Human α-fetoprotein as a Zn 2+-binding protein. Tight cation binding is not accompanied by global changes in protein structure and stability

• Published in: Biochimica et biophysica acta. Molecular basis of disease Vol. 1586; no. 1; pp. 1 - 10
• Main Authors: Permyakov, Serge E., Oberg, Keith A., Cherskaya, Alexandra M., Shavlovsky, Mikhail M., Permyakov, Eugene A., Uversky, Vladimir N.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 2002
• Subjects: Carcinoembryonic protein; Conformational stability; Protein structure; Structural transition; Zn 2+ binding; α-Fetoprotein; Conformational stability; α-Fetoprotein; Protein structure; Carcinoembryonic protein; Structural transition; Zn 2+ binding
• ISSN: 0925-4439; 1879-260X
• DOI: 10.1016/S0925-4439(01)00079-5

The binding of zinc to human α-fetoprotein (AFP) isolated from human umbilical cord serum was studied by fluorimetric Zn 2+-titration. We found that the total number of strong binding sites for zinc on this protein was 5: AFP has one very strong (dissociation constant, K d<10 −8 M) and at least four lower affinity zinc binding sites ( K d<10 −5 M). Fourier transform infrared (FTIR) analysis revealed that aspartate and histidine residues could be involved in the strong coordination of zinc. Intriguingly, binding of zinc to the protein does not induce structural changes that can be detected by circular dichroism, FTIR, intrinsic fluorescence or (1,1′)-bi-(4-anilino)naphthalene-5,5′-disulfonic acid (bis-ANS) binding. Finally, scanning microcalorimetry measurements showed that stability of the protein is also unaffected by zinc binding in spite of the strength of the coordination. Such strong interactions without major structural consequences are highly unusual, and AFP may therefore be the first characterized representative of a new class of ligand-binding proteins.