Alpha Helix

Alpha helix (α-helix) is a right- or left-handed coiled conformation, resembling a spring, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier (i+4 \rightarrow i hydrogen bonding). This secondary structure is also sometimes called a classic Pauling-Corey-Branson alpha helix.


Structure

Geometry and hydrogen bonding

The amino acids in an α helix are arranged in a right-handed helical structure where each amino acid corresponds to a 100° turn in the helix (i.e., the helix has 3.6 residues per turn), and a translation of 1.5 Å (= 0.15 nm) along the helical axis. The pitch of the helix (the vertical distance between two points on the helix) is 5.4 Å (= 0.54 nm)which is the product of 1.5 and 3,6. Most importantly, the N-H group of an amino acid forms a hydrogen bond with the C=O group of the amino acid four residues earlier; this repeated i+4 \rightarrow i hydrogen bonding defines an α-helix. Similar structures include the 310 helix (i+3 \rightarrow i hydrogen bonding) and the π-helix (i+5 \rightarrow i hydrogen bonding). These alternative helices are relatively rare, although the 310 helix is often found at the ends of α-helices, "closing" them off. Transient i+2 \rightarrow i helices (sometimes called δ-helices) have also been reported as intermediates in molecular dynamics simulations of α-helical folding.

Residues in α-helices typically adopt backbone (φ, ψ) dihedral angles around (-60°, -45°). More generally, they adopt dihedral angles such that the ψ dihedral angle of one residue and the φ dihedral angle of the next residue sum to roughly -105°. Consequently, α-helical dihedral angles generally fall on a diagonal stripe on the Ramachandran plot (of slope -1), ranging from (-90°, -15°) to (-35°, -70°). For comparison, the sum of the dihedral angles for a 310 helix is roughly -75°, whereas that for the π-helix is roughly -130°. The general formula for the rotation angle Ω per residue of any polypeptide helix with trans isomers is given by the equation.


The α-helix is tightly packed; there is almost no free space within the helix. The amino-acid side chains are on the outside of the helix, and point roughly "downwards" (i.e., towards the N-terminus), like the branches of an evergreen tree (Christmas tree effect). This directionality is sometimes used in preliminary, low-resolution electron-density maps to determine the direction of the protein backbone.


Stability

Helices observed in proteins can range from four to over forty residues long, but a typical helix contains about ten amino acids (about three turns). Short polypeptides generally do not exhibit much alpha helical structure in solution, since the entropic cost associated with the folding of the polypeptide chain is not compensated for by a sufficient amount of stabilizing interactions. The backbone hydrogen bonds of α-helices are generally considered slightly weaker than those found in β-sheets, and are readily attacked by the ambient water molecules. However, in more hydrophobic environments such as the plasma membrane, or in the presence of co-solvents such as trifluoroethanol (TFE), or isolated from solvent in the gas phase,[6] oligopeptides readily adopt stable α-helical structure.

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