Defects in protein Z lead to increased factor Xa activity and a propensity for thrombosis.

Its name implies that it requires protein Z, another circulating protein, to function properly, but this only applies to its inhibition of factor X.

In this case we would be interested in the equilibrium concentration of the protein Z (Goldbeter-Koshland kinetics only describe equilibrium properties, thus no dynamics can be modeled).

The presence of serpins in plants has long been recognised, indeed, an abundant barley grain serpin (barley Protein Z) is one of the major protein components in beer.

One example would be a protein Z that exists in a phosphorylated form Z and in an unphosphorylated form Z; the corresponding kinase Y and phosphatase X interconvert the two forms.

The affinity of this protein for factor Xa is increased 1000-fold by the presence of protein Z, while it does not require protein Z for inactivation of factor XI.

Inhibitors of factor XIa include protein Z-dependent protease inhibitor (ZPI, a member of the serine protease inhibitor/serpin class of proteins), which is independent of protein Z (its action on factor X, however, is protein Z-dependent, hence its name).