alpha helix

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helix

 [he´liks] (pl. he´lices, helixes) (Gr.)
1. a winding structure; see also coil and spiral.
2. the superior and posterior free margin of the pinna of the ear.
α-helix (alpha helix) the complex structural arrangement of parts of protein molecules in which a single polypeptide chain forms a right-handed helix.
double helix (Watson-Crick helix) the structure of deoxyribonucleic acid (DNA), consisting of two coiled chains, each of which contains information completely specifying the other chain.

α he·lix

the helical form (commonly right-handed) present in many proteins, deduced by Pauling and Corey from x-ray diffraction studies of proteins such as α-keratin; the helix is stabilized by hydrogen bonds between, differet eupeptide bonds, for example, R1R2C = O and HN(R3)R4' groups, symbolized by the center dot in R1R2C = O · HN(R3)R4. In a true α helix, there are 3.6 amino acid residues per turn of the helix and an increase of 1.5 Ǻ per residue.

alpha helix

n.
A secondary structure of proteins, characterized by a single, spiral chain of amino acids stabilized by hydrogen bonds.

al′pha-hel′i·cal (-hĕl′ĭ-kəl, -hē′lĭ-) adj.

alpha helix

A specific form of folding of the polypeptide backbone in fibrous and globular proteins; right-handed corkscrew arrangement of a polypeptide chain is maintained by intrachain hydrogen bonding. The alpha helix is a major structural protein motif deduced by Pauling and Corey, and it occurs as bundles in keratin, myosin, fibrin and the epidermis.

alpha helix

A coiled configuration of a POLYPEPTIDE chain found in many proteins. This is one of the commonest forms of secondary structure in proteins.

alpha helix

a twisted polypeptide chain which forms a helical structure in many proteins, with 3.6 amino-acid residues per turn of the helix. Successive turns of the helix are linked by weak hydrogen bonds and the structure is much more stable than an untwisted polypeptide chain. Long-chain alpha-helix construction is characteristic of structural ‘fibrous’ proteins, found in hair, claws, fingernails, feathers, wool and horn. Proteins that are intracellular are usually of the ‘globular’ type with short segments of alpha helixes.

Pauling,

Linus C., U.S. chemist and Nobel laureate, 1901-1994.
Pauling theory - a theory of narcosis pertaining to nonhydrogen-bonding agents. Synonym(s): hydrate microcrystal theory of anesthesia
Pauling-Corey helix - the helical form present in many proteins. Synonym(s): α helix
References in periodicals archive ?
Even though the most commonly distributed kind of secondary structural motif (beta strand-major binder-random coil) is characteristic for Glu residues binding [Mn.sup.2+], in proteins encoded by GC-rich genes glutamic acid residues from alpha helices became able to bind that ion too.
The difference is significant only for Glu residues in alpha helices: the usage of Lys around Glu residues which are not involved in binding is about 3 times higher than that around Glu residues binding [Mn.sup.2+].
In contrast, the usage of amino acids in alpha helices is 1.66 times lower among [Mn.sup.2+] binding residues than among all the residues (P < 0.001).
Amino acids binding [Mn.sup.2+] ions are significantly overrepresented in regions of coil between two beta strands (BCB) and significantly underrepresented in regions between two alpha helices (HCH) (see Figure 5).
In proteins encoded by GC-rich genes, the percentage of binding Glu residues situated in alpha helices increased significantly, while most of those residues bind the ion together with at least one amino acid from characteristic "beta strand-major binder-random coil" motif.
It has been shown that random coil regions can be considered to function as "connecting bridges" between major elements of secondary structure (alpha helices and beta strands) [12].
Are they nothing but random coil with two occasionally overlapping instable "i-i + 3" "main chain-main chain" hydrogen bonds, or are they short alpha helices (or even their parts) with different pattern of hydrogen bonding?
So, according to both classifications, the most of the proteins from this study contain both alpha helices and beta strands.
We constructed a histogram representing the distribution of lengths for 4272 alpha helices from the set of proteins used in this study.
Information entropy [19] of amino acid content distribution in 3/10 helices composed of 5 residues and of N-termini (the first five residues) of alpha helices was calculated according to the following equation:
So 3/10 helices from regions between two alpha helices have amino acid content very similar to the amino acid content of coils between alpha helices.
Photo: Part of the tertiary structure of the enzyme lactate dehydrogenase, showing the spatial arrangement of its alpha helices and beta-pleated sheets.