Omega loops: nonregular secondary structures significant in protein function and stability.

Omega (omega-) loops, a nonregular secondary structure found in globular proteins, are characterized by a polypeptide chain that follows a loop-shaped course in three-dimensional space. They do not contain repeating backbone dihedral angles or regular patterns of hydrogen bonding; however, many omega-loops contain a large number of hydrogen bonds, therefore it is not correct to think of omega-loops as structures lacking in hydrogen bonds. omega-Loops are found almost exclusively at the protein surface and exhibit amino acid preferences consistent with this observation. Since the first description of omega-loops in 1986, experiments have been conducted to probe the role of these structures in protein function, stability, and folding. It has become clear that omega-loops are often involved in protein function and molecular recognition. One motif, an omega-loop lid, that is flexible and mobile until substrate or inhibitor is bound and which probably plays a role in one or more steps of enzymatic catalysis, has been described in a variety of enzymes. Because they lack the periodic hydrogen bonding patterns of the regular secondary structures, some omega-loops are well suited for such functional roles in proteins. However, loops with a higher-than-average number of hydrogen bonds or hydrophobic contacts may play roles in protein stability or folding. Rather than determining further geometric definitions of loops, it may be instructional to group them according to their roles in protein structure, i.e., as categories of functional omega-loops, stability omega-loops, and folding omega-loops.