This is a zinc finger domain from a larger transcription factor. This is the most prevalent DNA-binding motif found in the human genome. The Zinc holds the 2 strands, sometimes seen as beta-strands, next to the alpha-helix The 2 His residues use Nitrogens to ligate the zinc, and the 2 Cys residues use Sulfurs. Notice that the Zinc disappears into the protein in a spacefilling view.

Elrod-Erickson, M., Rould, M. A., Nekludova, L., Pabo, C. O.: Zif268 protein-DNA complex refined at 1.6 A: a model system for understanding zinc finger-DNA interactions. Structure 4 pp. 1171 (1996) (PubMed)

1aay (PDB)