Probing different conformational states of pregnancy‐zone protein

The binding of the fluorescence probe 4,4′‐bis(8‐anilino‐1‐naphthalenesulphonate) (bis‐ANS) to the human proteinase inhibitor pregnancy‐zone protein (PZP) and its complexes with methylamine and chymotrypsin were investigated. The existence of dimeric PZP‐chymotrypsin complex was demonstrated and both the dimeric and the tetrameric PZP‐chymotrypsin complexes could be studied separately. The fluorescence data indicate that bis‐ANS binds to two different sites on PZP and its complexes. The values of the dissociation constant, K d1 , for the binding to the high‐affinity site were determined to be 231 ± 14, 220 ± 28, 114 ± 15 and 49 ± 1 nM, for the binding to native PZP, PZP‐methylamine and dimeric and tetrameric PZP‐chymotrypsin, respectively. An 11–30‐fold decrease was observed in the affinity for the second site, the corresponding values of the dissociation constant, K d2 , being 1.5 − 2.8 ± 1.0 μM, which are not significantly different for PZP and its derivatives. The results suggest that the probe bis‐ANS discriminates between the different conformational states of PZP and that while the conformation of the complex with methylamine does not differ much from that of the native protein, there is a significant change in conformation when chymotrypsin cleaves the bait region. This is substantiated by a 30%–45% decrease in the maximum enhancement of fluorescence intensity when PZP is treated with chymotrypsin. Although the dimeric and tetrameric forms of PZP‐chymotrypsin complexes differ in K d1 values, the difference in the maximum enhancement of the fluorescence of bis‐ANS by the two forms is not significant. This indicates that dimer‐dimer interaction in the tetrameric form does not involve hydrophobic sites. The necessity of bait‐region cleavage for extensive conformational changes in PZP distinguishes it from α 2 ‐macroglobulin, the other α‐macroglobulin in human plasma.