Venom metalloproteinase (M12B) family

General Activity
Metalloproteinases, or metalloproteases (MPs), are any protease whose catalytic mechanism involves a metal. Most metalloproteases require zinc, but some use cobalt. They are widely found across the animal kingdom, regrouping more than 50 families classified to date.

In venom, metalloproteinases are mostly found in snakes, although a few have been found in scorpions, insects and spiders. Snake venom metalloproteinases (SVMPs) are present in large quantities in viperid and in some elapid and colubrid venoms. A number of biological activities have been attributed to them including hemorrhage, edema, inflammation, hypotension and necrosis. They act by interfering at different levels of hemostasis. The metalloproteinase (domain) (see below) is responsible for degradation of matrix proteins, which thus results in the weakening of the endothelial cell attachment, and also for digestion of large proteins involved in hemostasis, such as fibrinogen.

Two subfamilies of SVMPs (P-II and P-III) also contain a disintegrin (Dis) domain (see the structure topic below). Disintegrins are non-enzymatic cysteine-rich peptides which have, among other properties, high affinity for the platelet alpha-IIb/beta-3 integrin and inhibit platelet aggregation by blocking the binding of fibrinogen to this receptor.

Nomenclature
SVMPs are part of the M12 subfamily of metalloproteinases, also termed reprolysins, which belong to the family of zinc-dependent metalloproteinases. All of them contain characteristic zinc-binding regions (consensus sequence HEXXHXXGXXH) and the Met-turn. SVMPs are synthesized as multidomain precursors, whose enzymatic activity is inhibited by a Cys-switch mechanism. They contain multiple disulfide bonds, which stabilize their structure and functional integrity in the oxidative extracellular environment.
They are classified into three major classes, P-I, P-II and P-III, depending on their general domain organization. P-I SVMPs (20-30 kDa) are composed of a single catalytic metalloproteinase (MP) domain. P-II SVMPs (30-60 kDa) are composed of one MP and one disintegrin domain. In addition to these two domains, P-III SVMPs (60-100 kDa) also contain a Cys-rich domain. P-II and P-III SVMPs are divided into several subclasses, based on proteolytic processing and dimerization. P-II SVMPs undergo proteolytic processing, being split into an MP domain and a non-enzymatic disintegrin, and in a similar manner some P-III SVMPs are processed into a disintegrin-like/Cys-rich containing protein, while the MP domain is degraded. For a schematic representation of these 3 classes, see Fig.1.

It is noteworthy that a former P-IV class has been described. It consists of a P-III structure plus a C-type lectin-like subunit composed of two chains disulfide-linked to one another and to the Cys-rich domain. It has been re-classified into a P-IIId subclass, since so far, no P-IV mRNA transcript has been found. The P-IIId subclass of SVMPs is therefore considered to be a posttranslationally modified P-III SVMP.

P-I

P-Ia

				P-IIa
				P-IIb
P-II				P-IIc
				P-IId
				P-IIe
				All

				P-IIIa
P-III				P-IIIb
				P-IIIc
				P-IIId
				All

Figure 1: Schematic representation of SVMP classes. Sig, signal peptide; Pro, propeptide; S, spacer region; Dis, disintegrin; Dis-l, disintegrin-like region; Cys-r, cysteine-rich region; Lec, lectin. UniProt logo is clickable and gives access to UniProtKB/Swiss-Prot entries that belong to the indicated subfamilies (classification from Fox and Serrano, 2008).

Activity:myotoxin Several SVMPs have been described to induce myotoxicity (See entries.

Activity:hemorrhagic toxin SVMPs cause hemorrhage by disturbing interactions between endothelial cells and the basement membrane, as a consequence of degradation of the basement membrane protein components fibronectin, laminin, type IV collagen and nidogen (entactin), together with endothelial cell membrane proteins integrins and cadherins (Sajevic et al., 2011 and references therein).

Activity:platelet aggregation inhibiting toxin P-I and P-III SVMPs have been shown to interfere with von Willebrand factor (vWF)-dependent platelet activation by binding to and/or hydrolyzing vWF and GPIb receptor (see Platelet role in hemostasis for description). P-II SVMPs are mostly proteolytically processed to their separate MP and Dis domains (See Disintegrins for Dis description). Non-processed P-II class SVMPs can, like disintegrins, inhibit platelet aggregation as they possess an RGD sequence. P-III class SVMPs possess a Dis-like domain with a D/ECD, instead of the usual RGD, motif and are also able to inhibit collagen-induced platelet aggregation. Inhibition of the interaction between collagen and alpha-2/beta-1 integrin probably results from a combination of cleavage and physical blocking. All three domains of P-III SVMPs appear to possess motifs that could interact with alpha-2/beta-1 integrin and/or its ligands. SVMPs also inhibit collagen-induced platelet aggregation by cleaving or binding to the platelet glycoprotein VI (GP6).
SVMPs also interact with proteins containing vWF-A1 domains, via their Cys-rich domain. This suggests an additional mechanism by which SVMPs interfere with platelet adhesion, which may promote the hemorrhage caused by SVMPs (Sajevic et al., 2011 and references therein).

Activity: platelet aggregation activating toxin Only the metalloprotease-disintegrin alborhagin has been shown to induce platelet aggregation. This protein selectively targets the platelet glycoprotein VI (GP6), and induces GP6-dependent signaling and platelet aggregation (Andrews et al., 2001). It is noteworthy that this protein induces metalloprotease-mediated shedding of GP6 from platelets, and that the mechanism of shedding involves activation of endogenous platelet metalloproteases and not a direct cleavage of GP6 (Wijeyewickrema et al., 2007).

Activity: blood coagulation cascade activating toxin Two types of blood coagulation cascade activators have been isolated from snake venoms, the prothrombin and the factor-X activators.
Prothrombin activators cleave the Arg320-Ile321 bond in human prothrombin to produce meizothrombin, which is then converted to alpha-thrombin by autolysis. They are classified in two groups, those that function without cofactors, and those that function with calcium. The second group belongs exclusively to the P-IIId class of SVMPs (See entries.
Factor X activators belong to P-IIId class SVMPs. With their C-type lectin-like subunits, they bind to the Gla-domain of factor X in a calcium-dependent manner. Then, with their catalytic MP domain, they cleave the heavy chain of FX at Arg52-Ile53, the same site as physiological activators (See entries.

Activity: fibrin(ogen)olytic toxin Numerous fibrin(ogen)olytic enzymes, belonging to all three structural classes of SVMPs have been described. These enzymes cleave the Aalpha- and Bbeta-chain of fibrinogen, usually having a preference for the Aalpha-, or being exclusively for the Aalpha-chain (Sajevic et al., 2011 and references therein).

Activity: complement system impairing toxin Several SVMPs have been described to impair the complement system (See entries.