Alpha helix

Helice alfa

The alpha helices are secondary structures of proteins. This helix maintains its shape by the formation of hydrogen bonds between the oxygen atoms of the carbonyl group of one amino acid and the hydrogen atom of the amino group of another amino acid located four amino acids away in the chain. The R groups extend out of the helix. It is an amphipathic structure because it has a hydrophilic part and a hydrophobic part, which causes the coiling of this structure, so that the hydrophobic part does not interact with water.

In proteins, the α-helix is the main motif of secondary structure. It was first postulated by Linus Pauling, Robert Corey, and Herman Branson in 1953 based on the then-known crystallographic structures of amino acids and peptides and on Pauling's prediction of the planar shape of peptide bonds.

Description

The amino acids in an α-helix are arranged in a right-handed helical structure, with about 3.6 amino acids per turn. Each amino acid involves a turn of about 100° in the helix, and the Cαs of two contiguous amino acids are separated by 1.5Å. The propeller is closely packed; so that there is almost no free space inside the helix. All amino acid side chains are arranged towards the outside of the helix.

The N-H group of the amino acid (n) can establish a hydrogen bond with the C=O group of the amino acid (n+4). In this way, each amino acid (n) of the helix forms two hydrogen bonds with its peptide bond and the peptide bond of the amino acid in (n+4) and in (n-4). In total there are 7 hydrogen bonds per turn. This greatly stabilizes the propeller. It is within the levels of organization of the protein.

Stability

The first four amino acids of the helix, known as alpha, and the last four can only form one hydrogen bond instead of two, therefore the α helix is usually more stable in the central area than at the ends. To compensate for this loss, the amino acids at the ends are often polar and form H-bonds with their side chains and the side chains of other amino acids in the helix. When two alpha helices approach each other they tend to interact at angles of -30 and 60o.

In the helix the dipole moments of all the amino acids are perfectly aligned, thus forming a total dipole with a partial positive charge at the N-terminal end and a partial negative charge at the C-terminal end.

In an α-helix, the side chains of the amino acids in position (n) and in position (n+4) are aligned. So if we put two charged amino acids of the same sign or very large in those positions, the helix is destabilized.

Some amino acids, called helix disruptors, can destabilize the helical structure. One of them is proline, which, being an imino acid (although some authors question that proline is not strictly an imino acid), the N of its peptide bond does not have an H attached to form a hydrogen bond with the amino acid in (n+4). In addition, the methylene attached to the N of the peptide bond also causes steric hindrances that make the helix tend to break at the point where the proline is, although it will not if the proline is long and stable enough. Glycine, by providing great flexibility, since its side chain is only one H, is usually in the kinks at the end of the helix.

The first amino acid of a helix at the N-terminal end is called N-cap and the last amino acid of the helix, at the C-terminal end, is called C- cap. In the N-cap position, uncharged polar amino acids usually appear, such as asparagine, or negatively charged, such as glutamic acid, so as to compensate for the loss of a peptide bond at the ends of the helix that we have already commented on and in the In the case of glutamic acid, the negative charge of its side chain interacts with the partial positive charge of the N-terminal end of the helix.

Glycine and proline are frequent in the C-cap, which, as we have already mentioned, break the structure of the helix, and also positively charged amino acids, such as lysine, whose positive charge interacts with the partial negative charge of the C-terminal -propeller terminal.

Short polypeptides are usually not capable of adopting the alpha-helix structure, since the entropic cost associated with folding the polypeptide chain is too high.

Importance

The α-helices, in addition to being the most common type of secondary structure in proteins, are of great importance in DNA-binding structural motifs, such as helix-turn-helix motifs and zinc fingers. This is because the 12Å diameter of the α-helix coincides with the width of the largest cleft of B-form DNA or B-DNA.

Other types of propeller

There are other types of helical structures similar to the α-helix in proteins, but much less common:

• Hélice 310{displaystyle 3_{10}}: similar to the α helix but with 3 amino acids per spin (more extended and closer).

• Helice π: is more compressed and is wider than the propeller α (4.4 waste per spin).

The alpha helix in art

Julian Voss-Andreae Alpha Propeller in tribute to Linus Pauling (2004) with polished steel cover and 3 meters high. The sculpture is located in front of Pauling's childhood home in Portland, Oregon (United States).

Julian Voss-Andreae is a German sculptor with degrees in experimental physics and sculpture. Since 2001 Voss-Andreae has been creating "protein sculptures" inspired by protein structure, the α helix being one of his favorite objects. This artist has made α-helix sculptures from various materials, including bamboo and other trees. In 2004 he made a monument in memory of Linus Pauling, discoverer of the alpha helix, designed from a large steel beam rearranged according to the shape of the alpha helix structure. The bright red, 10-foot-tall sculpture stands in front of Pauling's childhood home in Portland, Oregon.

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