protein z-related protease inhibitor protein z-related protease inhibitor protein z-dependent protease inhibitor (zpi) is a protein circulating in the blood which inhibits factors xa and xia of the coagulation cascade. it is a member of the class of the serine protease inhibitors (serpins). its name implies that it requires protein z, another circulating protein, to function properly, but this only applies to its inhibition of factor x. crystal structure of protein z-dependent peptidase inhibitor (red) in complex with protein z (blue). rendered from pdb 3f1sidentifierssymbolserpina10ncbi gene51156hgnc15996omim605271refseqnm_016186uniprotq9uk55other datalocuschr. 14 q32.1search forstructuresswiss-modeldomainsinterpro it is about 72 kda heavy and 444 amino acids large. it is produced by the liver. role in disease water et al. found deficiency of zpi in 4.4% of a cohort of patients with thrombophilia (a tendency to thrombosis). history han et al. first described zpi in 1998. the same group further characterised it in 2000.