What causes the insulin molecule to fold into the alpha helix and beta sheet pattern

In the following we will focus on the full general aspects of protein secondary structure. Many of the features discussed hither are essential for practical applications − for example in sequence alignment and analysis, homology modelling and analysis of model quality, in planning mutations or when analyzing protein-ligand interactions. The α-helix
The most common blazon of secondary construction in proteins is the α-helix. Linus Pauling was the first to predict the existence of α-helices. The prediction was confirmed when the first three-dimensional structure of a protein, myoglobin (past Max Perutz and John Kendrew) was determined by X-ray crystallography. An case of an α-helix is shown on the paradigm below. This type of representation of a poly peptide structure is called "sticks representation". To become a improve impression of how a helix looks like, just the main chain of the polypeptide is shown, no side chains. There are 3.half dozen residues/plow in an α-helix, which ways that at that place is i residue every 100 degrees of rotation (360/3.half dozen). Each remainder is translated i.5 Å along the helix axis, which gives a vertical distance of five.4 Å between structurally equivalent atoms in a turn (pitch of a turn). The repeating structural pattern in helices is a result of repeating (like) φ and ψ values, which is reflected in the clustering of the torsion angles within the helical region of the Ramachandran plot . When looking at the helix in the figure below, notice how the carbonyl (C=O) oxygen atoms (shown in crimson) signal in one direction, towards the amide NH groups 4 residues away (i, i+4). Together these groups form a hydrogen bond, one of the main forces in the stabilization of secondary construction in proteins. The hydrogen bonds are shown on the figure as dashed lines.

The α-helix is non the only helical structure in proteins. Other helical structures include the 3_10 helix, which is stabilized past hydrogen bonds of the type (i, i+3) and the π-helix, which is stabilized by hydrogen bonds of the type (i, i+5). The 3_10 helix has a smaller radius, compared to the α-helix, while the π-helix has a larger radius. The beginning detailed analysis of the occurrence of the π-helix in proteins, based on the analysis of entries in the Poly peptide Data Depository financial institution (PDB), was published by Fodje & Al-Karadaghi, 2002 .
Nosotros should also annotation that in addition to the "simple" helical structures mentioned hither, there is a number of so-called coiled-whorl structures, in which 2 or more α-helices together build higher-order helical structures. The β-sheet
The 2d major secondary structure chemical element in proteins is the β-sheet. β-sheets consist of several β-strands, stretched segments of the polypeptide chain kept together by a network of hydrogen bonds between side by side strands. An case of a β-sheet, with the stabilizing hydrogen bonds between side by side strands (shown as dotted lines), is shown in the prototype below:

Information technology is of import to note that unlike in helices, the residues informing hydrogen bonds betwixt the adjacent strands are separated from each other by long segments of the amino acid sequence.

In the following image the same β-sheet is shown, this time in the context of the 3D structure to which it belongs and in a then-called "ribbon" representation (the coloring here is co-ordinate to secondary structure

- β-sheets in yellow and helices in magenta). Each β−strand is represented by an arrow, which defines its direction starting from the Due north- to the C-terminus. When the strand arrows point in the same management, we call such β-sail parallel (the protein PDB code is 1G8P, BchI subunit of magnesium chelatase). You lot may likewise find a β-hairpin, two strands connected by a loop in the left corner of the image:

In the prototype below you can come across that the strand arrows point in opposite directions, which is a characteristic of an anti-parallel β-sheet (this protein PDB code is 1USR, Newcastle illness virus hemagglutinin-neuraminidase).

Loops, turns and hairpins
When there are but 2 anti-parallel β-strands, like in the figure beneath, it is called a β-hairpin.

The loop between the two strands is chosen a β-turn. Short turns and longer loops play an important office in poly peptide 3D structures, connecting together strands to strands, strands to α-helices, or helices to helices. The amino acid sequences in loop regions are oftentimes highly variable inside a poly peptide family. But in some cases, when a loop has some specific role, for example interaction with another protein, the sequence may be conserved. Loop length in proteins from organisms living at elevated temperatures (thermophilic organisms) is usually shorter than in protein from lower-temperature family members, presumably to give a poly peptide additional stability at high temperatures, preventing its unfolding and denaturation. During sequence alignment and homology modeling , when it is essential to have an accurate sequence alignment, the highly variable length of loop regions justifies the localization of insertions and deletions in the amino acid sequence to loop regions.

Structural motifs that contain combinations of helices, helices and strands, etc., are closely linked to protein fold. For this reason, when viewing a poly peptide 3D structures, it is an advantage to exist able to recognize the secondary construction elements and to identify structural motifs. In the next section nosotros will examine some of the ways by which secondary structure elements connect to each other, forming mutual structural

motifs and folds .

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Source: https://proteinstructures.com/structure/secondary-structure/