Pregnancy zone protein (PZP), also known as the pregnancy-associated α2-glycoprotein (α2-PAG or PAα2G), is a protein which in humans is encoded by the PZP gene on chromosome 12. PZP is part of the alpha-2 globulin family of proteins.Wong SG, Dessen A (2014). "Structure of a bacterial α2-macroglobulin reveals mimicry of eukaryotic innate immunity". Nature Communications. 5: 4917. It is often associated with pregnancy, during which it can be the most abundant among the plasma proteins.Chiabrando GA, Vides MA, Sanchez MC (2002). “Differential binding properties of human pregnancy zone protein– and a2-macroglobulin–proteinase complexes to low-density lipoprotein receptor-related protein”. Archives of Biochemistry and Biophysics. 398(1): 73–78. PZP is believed to play a role in immune-regulation during pregnancy, however many aspects of its mechanism, function and structure are yet to be determined.Oh JW, Kim SK, Cho K-C, Kim M-S, Suh CS, Lee JR, Kim KP (2017). “Proteomic analysis of human follicular fluid in poor ovarian responders during in vitro fertilization”.Proteomics. 17(6). Charkoftaki G, Chen Y, Han M, Sandoval M, Yu X, Zhao H, Orlicky DJ, Thompson DC, Vasiliou V
(2017). “Transcriptomic analysis and plasma metabolomics in Aldh16a1-null mice reveals a potential role of ALDH16A1 in renal function”. Chemico-Biological Interactions. 276: 15-22. Recent research has largely focused on determining how dysregulated PZP levels can act as a markers of various diseases.do Nascimento de Oliveira V, Lima-Neto ABM, van Tilburg MF, de Oliveira Monteiro-Moreira AC, Duarte Pinto Lobo M, Rondina D, Fernandes VO, Montenegro APDR, Montenegro RM Júnior, Guedes MIF (2018) “Proteomic analysis to identify candidate biomarkers associated with type 1 diabetes”. Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy. 11: 289-301.
Discovery The first publication reporting PZP was produced in 1959 by O. Smithies.Smithies O (1959). “Zone electrophoresis in starch gels and its application to studies of serum proteins”. Advances in Protein Chemistry. 14: 65–113. cited in Beckman L, Beckman G, Stigbrand T (1970). “Relation between the pregnancy zone protein and fetal sex”. Human Heredity. 20: 530-534. This was a result of an experimental process using starch gel zone-electrophoresis, which detected a protein band in the sera of 10% of the studied women in late pregnancy and after delivery.Sottrup-Jensen L, Folkersent J, Kristensen T, Tack BF (1984). “Partial primary structure of human pregnancy zone protein: Extensive sequence homology with human a2-macroglobulin (plasma proteins/evolution/acute-phase proteins)”. Proceedings of the National Academy of Sciences of the United States of America. 81: 7353-7357. Over the following years the detection of the protein was improved by researchers such as J. F. Afonso and R. R. Alvarez.Afonso JF, de Alvarez RR (1963). “Further starch gel fractionation of new protein zones in pregnancy”. The American Journal of Obstetrics and Gynecology. 86: 815-9. cited in Beckman L, Beckman G, Stigbrand T (1970). “Relation between the pregnancy zone protein and fetal sex”. Human Heredity. 20: 530-534.
Nomenclature Since its discovery, PZP has been synonymously referred to by numerous names in scholarly literature. These include the following: α2-pregnoglobulin, pregnancy-associated α2-glycoprotein, α-acute-phase glycoprotein, alpha-2-macroglobulin like, Xh-antigen, Schwangerschaftsprotein-3, Pα-1, pregnancy-associated α-macroglobulin, pregnancy-associated globulin, and α2-PAG.Wyatt AR, Cater JH, Ranson M (2016). “PZP and PAI-2: Structurally-diverse, functionally similar pregnancy proteins?”. The International Journal of Biochemistry & Cell Biology. 79: 113-117. Sand O, Folkersen J, Westergaard JG, Sottrup-Jensen L (1985). “Characterization of human pregnancy zone protein. Comparison with human alpha 2-macroglobulin”. Journal of Biological Chemistry. 15;260 (29): 15723-35.Poulsen OM, Hau J (1988). “Interaction between pregnancy zone protein and plasmin”. Archives of Gynecology and Obstetrics. 243(3): 157-164.
Gene expression and protein localisation The PZP gene contains 36 exons and is located on the short arm of chromosome 12 at position 12-13. The PZP gene encodes a transcript of 1482 base pairs for translation. The Protein Data Bank provides information on a coding sequence length of 4446 nucleotides.Protein Data Bank csb.org/pdb/gene/PZP Accessed 8/11/18 citing Berman, HM, Westbrook J, Feng Z, Gilliland G, Bhat TN, Weissig H, Shindyalov IN, Bourne PE (2000). The Protein Data Bank Nucleic Acids Research, 28: 235-242. A number of tissues, such as the uterus, liver, and brain, have PZP expressed in them. The presence of PZP has been detected in blood plasma, cerebral spinal fluid, and synovial fluid.Tayade C, Esadeg S, Fang Y, Croy BA (2005). “Functions of alpha 2 macroglobulins in pregnancy”. Molecular and Cellular Endocrinology. 245: 60-66.
In healthy adults and children in normal conditions, both male and female, PZP itself has been found to be present in low levels.Sarcione EJ, Biddle WC (2001). “Elevated serum pregnancy zone protein levels in HIV-1-infected men”. AIDS Issue. 15(18): 2467-2469. The amount of PZP in the plasma of healthy people in normal conditions has been identified as <0.03 mg/mL while for pregnant women this can rise to be at concentrations of 0.5 - 3.0 mg/mL.Ekelund L, Laurell CB (1994). “The pregnancy zone protein response during gestation: a metabolic challenge”. Scandinavian Journal of Clinical and Laboratory Investigation. 54: 623-629.
A 1976 study observing the roles of contraceptives on PZP amounts, resulted in speculation that estrogen plays some role in controlling PZP expression.Damber MG, von Schoultz B, Solheim F, Stigbrand T (1976). “A quantitative study of the pregnancy zone protein in sera of woman taking oral contraceptives”. The American Journal of Obstetrics and Gynecology. 124: 289-292. An earlier, 1971 study involving contraceptives, has noted that different PZP responses were created by different types of pills, posing questions about the effect of pill composition on the results.Beckman L, von Schoultz B, Stighrand T (1971). “Induction of the “Pregnancy Zone” protein by oral contraceptives”. Acta Obstetricia et Gynecologica Scandinavica. 50(4): 369-371.
The clone EPZP6 isolated from a human genomic library, has been observed to be a genomic clone of PZP.Devriendt K, Zhang J, van Leuven F, van den Berghe H, Cassiman J-J, Marynen P (1989). “A cluster of alpha 2-macroglobulin-related genes (alpha 2 M) on human chromosome 12p: cloning of the pregnancy-zone protein gene and an alpha 2M pseudogene”. Gene. 81: 325-334.
Protein structure Pregnancy zone proteins (PZPs) exist in the functionally active form as homodimers of 360 kDa molecular weights. This dimerisation of the constituent 180 kDa monomers happens by means of disulfide bridge bonds. PZPs are secreted in highly glycosylated form.Devriendt K, Van den Berghe H, Cassiman JJ, Marynen P (1991). “Primary structure of pregnancy zone protein. Molecular cloning of a full-length PZP cDNA clone by the polymerase chain reaction”. Biochimica et Biophysica Acta. 1088(1): 95-103. The structure has been confirmed experimentally when denaturation of PZP demonstrated the 360 kDa entity, while subsequent reduction - subunits of 180 kDa. Of the latter, some were shown to be partially fragmented to 90 kDa as a result of cleavage.Bohn H, Winckler W (1984). “Isolierung und Charakterisierung des schwangerschafts-assoziierten alpha2-Glykoproteins (alpha2-PAG)”. Blut. 33(6): 377–388. cited in Sottrup-Jensen L, Folkersent J, Kristensen T, Tack BF (1984). “Partial primary structure of human pregnancy zone protein: Extensive sequence homology with human a2-macroglobulin (plasma proteins/evolution/acute-phase proteins)”. Proceedings of the National Academy of Sciences of the United States of America. 81: 7353-7357.
On each PZP subunit there is a thiol-ester group, a signal domain, a bait region and a receptor binding domain. The bait region has numerous protease cleavage sites, while the binding domain is for low density lipoprotein receptor-related protein (LRP). A study of the bait domain has shown a rare polymorphism occurring, with either Valine or Methionine being at the sixth amino acid position. Also, a polymorphism of the nucleotides Adenine/Guanine has been observed at base pair 4097, while a Proline/Threonine polymorphism has been noted at the amino acid position 1180.
A PZP contains numerous sites of N-glycosylation, noted at the following residues: Asn1430, Asn997, Asn932, Asn875, Asn753, Asn406, Asn392, Asn246, Asn69, Asn54. The protein has an amino acid sequence which is 71% equivalent to that of alpha-2-Macroglobulin (α2M).
The minimal knowledge available about the PZP multi-domain fold allows only an approximation of its tertiary structure based on that of a transformed α2M at 4.3 Å resolution. As of November 2018, no crystal structure of PZP is available at the Protein Data Bank.Protein Data Bank http://www.rcsb.org/pdb/protein/P20742 Accessed 8/11/18 citing H.M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T.N. Bhat, H. Weissig, I.N. Shindyalov, P.E. Bourne. (2000) The Protein Data Bank Nucleic Acids Research, 28: 235-242.
A 1988 study of PZPs and three of its derivatives using monoclonal antibodies, showed that there is a possibility of a minimum of three various conformational states for PZP and its derivatives.Carlsson-Bostedt L, Moestrup SK, Gliemann J, Jensen, LS, Stigbrand T (1988). “Three different conformational states of Pregnancy Zone Protein identified by monoclonal antibodies”. The Journal of Biological Chemistry and Molecular Biology. 263(14): 6738-6741. In another study, experimentally, the hydrophobic surfaces displayed by α2M and PZP have been used to note that the conformational states of these two proteins display significant differences.Jensen, PEH, Hägglöf E-M, Arbelaez LF, Stigbrand T, Shanbhag VP (1993). “Comparison of conformational changes of pregnancy zone protein and human α2-macroglobulin, a study using hydrophobic affinity partitioning”. Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology. 1164(2): 152–158.
Biological activity and functions Over many years PZP has been classified as a protease inhibitor, however, as was observed in a paper in 2016, more recently, the suggestion of the roles of a T-helper cell modulator and/or an extracellular chaperone has been made. In a wider context, however, the specific biological significance of PZP is yet to be determined.
Despite early claims of PZP playing a role analogous to α2M, the latter shows inhibitory action against many more proteases than the former. In contrast to this, while PZP and PAI-2 display no obvious structural similarities, they show functional commonalities and complementary activity in extracellular fluids.