Exploring human sperm nuclear basic protein–DNA interactions: could hexavalent chromium play an interfering role?

In human, sperm nuclear basic proteins (SNBP) are protamines (∼85 %, P1 and P2) and histones (∼15 %). The interaction with DNA is prevalently mediated by protamines, through guanidinium-phosphate salt bridges, being these proteins very arginine rich. In this work, the binding of SNBP to DNA was investigated in the presence of hexavalent chromium [Cr(VI)], a known disruptor of reproductive function. With Cr(VI) treatment, electrophoretic mobility shift assays indicated markedly impaired of SNBP-DNA complex, SDS and native-PAGE showed SNBP aggregation and fluorescence spectroscopy analyses revealed significant rearrangements in the polar surface exposition. To further probe the role of arginine, SNBP were deguanidinated with hydrazine, producing changes in DNA-binding similar to those caused by Cr(VI). The combination of deguanidinated derivatives of SNBP with Cr(VI) resulted in a worsening of the DNA-binding. All this emphasizes the importance of arginine integrity in the function of SNBP. In silico molecular docking revealed that Cr(III), the most stable reduced form of Cr(VI), forms coordination complexes with the guanidinium groups of arginine residues, thereby affecting DNA binding. These Cr(III) complexes are the primary agents responsible for the genotoxic effects on DNA. Additionally, this approach showed that Cr(III) can form stable bonds with guanine bases in GC-rich sequences and less stable bonds with AT-rich sequences, consistent with available experimental data reported in the literature. All results highlight the importance of a correct SNBP-DNA binding for sperm chromatin organisation and human reproductive health.