Sandbox Reserved 596
From Proteopedia
| This Sandbox is Reserved from Feb 1, 2013, through May 10, 2013 for use in the course "Biochemistry" taught by Irma Santoro at the Reinhardt University. This reservation includes Sandbox Reserved 591 through Sandbox Reserved 599. |
To get started:
More help: Help:Editing |
Protein Z
Table of Contents
Background
The human body maintains the many aspects of homeostasis, or biological equilibrium, through multiple physiological pathways. Hemostatic mechanisms have evolved over 400 million years to protect against the ever-present danger of fatal hemorrhage. [1] When inflicted by a wound resulting in internal or external bleeding, one of the two converging coagulation cascade branches is triggered to maintain hemostasis. If the wound is just an epithelial surface wound, the body reacts with the intrinsic pathway but if the wound is more traumatic, resulting in more extensive vascular and tissue damage, the extrinsic branch is used. This coagulation pathway is known as a cascade because the product of the previous reaction acts as an enzyme for the following reaction, a flow that can be attributed to the specificity of each enzyme along with positive and negative feedback loops to further control the directionality. [2] The coagulation cascade involves many coagulation factors that act as serine proteases with active sites centered around their serine residues. These serine protease coagulation factors, like factors VII, IX, X, and protein C are vitamin K-dependent proteins, meaning their γ-carboxyglutamic acid residue (Gla) construction within the liver requires fat-soluble vitamin k. Factor Xa (FXa), an essential enzyme found at the intersection of the two coagulation pathways, is formed on platelet membranes through the interaction of factor IXa, complexed with its cofactor VIIIa, on factor X. FXa complexed with its cofactor Va promotes thrombin production from the cleaved zymogen prothrombin. The cascade continues to flow towards its final processes of prothrombin being cleaved into thrombin and thrombin then cleaving fibrinogen to form a meshwork of fibrin until a blood clot stops the bleeding. To ensure that a clot does not form anywhere other than the injury site, any FXa that dissociates from the membrane must be inhibited. FXa is regulated by two serine protease inhibitors (serpins), antithrombin and protein z-dependent inhibitor (ZPI) that are inactive to FXa until paired with their cofactors heparin and protein z (PZ) respectively. [3]
Structure
|
PZ is a single-chain protein with a N-terminal domain rich in γ-carboxyglutamic acids (Gla), two epidermal growth factor-like (EGF) domains and a C-terminal serine protease-like domain and has a molecular weight of 62 kDa. PZ genetically and structurally mimics other factors of the coagulation cascade but it is not a catalyticaly active enzyme. PZ is not like the other vitamin k-dependent coagulation factors because it does not have an active center, thus it lacks the Ser (S195) and His (H57) of the catalytic triad needed for the active site of serine proteases, like the coagulation factors VII, IX, X, and protein C. PZ's 360 amino acid sequence is N-terminally analogous to the serine protease coagulation factors, whose similarly composed genes are found clustered together along with PROZ on chromosome 13. PZ is 33% homologous in amino acid sequence to the coagulation factor it aids in inhibiting, factor Xa (FXa). PZ's binding interface is similar to that of thrombin, an anionic heparin binding site named exocite II.
The crystal structure of PZ shows a serine fold with a distorted oxyanion hole and S1 packet. The two EGF domains in PZ are closely drawn to the C-terminal serine protease-like domain. PZ is found in circulation as a complex with protein Z-dependent protease inhibitor (ZPI). The crystal structure of ZPI shows a serpin fold, with 3 central β-sheets, and a catalytic active center. ZPI has a negatively charged Asp213 residue in a hydrophobic core, which gives it maximum inhibition. Ionic and hydrophobic interactions between the PZ's C-terminal along with its two neighboring EGF serves as a ZPI binding site while its N-terminal serves as a binding site to the membranous phospholipids. The PZ's N-terminal Gla domain is stabilized by the available calcium ions as it binds to the FXa and phospholipid hyprophobic loops.
Function
The function of PZ was studied in 1991 by Hogg and Stenflo, who initially hypothesized PZ to amplify the coagulation cascade but found that bovine PZ has a higher affinity for thrombin than human PZ due to a 36 amino acid addition to the bovine PZ's C-terminus. Furthermore, they found that PZ virtually had no involvement in binding thrombin to phospholipids. It was not until 1998 that Han et al. described PZ present in the body as a complex with protein Z-dependent protease inhibitor (ZPI). The ZPI and PZ complex acts as an inhibitor of factor Xa (FXa), an important enzyme in prothrombin activation, attached to platelets and other phopholipid surfaces (forming a calcium-dependent teritary complex). FXa is inhibited by two serine protease inhibitors, or serpins, antithrombin and ZPI with their cofactors heparin and PZ respectively. PZ binds to the serpin ZPI on the opposite side as where the allosterically activating heparin binds to the serpin antithrombin. When analyzing the antithrombin-FXa Michaelis complex, it is conclusive that FXa can bind to ZPI in a similarly configured complex.
It has been shown that PZ and ZPI travel the body in plasma already bonded together as a complex and later binds to activated FXa bound to the phospholipid membrane. PZ has a high affinity for ZPI and speeds up its interaction FXa by 1000x with calcium and phospholipids present. While ZPI requires the presence of PZ, calcium, and phospholipids to inhibit FXa, ZPI does not need other components to inhibit another coagulation factor, factor XI. Because this tertiary complex of PZ, ZPI, and FXa has dependency on other cofactors, it is seen as a template mechanism. The PZ catalytically stimulates interaction of ZPI and FXa by associating into the tertiary complex, formed by PZ's C-terminal bonding with ZPI's helix G, more specifically with Tyr240 and Asp 293 and ZPI's oppositely charged top region bound to the FXa's autolysis loop made up of acidic residues. Once the inhibitory complex is formed with the necessary cofactors present, the PZ dissociates to be used again.
Clinical Relevance
References
