The alpha helix is the simplest secondary structure that a protein can adopt in space according to the rigidity and freedom of rotation of the bonds between its amino acid residues.
It is characterized by the spiral shape in which the amino acids are arranged, which seem to be arranged around an imaginary longitudinal axis with the R groups facing the outside of it.
Diagram of the alpha helix structure (Alejandro Porto, via Wikimedia Commons)
Alpha helices were first described in 1951 by Pauling et al., Who used available data on interatomic distances, bond angles, and other structural parameters of peptides and amino acids to predict the most likely configurations that the chains could assume. polypeptides.
The description of the alpha helix arose from the search for all possible structures in a peptide chain that were stabilized by hydrogen bonds, where the residues were stoichiometrically equivalent and the configuration of each one was planar, as indicated by the data from resonance of the peptide bonds that were available to date.
This secondary structure is the most common among proteins, and it is adopted by both soluble proteins and integral membrane proteins. More than 60% of proteins are believed to exist in the form of an alpha helix or beta sheet.
Structure
In general, each turn of an alpha helix has an average of 3.6 amino acid residues, which is roughly 5.4 Å in length. However, turn angles and lengths vary from protein to protein with strict dependence on the amino acid sequence of the primary structure.
Most alpha helices have a right-hand turn, but it is now known that proteins with alpha helices with left-hand turns can exist. The condition for one or the other to occur is that all the amino acids are in the same configuration (L or D), since they are responsible for the direction of the spin.
The stabilization of these important structural motifs for the protein world is given by hydrogen bonds. These bonds occur between the hydrogen atom attached to the electronegative nitrogen of a peptide bond and the electronegative carboxylic oxygen atom of the amino acid four positions further, in the N-terminal region with respect to itself.
Each turn of the helix, in turn, is joined to the next by hydrogen bonds, which are essential to achieve the overall stability of the molecule.
Not all peptides can form stable alpha helices. This is given by the intrinsic capacity of each amino acid in the chain to form helices, which is directly related to the chemical and physical nature of its substituent R groups.
For example, at a certain pH, many polar residues can acquire the same charge, so they cannot be placed consecutively in a helix since the repulsion between them would imply a great distortion in it.
The size, shape, and position of amino acids are also important determinants of helical stability. Without going any further, residues such as Asn, Ser, Thr, and Cys positioned closely within the sequence could also have a negative effect on the alpha helix configuration.
In the same way, the hydrophobicity and hydrophilicity of the alpha helical segments in a given peptide depend exclusively on the identity of the R groups of the amino acids.
In integral membrane proteins alpha helices abound with residues of strong hydrophobic character, strictly necessary for the insertion and configuration of the segments between the apolar tails of the constituent phospholipids.
Soluble proteins, on the other hand, have alpha helices rich in polar residues, which make possible a better interaction with the aqueous medium present in the cytoplasm or in the interstitial spaces.
Functional importance
Alpha helix motifs have a wide range of biological functions. Specific interaction patterns between the helices play a critical role in the function, assembly, and oligomerization of both membrane proteins and soluble proteins.
These domains are present in many transcription factors, important from the point of view of the regulation of gene expression. They are also present in proteins with structural relevance and in membrane proteins that have transport and / or signal transmission functions of various kinds.
Here are some classic examples of proteins with alpha helices:
Myosin
Myosin is an actin-activated ATPase that is responsible for muscle contraction and a variety of forms of cell mobility. Both muscular and non-muscular myosins consist of two globular regions or "heads" linked together by a long alpha helical "tail."
Collagen
One third of the total protein content of the human body is represented by collagen. It is the most abundant protein in the extracellular space and its distinctive feature is a structural motif composed of three parallel strands with a left-handed helical configuration, which join to form a dextrorotatory sense triple helix.
Keratin
Keratins are a group of filament-forming proteins that are produced by some epithelial cells in vertebrates. They are the main component of the nails, hair, claws, the shell of turtles, horns and feathers. Part of its fibrillar structure is made up of alpha helix segments.
Keratin structuring (Mlpatton, from Wikimedia Commons)
Hemoglobin
Oxygen in the blood is carried by hemoglobin. The globin portion of this tetrameric protein consists of two identical alpha helices of 141 residues each, and two beta chains of 146 residues each.
'Zinc finger' type proteins
Eukaryotic organisms possess a great wealth of zinc finger-like proteins, which function for different purposes: DNA recognition, RNA packaging, transcriptional activation, apoptosis regulation, protein folding, etc. Many zinc finger proteins have alpha helices as the main component of their structure and which are essential for their function.
References
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