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Protein Z
Table of Contents
Background
Protein Z (PZ) is a vitamin k-dependent glycoprotein made in the liver and involved in the blood-clot-forming coagulation cascade. Bovine PZ was first identified by Prowse and Esnouf in 1977 and Human PZ was first isolated and studied by Broze Jr. and Miletich in 1984. The gene coding for PZ, called PROZ, was found in 1998 on chromosome 13 at location 13q34 and is composed of nine exons (one being an alternative exon).
Structure
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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 390 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 nteractions 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
