Nomenclature of Factor XI and the Contact System

The contact system (CS) consists of three proteins [factor XII (FXII), prekallikrein (PK) and H-kininogen (HK)] that comprise the initiators of the so-called intrinsic blood coagulation system. The first protein in the hemostatic pathway of intrinsic blood coagulation associated with bleeding is factor XI (FXI), a substrate of FXIIa. This report, which is focused on introducing a consensus nomenclature for the three CS proteins and FXI, will describe each protein. However, this consensus report is not intended to be a review of the field; hence, the statements are not referenced. FXII is a zymogen of a serine protease factor XIIa (FXIIa, EC 3.4.21.38) that becomes an enzyme by autoactivation on artificial and biologic surfaces or by activation by another serine protease such as plasma kallikrein (PKa, EC 3.4.21.34) or plasmin. Plasma PK is a zymogen that is activated to PKa by FXIIa and other proteases. PK and FXI circulate and bind to cells in the intravascular compartment in complex with H-kininogen (HK, high molecular weight kininogen). In addition to its carrier function, HK is a cofactor for FXIIa and PKa activities. It is also the parent protein of the vasoactive peptide bradykinin (BK) that is mostly liberated from HK by PKa. It is well-recognized that deficiencies in CS proteins, although having a critical role in in vitro intrinsic blood coagulation assays, such as the activated partial thromboplastin time (aPTT), are not associated with excess bleeding. FXI deficiency is a bleeding state that is called hemophilia C.

Recently, there is new-found interest in the CS due to the fact that several biologic substances, both physiologic and pathophysiologic, have been recognized as activators of FXII and, in turn, the intrinsic pathway of blood coagulation. These activators may include collagen, DNA, polyphosphates, exosomes, and denatured and misfolded proteins. Additionally, when CS protein expression was genetically ablated, FXII, PK, and HK deficient mice have reduced and/or defective vascular occlusion when compared to wildtype animals in a variety of models of arterial and venous thrombosis. These observations suggested that CS proteins may be targets to prevent thrombosis without altering hemostasis. This novel realization has attracted new interest and investigators to the field. As a result, there is a plethora of old and new abbreviations adulterating the literature by the many new investigators resulting in situations where the reader does not know if the writer is describing a zymogen or active enzyme of a protein. Thus, the Subcommittee on Factor XI and the Contact System agreed at the 62^nd Annual ISTH SCC 2016 meeting in Montpellier, France to form a working group to review the nomenclature and recommend abbreviations that can be utilized in the communications and investigations. The members of the working group are the authors of this publication. This paper is the first comprehensive report and recommendation of abbreviations for all the CS proteins and FXI. Previously, this subcommittee of the SSC had recommended nomenclature for both H- and L-kininogens (low molecular weight kininogen, LK) and their peptide fragments¹. The present report mostly accepts the previous report and builds upon it. The complete recommendations of the abbreviations of for all the CS proteins and FXI are shown in Table 1.

Table 1.

Proposed Nomenclature for Factor XI and the Contact System^*

Preferred protein designation	Preferred abbreviation	Synonyms
H-kininogen	HK	high molecular weight kininogen
		high molecular mass kininogen
		HMW kininogen
		alpha1-cysteine proteinase inhibitor
		alpha1-thiol proteinase inhibitor
		Williams or Fitzgerald Factor
Cleaved H-kininogen	cHK	Cleaved high molecular weight kininogen
		Activated H-kininogen
		Bradykinin-free H-kininogen
		HKa
Bradykinin	BK	Bk
Plasma prekallikrein	PK	prekallikrein
		plasma prokallikrein
		Fletcher Factor
		PPK
Plasma kallikrein	PKa	plasma kallikrein
		PK
Factor XII	FXII	Zymogen Factor XII
		XII, αFXII
		Hageman Factor
Factor XIIa	FXIIa	Activated Factor XII
		Activated Hageman Factor
	αFXIIa	Activated Factor XII with full heavy chain
	βFXIIa	Activated Factor XII with small heavy chain
		Factor XIIa fragment
		Hageman Factor fragment, HFf
Factor XI	FXI	Factor XI
		XI, Rosenthal Factor
Factor XIa	FXIa	Activated FXI

^*Approved at the meeting of the SSC subcommittee on Factor XI and the Contact System in Dublin, Ireland in 2018

Human H-kininogen (HK, Table 1) is a single-chain 120 kilodalton (kDa) protein of six domains that when cleaved by PKa consists of a so-called heavy chain of approximately 65 kDa consisting of domains D1–D3 and a light chain of approximately 56 kDa (C-terminal domain of D4 and D5 and D6, Figure 1A). PKa cleaves HK liberating the vasoactive peptide bradykinin (BK, Table 1) that consists of 9 amino acids, RPPGFSPFR, from the small 21 amino acid domain 4 of HK (D4, Figure 1A). The residual HK is called cleaved HK (cHK). Human HK is one of two kininogens (H-and L-kininogen) that arise from a single gene (Kgn1), but alternative splicing in exon 10 produces two proteins¹. LK has a short light chain and does not bind to PK or FXI and has no procoagulant activity¹. As a result, it is not a “classical” member of the CS family and is not considered further in this report. When HK is cleaved and BK is liberated, the residual BK-free HK has a considerably long half-life in the intravascular compartment compared to the BK peptide that is degraded within minutes. Cleaved HK (cHK, Table 1), as detected on immunoblot or ELISA or by its fragments on mass spectrometry, is now recognized as a relatively stable plasma biomarker of prior CS activation (Figure 1A).

Figure 1.

Cartoon characterization of each of the proteins of the human contact system. Panel A. The structure of H-kininogen (HK) is shown. It is a single chain polypeptide consisting of a heavy chain (H-chain) (domains 1–3) and a light chain (L-chain) (the C-terminal part of domain 4 and domains 5 and 6). Domain 4 consists of the bradykinin (BK) peptide RPPGFSPFR and an additional 12 amino acids at the C-terminus. These latter 12 amino acids of D4 are considered part of the light chain (L-chain) of HK. Domain 5 of HK starts after these 12 amino acids. When cleaved to liberate BK, the residual cleaved H-kininogen (cHK) functions as a biomarker for contact system activation. Panel B. Plasma prekallikrein (PK) is a single chain zymogen. When activated to a serine protease, plasma kallikrein (PKa) has a heavy (H) and light (L) chain held together by an intramolecular disulfide bond. Panel C. Factor (FXII) also is a single chain zymogen. When formed into an active enzyme (αFXIIa), it too has a heavy (H) and light (L) chain held together by an intramolecular disulfide bond. If the heavy chain for αFXIIa is lost, the enzyme loses its ability to attach to surfaces and becomes a fluid-phase activator (βFXIIa) of its substrates. Panel D. Factor XI (FXI) is a dimer of two single chain identical polypeptides. Upon activation to factor XIa (FXIa), the protein consists of four polypeptides, two heavy (H) and two light (L) chains, held together by intramolecular disulfide bonds.

Plasma prekallikrein (PK, Table 1) is an 88 kDa single-chain protein zymogen. When PK is activated to plasma kallikrein (PKa, EC 3.4.21.34), it consists of a N-terminal heavy (50 kDa) and C-terminal light (30 kDa) chains (Figure 1B) linked by a disulfide bond. The enzymatic activity of PKa is contained in its C-terminal light chain (protease domain). The abbreviation PK for plasma prekallikrein was chosen due to its long-standing use and recognition in the field. PK is converted into the serine protease plasma kallikrein (PKa, Table 1) by FXIIa. The major substrates of PKa are zymogen factor XII and HK. Additional potential substrates of PKa include plasminogen, plasma prorenin, complement factor 3 and protease activated receptor-1. The activation of factor XII by PKa leads to additional PK activation and reciprocal amplification of activation of the entire system. Cleavage of HK with the liberation of BK results in activation of the plasma kallikrein/kinin system and stimulation of the vasculature’s BK receptor system. The abbreviation of PKa for plasma kallikrein was agreed upon because it was the least ambiguous and is consistent with the convention of nomenclature of activated forms of the serine proteases of the plasma blood coagulation system. PK is encoded by the KLKB1 gene and is distinct from other kallikreins including the KLK1 gene product tissue kallikrein, which displays kininogenase activity cleaving L-kininogen to generate Lys-bradykinin.

Factor XII (FXII) (Table 1) also is a single-chain 80 kDa protein zymogen (Figure 1C). Structurally it is similar to single-chain urokinase and hepatocyte growth factor activator. Following zymogen activation, the active serine protease FXIIa consists of a heavy chain of 50 kDa linked to a light chain of 30 kDa by a disulfide bond. The serine protease active site of FXIIa is contained in its light chain. When plasma factor XII is activated to a serine protease (FXIIa, EC 3.4.21.38) (Table 1, Figure 1C) by PKa or plasmin it acts on various substrates that include PK, FXII, plasminogen, C1r, C1s, and factor VII. It is recognized that there are at least two forms of activated FXII (FXIIa). αFXIIa (Table 1) retains a fully intact heavy chain (Figure 1C). βFXIIa (Table 1) is also two chain, but it is missing the majority of its heavy chain and has been called FXIIa fragment or Hageman Factor fragment (Figure 1C). βFXIIa has potent enzymatic activity and is a fluid-phase activator of PK and plasminogen. The subcommittee recognizes that much work currently is being performed on factor XII as a single-chain protein with detectable enzymatic activity in the absence and presence of surfaces. This area is under active investigation and it was agreed to withhold recommendations of new abbreviation designations for future nomenclature revisions.

Factor XI (FXI) (Table 1) is a two-chain 160 kDa protein consisting of two identical 80 kDa chains held together by a single intramolecular disulfide bond, although the disulfide is not essential for factor XI to be a dimer (Figure 1D). Structurally each FXI subunit is similar to PK. When FXI is activated to an active serine protease (FXIa, EC 3.4.21.27, Table 1) by FXIIa or thrombin, each monomer is converted to a heavy chain of 50 kDa and a light chain of 30 kDa (Figure 1D) that remain connected by a disulfide bond. FXIa activates zymogen blood coagulation factor IX in the absence of any known protein cofactor.

The nomenclature proposal contained in this document was repeatedly circulated to the working group members between 2016 and 2019. After the 2017 ISTH meeting in Berlin, the subcommittee circulated the nomenclature proposal (Table 1) to the more than 200 contributors of the Factor XI and the Contact System Subcommittee. The proposal was adopted unanimously by the Subcommittee on Factor XI and the Contact System at its business meeting during the SSC meeting in Dublin, Ireland in 2018.