Final answer:
Reverse turns, also known as beta turns or hairpin turns, are irregular structures in proteins that cause a reversal in the peptide backbone. They play a role in connecting regular secondary structures and contribute to the folding and stability of protein structure. Reverse turns are classified into different types based on their backbone torsion angles.
Step-by-step explanation:
Reverse turns, also known as beta turns or hairpin turns, are a type of secondary structure in proteins. They are irregular structures that cause a reversal in the peptide backbone. Reverse turns consist of four amino acid residues in a non-helical segment, with a distance of less than 7 Å between Ca and C α. The main role of reverse turns is to connect regular secondary structures, such as alpha helices and beta sheets, in proteins. They allow for changes in direction, which helps in the folding and stability of the protein structure. The angles of the i+1 and i+2 residues in a reverse turn are important for its stability and conformation. Bonds between the carbonyl oxygen (O) of residue i and the amide nitrogen (N) of residue i+3 stabilize the turn. These stabilizing bonds are mainly CO-NH hydrogen bonds. Reverse turns can be classified into different types based on the values of the backbone torsion angles. The most common types are Type I, Type II, and Type VIII.