Protein Z is a vitamin K-dependent plasma protein of unknown function. Its modular structure is identical with those of factors VII, IX, X, and protein C. These proteins have an N-terminal γ-carboxyglutamic acid (Gla)-containing module which binds six to ten Ca2+. In factors IX, X, and protein C, the adjacent epidermal growth factor (EGF)-like module binds one Ca2+ whereas the EGF-like module in protein Z does not. We have compared the Ca2+ binding properties of a fragment of protein Z comprising the Gla and N-terminal EGF-like modules (pZ-GlaEGFN) with those of intact protein Z and the isolated Gla module by measuring the Ca2+-induced quenching of the intrinsic protein fluorescence. The similar Ca2+ affinities of pZ-GlaEGFN and protein Z indicate that pZ-GlaEGFN has a native conformation and normal Ca2+ binding properties. A comparison of the Ca2+ binding to pZ-GlaEGFN with those to the corresponding fragments of factors IX, X, and protein C indicate that Ca2+ binding to the N-terminal EGF-like modules in the latter proteins does not influence the folding and Ca2+ binding properties of their Gla modules. Furthermore, the Ca2+-induced fluorescence enhancements of GlaEGF fragments from factors IX, X, and protein C appear to be caused by Ca2+ binding to the site in the EGF-like modules since it is not observed for pZ-GlaEGFN.
