Breast cancer has the highest incidence and mortality rate for women in the world. 70% of breast tumors are estrogen receptor alpha (ERα)-positive. The overexpression of ERα induces tumor growth, making ERα a target for breast cancer treatment. In the ERα transcription factor DNA binding domain (DBD) homodimer, each monomer has two conserved (Cys)₄ liganded zinc-fingers, ZF_I (C-terminal) and ZF_II(N-terminal). Although the Zn²⁺ is not redox-active, the sulfur atom in the coordinating Cys can be oxidized, causing the Zn²⁺ to be ejected. Electrophilic agents inhibit breast cancer growth by preferentially ejecting Zn²⁺ from ZF_II, preventing dimerization and DNA binding, suggesting that ZF_II is more susceptible to electrophilic agents. Neither ZF is vulnerable to oxidation when the dimer is bound to DNA. Elucidating the factors that determine a Cys lability is vital for drug discovery.
Multi-microsecond molecular dynamics (MD) simulations of ERα DBD monomer and DNA-bound dimer were carried out to examine the factors suggested to affect ZFs reactivity. In the monomer, ZF_II is more flexible than ZF_I, as demonstrated by the higher deviation from the root mean square (rmsd) from the crystal structure. ZF_I had more than twice the fraction of hydrogen bonds to the thiolate from neighboring AAs compared to ZF_II, making ZF_I’s Cys more electronically screened. These hydrogen bonds in ZF_II were more transient, suggesting that the greater susceptibility to electrophiles could be attributed to the increased flexibility of ZF_II. ZF_I is also less solvent accessible than ZF_II, contributing to its lack of reactivity. In the dimer bound to DNA, both ZF_I and ZF_II were less flexible and less solvent accessible and the fraction of hydrogen bonds to Cys in ZF_II was slightly increased compared to the monomer. The binding of the dimers to each other and binding to the DNA leads to lower flexibility and less solvent accessibility, protecting the DNA-bound dimer from oxidation and loss of Zn²⁺.

Wesley Coombs

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