The effects of salt and pH on the spermatozoa agglutinating activity of carp ovum cystatin by molecular dynamics simulations

摘要

In order to elucidate how the spermatozoa agglutinating activity of carp ovum cystatin is significantly affected by salt and pH in the atomic scale, an homology model of carp ovum cystatin was constructed based on the crystal structure of chicken egg white cystatin in this study. Although these two proteins exhibit only 36.11% of sequence identity, they share similar folds. Our homology model also shows that 16 positively charged residues are equally distributed on the surface of carp ovum cystatin, leading to agglutinating spermatozoa via electrostatic interaction. The structure of carp ovum cystatin is not significantly affected by salt and pH during 400 ps molecular dynamics simulations. However, the total accessible surface area (ASA) of the positively charged residues on the surface of carp ovum cystatin decreased with increasing salt concentration, leading to the abolishment of its spermatozoa agglutinating activity. Moreover, 13 Lys residues became deprotonated at pH 11, resulting in the decrease of the net positive charges on the surface of carp ovum cystatin and thus losing its spermatozoa agglutinating activity.