endothelial cells, causing vasodilation, increased vascular permeability, and pain. This system plays key roles in blood pressure regulation, inflammatory responses, and has been implicated in hereditary angioedema and septic shock pathophysiology.

MASP-1 and MASP-2 — lectin complement and prothrombin cross-talk

Mannose-binding lectin-associated serine proteases (MASPs) are the effector arm of the lectin pathway of complement activation. When mannose-binding lectin (MBL) or ficolins recognize pathogen-associated carbohydrate patterns, MASP-2 cleaves C4 and C2, forming the C3 convertase C4b2a and initiating the complement cascade. MASP-1 was initially considered a cofactor but is now known to activate MASP-2, cleave fibrinogen, and directly cleave prothrombin to thrombin independently of the canonical coagulation cascade, establishing a direct link between innate immunity and thrombosis.

MASP-3, encoded by alternative splicing of the MASP1 gene, lacks complement activity but cleaves pro-factor D in the alternative pathway. Our Anti-MASP-1, Anti-MASP-2, and Anti-MASP-3 Antibodies distinguish these isoforms in serum and tissue extracts, supporting work on lectin pathway dysregulation in ischemia-reperfusion injury and atypical hemolytic uremic syndrome.

What's the difference between MASP-1 and MASP-2?

MASP-1 and MASP-2 are both mannose-binding lectin-associated serine proteases that activate the lectin complement pathway. MASP-2 is the primary C4 and C2 cleaving enzyme, directly forming the C3 convertase. MASP-1 functions upstream by activating MASP-2 through limited proteolysis and also has broader substrate specificity—it can cleave fibrinogen and prothrombin, linking complement activation to coagulation. MASP-1 and MASP-3 are splice variants of the same gene, whereas MASP-2 is encoded separately. Functionally, MASP-2 deficiency abrogates lectin pathway activity, while MASP-1 deficiency reduces but does not eliminate it.

TMPRSS family — type-II transmembrane serine proteases

The type-II transmembrane serine protease (TTSP) family comprises more than 20 enzymes anchored in the plasma membrane with extracellular catalytic domains. TMPRSS2 gained prominence during the COVID-19 pandemic as the protease that cleaves SARS-CoV-2 spike protein at the S1/S2 boundary, priming viral entry into airway epithelial cells. Beyond viral pathogenesis, TMPRSS2 is androgen-regulated and frequently fused to ETS transcription factors in prostate cancer (TMPRSS2-ERG fusions).

TMPRSS3 is required for auditory function; loss-of-function mutations cause congenital deafness (DFNB8/10). TMPRSS4 promotes epithelial-mesenchymal transition and metastasis in colorectal and gastric cancers. TMPRSS5 (spinesin) is expressed in the inner ear and may cooperate with TMPRSS3. The TMPRSS11 subfamily (11A, 11B, 11D, 11E, 11F) is enriched in keratinocytes and mucosa and implicated in barrier integrity and immune signaling. We supply antibodies to TMPRSS2, TMPRSS3, TMPRSS4, TMPRSS5, TMPRSS11A, 11B, 11D, 11E, 11F, TMPRSS12, and TMPRSS13.

What does TMPRSS2 do beyond SARS-CoV-2 entry?

TMPRSS2 is an androgen-regulated, type-II transmembrane serine protease expressed in prostate, lung, and gastrointestinal epithelia. In the prostate, TMPRSS2 is frequently fused to ERG or other ETS family oncogenes in gene rearrangements that drive prostate cancer. TMPRSS2 also activates epithelial sodium channels (ENaC), influencing fluid balance in the airway and kidney. In addition to priming SARS-CoV-2 spike, TMPRSS2 cleaves influenza hemagglutinin and other viral fusion proteins, serving as a broad viral entry factor. Its physiological substrates and roles in epithelial homeostasis remain active areas of investigation.

Matriptases (TMPRSS-related) in epithelial barrier biology

Matriptases are closely related TTSPs with essential roles in epithelial development and barrier function. Matriptase-1 (ST14, also known as epithin) is required for epidermal barrier formation; St14-null mice die perinatally with severe dehydration due to defective stratum corneum. Matriptase-1 activates prostasin (CAP1/PRSS8), another GPI-anchored serine protease, in a cascade that regulates ENaC-mediated sodium transport and desquamation. Loss-of-function mutations in human ST14 cause autosomal recessive ichthyosis with hypotrichosis.

Matriptase-2 (TMPRSS6) is a negative regulator of hepcidin expression. It cleaves hemojuvelin, a BMP co-receptor on hepatocytes, thereby suppressing hepcidin transcription and increasing iron absorption. Mutations in TMPRSS6 cause iron-refractory iron deficiency anemia. Matriptase-3 (TMPRSS7) is expressed in the inner ear and may contribute to auditory processing. Our Anti-Matriptase-1, Anti-Matriptase-2, and Anti-Matriptase-3 Antibodies enable detection of these proteases in epithelial tissues, liver, and inner ear sections.

Matriptase-1 activity is tightly controlled by HAI-1 (hepatocyte growth factor activator inhibitor-1, encoded by SPINT1) and HAI-2 (SPINT2). Genetic ablation of either HAI in mice phenocopies matriptase-1 overactivation, underscoring the importance of protease-inhibitor balance. We offer Anti-HAI-1 and Anti-HAI-2 Antibodies for co-detection studies.

HTRA proteases — quality control and cardiovascular relevance

The high-temperature requirement A (HTRA) family comprises four mammalian members (HTRA1–4) with both protease and chaperone activities. HTRA1 is secreted into the extracellular space and cleaves a broad range of matrix and signaling proteins, including fibronectin, aggrecan, and TGF-β family members. Gain-of-function mutations and promoter polymorphisms in HTRA1 are associated with cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL) and age-related macular degeneration, respectively.

HTRA2 (Omi) resides in the mitochondrial intermembrane space and is released into the cytosol during apoptosis, where it cleaves inhibitor of apoptosis proteins (IAPs) and promotes caspase activation. HTRA2 also degrades misfolded proteins in the mitochondria, and loss of function in mice causes progressive neurodegeneration. HTRA3 and HTRA4 are predominantly expressed in placenta and may regulate trophoblast invasion; dysregulation has been implicated in preeclampsia pathogenesis. Our Anti-HTRA-1, Anti-HTRA-2, Anti-HTRA-3, and Anti-HTRA-4 Antibodies are validated for immunoblot and IHC in cardiovascular, neural, and placental tissues.

Corin — cardiac ANP processing

Corin is a type-II transmembrane serine protease expressed on cardiomyocytes that converts pro-atrial natriuretic peptide (pro-ANP) into active ANP. ANP regulates blood volume and pressure by promoting natriuresis, diuresis, and vasodilation. Corin-deficient mice develop salt-sensitive hypertension and cardiac hypertrophy, and reduced plasma corin and ANP levels have been reported in human heart failure. Corin is also expressed in pregnant uterus, where it may activate local pro-ANP to regulate decidual spiral artery remodeling. Our Anti-Corin Antibody detects both the membrane-bound zymogen and shed ectodomain in cardiac lysates and plasma.

Serpin inhibitors and matched-pair antibody panels

Serine protease inhibitors (serpins) form covalent, suicide complexes with their target proteases through a conserved mechanism involving an exposed reactive center loop. The serpin superfamily includes more than 30 human members. SERPINA1 (alpha-1-antitrypsin) inhibits neutrophil elastase; deficiency causes emphysema and liver cirrhosis due to misfolded protein aggregation. SERPINA3 (alpha-1-antichymotrypsin) inhibits cathepsin G and mast cell chymase. SERPINA4 (kallistatin) inhibits tissue kallikrein and has anti-angiogenic activity. SERPINA5 (protein C inhibitor) regulates activated protein C, thrombin, and kallikrein.

For experiments examining protease-inhibitor dynamics, we supply antibodies to SERPINA1, SERPINA2, SERPINA3, SERPINA4, and SERPINA5, enabling co-immunoprecipitation and Western blot analysis of protease-serpin complexes. These reagents are particularly useful in studies of neutrophil extracellular traps (NETs), where protease release and inhibitor consumption alter the local proteolytic balance.

TPB's serine protease and serpin antibody catalog

Since 1994, Triple Point Biologics has maintained one of the most extensive rabbit polyclonal antibody panels for serine proteases and their inhibitors. Each antibody is raised against recombinant protein or synthetic peptide immunogen and affinity-purified. Datasheets include immunogen sequence, predicted species cross-reactivity based on homology, and representative blot or IHC images where available. All antibodies are research-use-only reagents supplied in PBS with preservative. Custom aliquoting and conjugation services are available on request. For laboratories profiling protease expression across disease models, bulk or matched-pair orders can be arranged through our technical support team.