Alpha helix

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Three-dimensional structure of an alpha helix in the protein crambin

An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix).

The alpha helix is the most common structural arrangement in the secondary structure of proteins. It is also the most extreme type of local structure, and it is the local structure that is most easily predicted from a sequence of amino acids.

The alpha helix has a

residues
earlier in the protein sequence.

Other names

The alpha helix is also commonly called a:

  • Pauling–Corey–Branson α-helix (from the names of three scientists who described its structure)
  • 3.613-helix because there are 3.6 amino acids in one ring, with 13 atoms being involved in the ring formed by the hydrogen bond (starting with amidic hydrogen and ending with carbonyl oxygen)
Protein secondary structureBeta sheetAlpha helix
The image above contains clickable links
The image above contains clickable links
Interactive diagram of
hydrogen bonds in protein secondary structure. Cartoon above, atoms below with nitrogen in blue, oxygen in red (PDB: 1AXC​
​)


Discovery

carbonyl groups are pointing upwards toward the viewer, spaced roughly 100° apart on the circle, corresponding to 3.6 amino-acid
residues per turn of the helix.

In the early 1930s,

ångströms (0.51 nanometres
).

Astbury initially proposed a linked-chain structure for the fibers. He later joined other researchers (notably the American chemist Maurice Huggins) in proposing that:

  • the unstretched protein molecules formed a helix (which he called the α-form)
  • the stretching caused the helix to uncoil, forming an extended state (which he called the β-form).

Although incorrect in their details, Astbury's models of these forms were correct in essence and correspond to modern elements of

Bragg and collaborators[5] to propose models of keratin
that somewhat resemble the modern α-helix.

Two key developments in the modeling of the modern α-helix were: the correct bond geometry, thanks to the crystal structure determinations of amino acids and peptides and Pauling's prediction of planar peptide bonds; and his relinquishing of the assumption of an integral number of residues per turn of the helix. The pivotal moment came in the early spring of 1948, when Pauling caught a cold and went to bed. Being bored, he drew a polypeptide chain of roughly correct dimensions on a strip of paper and folded it into a helix, being careful to maintain the planar peptide bonds. After a few attempts, he produced a model with physically plausible hydrogen bonds. Pauling then worked with Corey and Branson to confirm his model before publication.[6] In 1954, Pauling was awarded his first Nobel Prize "for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances"[7] (such as proteins), prominently including the structure of the α-helix.

Structure

Geometry and hydrogen bonding

The amino acids in an α-helix are arranged in a right-handed

Secondary Structure of Protein).[11]

Contrast of helix end views between α (offset squarish) vs 310 (triangular)

Similar structures include the 310 helix (i + 3 → i hydrogen bonding) and the π-helix (i + 5 → i hydrogen bonding). The α-helix can be described as a 3.613 helix, since the i + 4 spacing adds three more atoms to the H-bonded loop compared to the tighter 310 helix, and on average, 3.6 amino acids are involved in one ring of α-helix. The subscripts refer to the number of atoms (including the hydrogen) in the closed loop formed by the hydrogen bond.[12]

Ramachandran plot (φ, ψ plot), with data points for α-helical residues forming a dense diagonal cluster below and left of center, around the global energy minimum for backbone conformation.[13]

Residues in α-helices typically adopt backbone (φ, ψ)

Ramachandran diagram (of slope −1), ranging from (−90°, −15°) to (−70°, −35°). 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[14][15]

3 cos Ω = 1 − 4 cos2 φ + ψ/2

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 "downward" (i.e., toward 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.[16]

Stability

See also: Stapled peptide

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). In general, short

trifluoroethanol (TFE), or isolated from solvent in the gas phase,[17] oligopeptides readily adopt stable α-helical structure. Furthermore, crosslinks can be incorporated into peptides to conformationally stabilize helical folds. Crosslinks stabilize the helical state by entropically destabilizing the unfolded state and by removing enthalpically stabilized "decoy" folds that compete with the fully helical state.[18] It has been shown that α-helices are more stable, robust to mutations and designable than β-strands in natural proteins,[19] and also in artificially designed proteins.[20]

An α-helix in ultrahigh-resolution electron density contours, with oxygen atoms in red, nitrogen atoms in blue, and hydrogen bonds as green dotted lines (PDB file 2NRL, 17–32). The N-terminus is at the top, here.

Visualization

The 3 most popular ways of visualizing the alpha-helical secondary structure of oligopeptide sequences are (1) a helical wheel,[21] (2) a wenxiang diagram,[22] and (3) a helical net.[23] Each of these can be visualized with various software packages and web servers. To generate a small number of diagrams, Heliquest[24] can be used for helical wheels, and NetWheels[25] can be used for helical wheels and helical nets. To programmatically generate a large number of diagrams, helixvis[26][27] can be used to draw helical wheels and wenxiang diagrams in the R and Python programming languages.

Experimental determination

Since the α-helix is defined by its hydrogen bonds and backbone conformation, the most detailed experimental evidence for α-helical structure comes from atomic-resolution

NMR spectroscopy also show helices well, with characteristic observations of nuclear Overhauser effect
(NOE) couplings between atoms on adjacent helical turns. In some cases, the individual hydrogen bonds can be observed directly as a small scalar coupling in NMR.

There are several lower-resolution methods for assigning general helical structure. The

electron microscopy
is now capable of discerning individual α-helices within a protein, although their assignment to residues is still an active area of research.

Long homopolymers of amino acids often form helices if soluble. Such long, isolated helices can also be detected by other methods, such as

dipole moment
.

Amino-acid propensities

Different amino-acid sequences have different propensities for forming α-helical structure.

amino-acid 1-letter codes) all have especially high helix-forming propensities, whereas proline and glycine have poor helix-forming propensities.[28] Proline either breaks or kinks a helix, both because it cannot donate an amide hydrogen bond (having no amide hydrogen), and also because its sidechain interferes sterically with the backbone of the preceding turn – inside a helix, this forces a bend of about 30° in the helix's axis.[12] However, proline is often seen as the first residue of a helix, it is presumed due to its structural rigidity. At the other extreme, glycine
also tends to disrupt helices because its high conformational flexibility makes it entropically expensive to adopt the relatively constrained α-helical structure.

Table of standard amino acid alpha-helical propensities

Estimated differences in

kcal/mol
per residue in an α-helical configuration, relative to alanine arbitrarily set as zero. Higher numbers (more positive free energy changes) are less favoured. Significant deviations from these average numbers are possible, depending on the identities of the neighbouring residues.

Differences in free energy change per residue[29]
Amino acid 3-
letter 		1-
letter 		Helical penalty
kcal/mol
kJ/mol
Alanine Ala A 0.00 0.00
Arginine Arg R 0.21 0.88
Asparagine Asn N 0.65 2.72
Aspartic acid Asp D 0.69 2.89
Cysteine Cys C 0.68 2.85
Glutamic acid Glu E 0.40 1.67
Glutamine Gln Q 0.39 1.63
Glycine Gly G 1.00 4.18
Histidine His H 0.61 2.55
Isoleucine Ile I 0.41 1.72
Leucine Leu L 0.21 0.88
Lysine Lys K 0.26 1.09
Methionine Met M 0.24 1.00
Phenylalanine Phe F 0.54 2.26
Proline Pro P 3.16 13.22
Serine Ser S 0.50 2.09
Threonine Thr T 0.66 2.76
Tryptophan Trp W 0.49 2.05
Tyrosine Tyr Y 0.53 2.22
Valine Val V 0.61 2.55

Dipole moment

A helix has an overall

aspartate, or sometimes a phosphate ion. Some regard the helix macrodipole as interacting electrostatically with such groups. Others feel that this is misleading and it is more realistic to say that the hydrogen bond potential of the free NH groups at the N-terminus of an α-helix can be satisfied by hydrogen bonding; this can also be regarded as set of interactions between local microdipoles such as C=O···H−N.[31][32]

Coiled coils

Coiled-coil α helices are highly stable forms in which two or more helices wrap around each other in a "supercoil" structure.

electrostatic interactions. Fibrous proteins such as keratin or the "stalks" of myosin or kinesin often adopt coiled-coil structures, as do several dimerizing proteins. A pair of coiled-coils – a four-helix bundle – is a very common structural motif in proteins. For example, it occurs in human growth hormone and several varieties of cytochrome. The Rop protein
, which promotes plasmid replication in bacteria, is an interesting case in which a single polypeptide forms a coiled-coil and two monomers assemble to form a four-helix bundle.

Facial arrangements

The amino acids that make up a particular helix can be plotted on a

polar amino acids oriented toward the solvent
-exposed surface of the protein.

Changes in binding orientation also occur for facially-organized oligopeptides. This pattern is especially common in antimicrobial peptides, and many models have been devised to describe how this relates to their function. Common to many of them is that the hydrophobic face of the antimicrobial peptide forms pores in the plasma membrane after associating with the fatty chains at the membrane core.[33][34]

Larger-scale assemblies

The Hemoglobin molecule has four heme-binding subunits, each made largely of α-helices.

Myoglobin and hemoglobin, the first two proteins whose structures were solved by X-ray crystallography, have very similar folds made up of about 70% α-helix, with the rest being non-repetitive regions, or "loops" that connect the helices. In classifying proteins by their dominant fold, the Structural Classification of Proteins database maintains a large category specifically for all-α proteins.

Hemoglobin then has an even larger-scale

quaternary structure
, in which the functional oxygen-binding molecule is made up of four subunits.

Functional roles

DNA-binding helices: transcription factor Max (PDB
file 1HLO)
Bovine rhodopsin (PDB file 1GZM), with a bundle of seven helices crossing the membrane (membrane surfaces marked by horizontal lines)

DNA binding

α-Helices have particular significance in

coiled-coil (or leucine zipper) dimers of helices can readily position a pair of interaction surfaces to contact the sort of symmetrical repeat common in double-helical DNA.[35] An example of both aspects is the transcription factor
Max (see image at left), which uses a helical coiled coil to dimerize, positioning another pair of helices for interaction in two successive turns of the DNA major groove.

Membrane spanning

α-Helices are also the most common protein structure element that crosses biological membranes (

G protein–coupled receptors (GPCRs). The structural stability between pairs of α-Helical transmembrane domains rely on conserved membrane interhelical packing motifs, for example, the Glycine-xxx-Glycine (or small-xxx-small) motif.[37]

Mechanical properties

α-Helices under axial tensile deformation, a characteristic loading condition that appears in many alpha-helix-rich filaments and tissues, results in a characteristic three-phase behavior of stiff-soft-stiff tangent modulus.[38] Phase I corresponds to the small-deformation regime during which the helix is stretched homogeneously, followed by phase II, in which alpha-helical turns break mediated by the rupture of groups of H-bonds. Phase III is typically associated with large-deformation covalent bond stretching.

Dynamical features

Alpha-helices in proteins may have low-frequency accordion-like motion as observed by the Raman spectroscopy[39] and analyzed via the quasi-continuum model.[40][41] Helices not stabilized by tertiary interactions show dynamic behavior, which can be mainly attributed to helix fraying from the ends.[42]

Helix–coil transition

See also: Helix–coil transition model

Homopolymers of amino acids (such as polylysine) can adopt α-helical structure at low temperature that is "melted out" at high temperatures. This helix–coil transition was once thought to be analogous to protein denaturation. The statistical mechanics of this transition can be modeled using an elegant transfer matrix method, characterized by two parameters: the propensity to initiate a helix and the propensity to extend a helix.

In art

Julian Voss-Andreae's Alpha Helix for Linus Pauling (2004), powder coated steel, height 10 ft (3 m). The sculpture stands in front of Pauling's childhood home on 3945 SE Hawthorne Boulevard in Portland, Oregon, USA.

At least five artists have made explicit reference to the α-helix in their work: Julie Newdoll in painting and Julian Voss-Andreae, Bathsheba Grossman, Byron Rubin, and Mike Tyka in sculpture.

San Francisco area artist Julie Newdoll,[43] who holds a degree in microbiology with a minor in art, has specialized in paintings inspired by microscopic images and molecules since 1990. Her painting "Rise of the Alpha Helix" (2003) features human figures arranged in an α helical arrangement. According to the artist, "the flowers reflect the various types of sidechains that each amino acid holds out to the world".[43] This same metaphor is also echoed from the scientist's side: "β sheets do not show a stiff repetitious regularity but flow in graceful, twisting curves, and even the α-helix is regular more in the manner of a flower stem, whose branching nodes show the influence of environment, developmental history, and the evolution of each part to match its own idiosyncratic function."[12]

Julian Voss-Andreae is a German-born sculptor with degrees in experimental physics and sculpture. Since 2001 Voss-Andreae creates "protein sculptures"[44] based on protein structure with the α-helix being one of his preferred objects. Voss-Andreae has made α-helix sculptures from diverse materials including bamboo and whole trees. A monument Voss-Andreae created in 2004 to celebrate the memory of Linus Pauling, the discoverer of the α-helix, is fashioned from a large steel beam rearranged in the structure of the α-helix. The 10-foot-tall (3 m), bright-red sculpture stands in front of Pauling's childhood home in Portland, Oregon.

human growth hormone, and phospholipase A2.[46]

Mike Tyka is a computational biochemist at the University of Washington working with David Baker. Tyka has been making sculptures of protein molecules since 2010 from copper and steel, including ubiquitin and a potassium channel tetramer.[47]

See also

References

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Further reading

External links
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