Tertiary structure of Beta globin subunit of hemoglobin. The alpha helix is shown coiled inside a tubular structure; the helix may bend at proline corners The structure is held together by -S-S- disulfide bridges between non-djacent cysteine residues, and by electrostatic bonds. [The globin molecule happens to include an inorganic iron-containing heme group, which is not part of the tertiary structure of most proteins].