Protein Z (PZ or PROZ) is a Protein in humans which is encoded by the PROZ gene. Protein Z is a member of the coagulation cascade, the group of blood proteins that leads to the formation of blood clots. It is a glycoprotein. Protein Z functions to inhibit blood coagulation by binding to an inhibitor. It is a GLA domain protein and thus Vitamin K-dependent, and its functionality is therefore impaired in warfarin therapy. Physiology Although it is not enzymatically active, it is structurally related to several serine proteases of the coagulation cascade: Factors VII, IX, X and Protein C. The carboxyglutamate residues (which require Vitamin K) bind Protein Z to phospholipid surfaces. The main role of Protein Z appears to be the degradation of Factor Xa. This is done by Protein Z-related protease inhibitor (ZPI), but the reaction is accelerated 1000-fold by the presence of Protein Z. Oddly, ZPI also degrades Factor XI, but this reaction does not require the presence of Protein Z. ZPI activated by Protein Z does not appear to happen because of its conformation, but proximity to each other. When Protein Z in bound to ZPI, it will bind to the same phospholipid surface as Factor Xa. This is what promotes the inhibition of Factor Xa. In some studies, deficiency states have been associated with a propensity to thrombosis. Others, however, link it to bleeding tendency; there is no clear explanation for this, as it acts physiologically as an inhibitor, and deficiency would logically have led to a predisposition for thrombosis. Genetics It is 62 kDa large and 396 amino acids long. The PROZ gene has been linked to the thirteenth chromosome (13q34). It has four domains: a GLA-rich region, two EGF-like domains and a trypsin-like domain. It lacks the serine residue that would make it catalytically active as a serine protease. History Protein Z was first isolated in cattle blood by Christopher Prowse and Peter Esnouf in 1977, and Broze & Miletich determined it in human plasma in 1984. Protein Z found in humans was given the same name as the one found in cattle for a few reasons. When looking at these isolated proteins it was found that they both have similar molecular weight, a similar composition of amino acids, and a similar Amino Terminal sequences. These similarities in molecular composition of the protein found in cattle and humans was great enough that