Characterization of the estradiol-binding site structure of human protein disulfide isomerase (PDI)

Earlier studies showed that 17β-estradiol (E(2)), an endogenous female sex hormone, can bind to human protein disulfide isomerase (PDI), a protein folding catalyst for disulfide bond formation and rearrangement. This binding interaction can modulate the intracellular levels of E(2) and its biological actions. However, the structure of PDI's E(2)-binding site is still unclear at present, which is the focus of this study.

The E(2)-binding site structure of human PDI was studied by using various biochemical approaches coupled with radiometric receptor-binding assays, site-directed mutagenesis, and molecular computational modeling. Analysis of various PDI protein fragments showed that the [(3)H]E(2)-binding activity is not associated with the single b or b' domain but is associated with the b-b' domain combination. Computational docking analyses predicted that the E(2)-binding site is located in a hydrophobic pocket composed mainly of the b' domain and partially of the b domain. A hydrogen bond, formed between the 3-hydroxyl group of E(2) and His256 of PDI is critical for the binding interaction. This binding model was jointly confirmed by a series of detailed experiments, including site-directed mutagenesis of the His256 residue coupled with selective modifications of the ligand structures to alter the binding interaction.

The results of this study elucidated the structural basis for the PDI-E(2) binding interaction and the reservoir role of PDI in modulating the intracellular E(2) levels. The identified PDI E(2)-binding site is quite different from its known peptide binding sites. Given that PDI is a potential therapeutic target for cancer chemotherapy and HIV prevention and that E(2) can inhibit PDI activity in vitro, the E(2)-binding site structure of human PDI determined here offers structural insights which may aid in the rational design of novel PDI inhibitors.