Protein Secondary Structure Types

Alpha Helices

$\alpha$ helices are defined by dihedral angles of around (-60◦ ,-45◦ ). Generally $\alpha$-helices adopt dihedral angles such that the sum of the $\varphi$ angle of one residue and the $\psi$ angle of the next residue is approximately 105◦.

Also of great importance to this secondary structure element is the hydrogen bonding pattern.

In an α -helix the N−H of one residue is hydrogen bonded to the C − O of the residue four residues previous, giving a hydrogen bonding pattern of (i, i + 4). The combination of the dihedral angles and the hydrogen bonding pattern give rise to a helix with four residues per full 360◦ turn, meaning that the minimum length for this type of helix is four residues.

Beta Sheets

β -sheets consisting of two or more β -strands held together by inter-strand hydrogen bonds. The sheet is defined by dihedral angles of around (-135 ◦ ,135 ◦ ), though the actual values can vary greatly.

β -sheets can be described either as parallel or anti-parallel; in a parallel sheet the individual strands are aligned N-terminus to N-terminus, whereas in an anti-parallel sheet the strands are aligned N-terminus to C-terminus.

Unlike in the α -helix, the hydrogen bonds present in a β -sheet do not have to be between local or consecutive residues; in fact β -sheets are commonly formed from strands made up of residues throughout the amino acid chain.

3${}_{10}$ Helix

The 3${}_{10}$ helix is a tighter helix than the Alpha Helix due to its (i, i + 3) hydrogen bonding pattern.

Pi-helix

The $\pi$-helix is similar to the Alpha Helix but it has a looser (i, i + 5) hydrogen bonding arrangement.