Factor VIIa: Difference between revisions

{{STRUCTURE_1dan|  PDB=1dan  | SIZE=400| SCENE=Factor_VIIa/Cv/1 |right|CAPTION=Human factor VIIa heavy chain (pink) and light chain (cyan) complex with soluble tissue factor (dark red, dark green) and a peptide inhibitor, cacodylate, Cl- and Ca+2 ions [[1dan]] }}

[[Factor VIIa]] (FVIIa) is a single chain trypsin-like serine protease [http://en.wikipedia.org/wiki/Serine_protease](EC 3.4.21.21) of 406 residues. The FVII[http://en.wikipedia.org/wiki/Factor_VIIa] zymogen is a glycoprotein consisting of an amino-terminal (N-linked) γ-carboxyglutamic acid (Gla)[http://en.wikipedia.org/wiki/Carboxyglutamic_acid]domain followed by two epidermal growth factor-like (EGF1 and EGF2) domains, a short linker peptide, and a carboxy terminal serine protease domain<ref>PMID:10430872</ref>. The active form, FVIIa, is generated by a specific cleavage of a peptide bond between Arg152 and Ile153 at the end of the linker peptide by either factor Xa (FXa) or thrombin (IIa). This cleavage generates an N-terminal light chain of 152 residues linked to a heavy chain of 254 residues by a disulfide bridge <ref>PMID:6778860</ref>. Following cleavage the newly formed N-terminal inserts itself into a cavity, or the activation pocket, forming a salt bridge with Asp343 (Asp194 trypsin numbering).Formation of this salt bridge allows for the maturation of FVIIa to its active form. The images at the left and at the right correspond to one representative Factor VIIa, ''i.e.'' the crystal structure of Human factor VIIa complex with tissue factor and a peptide inhibitor ([[1dan]]).

==FVIIa mechanism==

FVIIa alone shows very little proteolytic activity and only becomes fully active when complexed to its obligatory cofactor, tissue factor (TF) and cations, mainly Ca++. TF, located in the vessel wall, is exposed to circulating FVIIa upon injury or some type of stimulus and forms a TF-FVIIa complex. A unique property of TF-FVIIa among other coagulation enzyme complexes is that phospholipids are not an obligate requirement for the assembly of the complex. However, the activity of the complex towards its substrates (FIX and FX) requires a lipid surface which is provided by the membrane-anchored TF. The TF-phospholipid complex enhances the efficiency (kcat/Km) of FVIIa-catalyzed reactions by the 10^10-fold. There are four distinct steps that are required for the full activity of the TF-FVIIa complex: 1) proteolytic activation of single-chained FVII to two-chain disulfide bridged FVIIa 2) binding of cations 3) interaction of TF with FVIIa 4) acidic-membrane association and proper orientation of substrate<ref>PMID:1537862</ref><ref>PMID:18640965</ref>.  

[[Image:FVIIa-TF_FX.jpg|left]]

===Allosteric activation===

   

[[Image:bajaj struc..jpg|right]]

====TF interaction - entropy trap====

The Gla domain of FVIIa binds to the C-domain of TF. The interaction is mainly hydrophobic termed the “hydrophobic stack”. EGF1 domain packs into a groove formed by the two modules of TF. This interface is the largest and  contributes the most energetically in binding of the cofactor to the protease domain. EGF2 and the catalytic domain interact with the N-domain of TF. Two “lock and key” interactions are observed. One is the side chain of Phe50 of TF which is trapped in the pocket formed by the end of EGF2 domain of FVIIa. Second, the side chain Met306 of FVIIa is enclosed by TF residues Arg74, Phe76, Glu92, Leu94. This Met306 residue is responsible for the thermodynamic coupling to the active site and is unique to FVIIa. Mutation of this residue results in the failure of TF to decrease the dissociation rate of the enzyme from the cofactor. Cofactor interactions, specifically through Met306 lead to subtle changes which then influence the position of Asp331. Asp331 is the specificity-determining residue in the binding pocket. Constraint and stabilization promote formation of a hydrogen bond between the amide of Arg315(170C) and carbonyl of Gly372(223). When bound to TF the activation region of FVIIa contains a large number of hydrogen bonds between the main chain and side chain atoms. At least 11 water molecules are identified in the catalytic domain region. An interesting observation is that the center of this region contains water, in contrast to the hydrophobic interactions in the other two interface regions. These hydrophilic interactions may be more efficient at mediating TF affinity and substantial conformational changes to induce activity of FVIIa. Therefore TF and cations are obligatory cofactors in the allosteric regulation of FVIIa activity by stabilizing the disordered, flexible FVIIa and restraining the enzyme for catalytic activity.

   

===Kinetics of action===

In general serine proteases have a three-step kinetic mechanim: 1) formation of an enzyme-substrate (E-S) complex(k+1, k-1), 2) acylation of the active site serine(k2), and 3) hydrolysis of the acylenzyme intermediate (k3).  

   

[[Image:kin mech1.jpg|left]]

===Catalytic domain===