Alpha helix

alpha helicesα-helicesalpha-helicesα-helixalpha-helicalhelicesalpha-helixalpha helicalα-helicalhelix
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located three or four residues earlier along the protein sequence.wikipedia
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Protein secondary structure

secondary structuresecondarystructure
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located three or four residues earlier along the protein sequence.
The two most common secondary structural elements are alpha helices and beta sheets, though beta turns and omega loops occur as well.

Linus Pauling

PaulingLinus Carl PaulingLinus C. Pauling
Although incorrect in their details, Astbury's models of these forms were correct in essence and correspond to modern elements of secondary structure, the α-helix and the β-strand (Astbury's nomenclature was kept), which were developed by Linus Pauling, Robert Corey and Herman Branson in 1951 (see below); that paper showed both right- and left-handed helices, although in 1960 the crystal structure of myoglobin showed that the right-handed form is the common one.
Pauling also worked on the structures of biological molecules, and showed the importance of the alpha helix and beta sheet in protein secondary structure.

William Astbury

William Thomas AstburyAstburyAstbury, William Thomas
In the early 1930s, William Astbury showed that there were drastic changes in the X-ray fiber diffraction of moist wool or hair fibers upon significant stretching.
His work on keratin provided the foundation for Linus Pauling's discovery of the alpha helix.

Robert Corey

CoreyRobert Brainard Corey
Although incorrect in their details, Astbury's models of these forms were correct in essence and correspond to modern elements of secondary structure, the α-helix and the β-strand (Astbury's nomenclature was kept), which were developed by Linus Pauling, Robert Corey and Herman Branson in 1951 (see below); that paper showed both right- and left-handed helices, although in 1960 the crystal structure of myoglobin showed that the right-handed form is the common one.
Robert Brainard Corey (August 19, 1897 – April 23, 1971) was an American biochemist, mostly known for his role in discovery of the α-helix and the β-sheet with Linus Pauling.

Helix

helicalheliceshelically
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located three or four residues earlier along the protein sequence.
Helices are important in biology, as the DNA molecule is formed as two intertwined helices, and many proteins have helical substructures, known as alpha helices.

Herman Branson

Herman Russell BransonBranson, Herman
Although incorrect in their details, Astbury's models of these forms were correct in essence and correspond to modern elements of secondary structure, the α-helix and the β-strand (Astbury's nomenclature was kept), which were developed by Linus Pauling, Robert Corey and Herman Branson in 1951 (see below); that paper showed both right- and left-handed helices, although in 1960 the crystal structure of myoglobin showed that the right-handed form is the common one.
Herman Russell Branson (August 14, 1914 – June 7, 1995) was an American physicist, chemist, best known for his research on the alpha helix protein structure, and was also the president of two colleges.

Glycine

GlyGglycinate
Short pieces of left-handed helix sometimes occur with a large content of achiral glycine amino acids, but are unfavorable for the other normal, biological L-amino acids.
Glycine is integral to the formation of alpha-helices in secondary protein structure due to its compact form.

310 helix

3 10 helix3 10 helices3 10 -helix
Similar structures include the 3 10 helix (i + 3 → i hydrogen bonding) and the π-helix (i + 5 → i hydrogen bonding).
Of the numerous protein secondary structures present, the 3 10 -helix is the fourth most common type observed; following α-helices, β-sheets and reverse turns.

Protein

proteinsproteinaceousstructural proteins
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located three or four residues earlier along the protein sequence.

Helical wheel

Two of these emphasize circular placement around the cylindrical cross-section: The first-developed such diagram is called the "helical wheel", and a more recent version is called the "wenxiang diagram".
A helical wheel is a type of plot or visual representation used to illustrate the properties of alpha helices in proteins.

Beta sheet

β-sheetbeta sheetsbeta strand
Although incorrect in their details, Astbury's models of these forms were correct in essence and correspond to modern elements of secondary structure, the α-helix and the β-strand (Astbury's nomenclature was kept), which were developed by Linus Pauling, Robert Corey and Herman Branson in 1951 (see below); that paper showed both right- and left-handed helices, although in 1960 the crystal structure of myoglobin showed that the right-handed form is the common one.
The hydrogen bonds lie roughly in the plane of the sheet, with the peptide carbonyl groups pointing in alternating directions with successive residues; for comparison, successive carbonyls point in the same direction in the alpha helix.

Pi helix

π helixpi helicesπ-helix
Similar structures include the 3 10 helix (i + 3 → i hydrogen bonding) and the π-helix (i + 5 → i hydrogen bonding).
Discovered by crystallographer Barbara Low in 1952 and once thought to be rare, short π-helices are found in 15% of known protein structures and are believed to be an evolutionary adaptation derived by the insertion of a single amino acid into an α-helix.

Structural motif

motifmotifsprotein motif
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located three or four residues earlier along the protein sequence. The first and especially the fourth residues (known as the a and d positions) are almost always hydrophobic; the fourth residue is typically leucine – this gives rise to the name of the structural motif called a leucine zipper, which is a type of coiled-coil.
Consists of alpha helices bound by a looping stretch of amino acids.

Wenxiang diagram

Two of these emphasize circular placement around the cylindrical cross-section: The first-developed such diagram is called the "helical wheel", and a more recent version is called the "wenxiang diagram".
The wenxiang diagram, also known as the wenxiang graph, was proposed in 1997 by Kuo-Chen Chou, Chun-Ting Zhang, and Gerald M. Maggiora for helping intuitively analyze the disposition of amphiphilic alpha helices in heteropolar environments It is closely related to the earlier 2D diagram called a "helical wheel", which is a slightly idealized projection of the Calphas down the helix axis, also with one-letter-code labels and color-coded symbols to show the sequence and its properties.

Integral membrane protein

integral membrane proteinsintegral proteinintegral
The helix net is not suitable for studying helix–helix packing interactions, but it has become the dominant means of representing the sequence arrangement for integral membrane proteins because it shows important relationships of the helical sequence to vertical positioning within the membrane even without knowledge of how the helices are arranged in 3D.
Their membrane-anchoring α-helices have been removed to facilitate the extraction and crystallization.

DSSP (hydrogen bond estimation algorithm)

DSSPDSSP (protein)DSSP algorithm
The α-helices can be identified in protein structure using several computational methods, one of which is DSSP (Define Secondary Structure of Protein).
The 3 10 helix, α helix and π helix have symbols G, H and I and are recognized by having a repetitive sequence of hydrogen bonds in which the residues are three, four, or five residues apart respectively.

Keratin

keratinizationkeratinouskeratinized
Neurath's paper and Astbury's data inspired H. S. Taylor, Maurice Huggins and Bragg and collaborators to propose models of keratin that somewhat resemble the modern α-helix.
Hair and other α-keratins consist of α-helically coiled single protein strands (with regular intra-chain H-bonding), which are then further twisted into superhelical ropes that may be further coiled.

Fiber diffraction

In the early 1930s, William Astbury showed that there were drastic changes in the X-ray fiber diffraction of moist wool or hair fibers upon significant stretching.
Fiber diffraction data led to several important advances in the development of structural biology, e.g., the original models of the α-helix and the Watson-Crick model of double-stranded DNA.

Lawrence Bragg

William Lawrence BraggBraggSir Lawrence Bragg
Neurath's paper and Astbury's data inspired H. S. Taylor, Maurice Huggins and Bragg and collaborators to propose models of keratin that somewhat resemble the modern α-helix.
Unlike myoglobin, in which nearly 80 per cent of the amino-acid residues are in the alpha-helix conformation, in lysozyme the alpha-helix content is only about 40 per cent of the amino-acid residues found in four main stretches.

Hydrogen bond

hydrogen bondinghydrogen bondshydrogen-bonding
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located three or four residues earlier along the protein sequence.
When the spacing of the amino acid residues participating in a hydrogen bond occurs regularly between positions i and i + 4, an alpha helix is formed.

Cell membrane

plasma membranemembranecell membranes
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, oligopeptides readily adopt stable α-helical structure.

Ramachandran plot

RamachandranRamachandran spacebackbone angles
As a consequence, α-helical dihedral angles, in general, fall on a diagonal stripe on the Ramachandran diagram (of slope −1), ranging from (−90°, −15°) to (−35°, −70°).
The most common regions are labeled: α for α helix, Lα for left-handed helix, β for β-sheet, and ppII for polyproline II.

Circular dichroism

CDcircular dichroism spectroscopyhelicity
The far-UV (170–250 nm) circular dichroism spectrum of helices is also idiosyncratic, exhibiting a pronounced double minimum at around 208 and 222 nm.
Therefore, the alpha helix of proteins and the double helix of nucleic acids have CD spectral signatures representative of their structures.

Aspartic acid

aspartateAspL-aspartate
α-helices often occur with the N-terminal end bound by a negatively charged group, sometimes an amino acid side chain such as glutamate or aspartate, or sometimes a phosphate ion.
In proteins aspartate sidechains are often hydrogen bonded to form asx turns or asx motifs, which frequently occur at the N-termini of alpha helices.

Leucine zipper

leucine zippersleucine-zipperBasic zipper
The first and especially the fourth residues (known as the a and d positions) are almost always hydrophobic; the fourth residue is typically leucine – this gives rise to the name of the structural motif called a leucine zipper, which is a type of coiled-coil.
They were first described by Landschulz and collaborators in 1988 when they found that an enhancer binding protein had a very characteristic 30-amino acid segment and the display of these amino acid sequences on an idealized alpha helix revealed a periodic repetition of leucine residues at every seventh position over a distance covering eight helical turns.