The defining characteristic is that residues that make up the beginning and end of the loop are close together in space with no intervening lengths of regular secondary structural motifs.
It is named after its shape, which resembles the upper-case Greek letter Omega (Ω).
Omega loops, being non-regular, non-repeating secondary structural units, have a variety of three-dimensional shapes.
Omega loop shapes are analyzed to identify recurring patterns in dihedral angles and overall loop shape to help identify potential roles in protein folding and function.
[12][13] A heritable coagulation disorder is caused by a single-site mutation in an omega loop of protein C.[14] Likewise, omega loops play an interesting role in the function of the beta-lactamases: mutations in the "omega loop region" of a beta-lactamase can change its specific function and substrate profile,[15][16][17] perhaps due to an important functional role of the correlated dynamics of the region.