Protein Z (PZ) is a vitamin K-dependent, plasma protein that serves as a cofactor for the inhibition of factor Xa by protein Z-dependent protease inhibitor (ZPI) in the presence of calcium ions and phospholipids. ZPI is a member of the serpin superfamily of proteinase inhibitors and inhibits factor Xa in a PZ-dependent fashion, but also inhibits factor XIa in the absence of cofactors. Read more…