EXAFS analysis of the zinc-binding domain of boar spermatidal transition protein 2

Boar Spermatidal Transition Protein 2 (TP2; 137 amino acid residues) is sup posed to play an important role in initiation of chromatin condensation and cessation of transcriptional activity during mammalian spermiogenesis. Boa r TP2 has three potential zinc finger motifs and binds three atoms of zinc per molecule. However the structure of the zinc-binding domain of boar TP2 has not been completely determined. To elucidate the local structure around the zinc atoms of boar TP2, we performed an X-ray absorption fine structur e (XAFS) measurement on the zinc-binding domain of TP2(TP2Z)( residues 1-10 3) in the fluorescence mode. By EXAFS analyses we have demonstrated that ea ch of the three zinc atoms is coordinated by approximately two sulfur and t wo nitrogen atoms on average. The average Zn-S and Zn-N distances were foun d to be 2.36 Angstrom and 2.01 Angstrom, respectively. The sulfur and nitro gen atoms are attributed to cysteine and histidine residues, respectively, from comparison of the EXAFS spectra with model compounds ZnS and ZnTPP(zin c(II) tetraphenylporphyrin).