No two atoms should be closer together than the sum of their covalent radii. All peptide bonds should remain planar The resulting structure must be capable of being stabilized by hydrogen bonding. The final three dimensional form into which the entire polypeptide chain folds is termed the tertiary structure . Globular proteins coil into a compact, roughly spherical shape with the hydrophilic groups on the outside, and the hydrophobic groups folded in toward the center of the structure. This structure allows a heme group that carries the oxygen binding site of myoglobin to be embedded in a suitable pocket and held in a configuration that allows for the uptake and release of molecular oxygen.
Fibrous proteins assume elongated forms and tend to exist as ribbons or filaments.
Most of them play structural roles . They include the major proteins of skin, connective tissue, and fibers such as silk and hair. Various of these molecules twine around one another to create the protofibrils and microfibriles that form the basis for the structure of hair. Such twisted cables are stretchy and flexible, and therefore perfectly serve their biological function. The beta sheet structure is most elegantly utilized in the fibers spun by silkworms and spiders.