how to draw proline in a peptide chain

Peptide Torsion Angles and Secondary Structure

Dorsum to Index

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Peptide Torsion Angles

The figure below shows the three principal chain torsion angles of a polypeptide. These are phi, psi and omega.

The planarity of the peptide bail restricts omega to 180 degrees in very nearly all of the primary chain peptide bonds. In rare cases omega = 0 degrees for a cis peptide bail which, every bit stated above, usually involves proline.

Development of a Model for Alpha-Helix Structure.

Pauling and Corey twisted models of polypeptides around to detect means of getting the backbone into regular conformations which would agree with alpha-keratin fibre diffraction information. The near elementary and elegant organisation is a correct-handed spiral conformation known as the 'alpha-helix'.

The effigy beneath shows how a right-handed helix differs from a left-handed one. An easy way to remember this is to hold both your easily in front of you lot with your thumbs pointing up and your fingers curled towards y'all. For each hand the thumbs indicate the management of translation and the fingers indicate the direction of rotation.

Properties of the blastoff-helix.

1. The construction repeats itself every v.four Angstroms along the helix axis, ie we say that the alpha-helix has a pitch of 5.iv Angstroms. Alpha-helices have 3.6 amino acrid residues per plough, ie a helix 36 amino acids long would form 10 turns. The separation of residues forth the helix centrality is 5.4/3.6 or i.v Angstroms, ie the alpha-helix has a rise per balance of 1.5 Angstroms.
2. Every mainchain C=O and N-H grouping is hydrogen-bonded to a peptide bond 4 residues away (ie O(i) to North(i+4)). This gives a very regular, stable system.
3. The peptide planes are roughly parallel with the helix axis and the dipoles within the helix are aligned, ie all C=O groups betoken in the same direction and all N-H groups point the other way. Side bondage point outward from helix centrality and are generally oriented towards its amino-final end.

   

4. All the amino acids have negative phi and psi angles, typical values being -threescore degrees and -50 degrees, respectively.

Distortions of alpha-helices.

The majority of alpha-helices in globular proteins are curved or distorted somewhat compared with the standard Pauling-Corey model. These distortions arise from several factors including:

1. The packing of buried helices confronting other secondary structure elements in the core of the protein.
2. Proline residues induce distortions of around twenty degrees in the direction of the helix axis. This is because proline cannot grade a regular alpha-helix due to steric hindrance arising from its cyclic side chain which also blocks the main chain Northward cantlet and chemically prevents information technology forming a hydrogen bond. Janet Thornton has shown that proline causes two H-bonds in the helix to be broken since the NH group of the following residue is also prevented from forming a skilful hydrogen bond. Helices containing proline are usually long perhaps because shorter helices would be destabilised past the presence of a proline balance besides much. Proline occurs more commonly in extended regions of polypeptide.
3. Solvent. Exposed helices are frequently bent away from the solvent region. This is because the exposed C=O groups tend to indicate towards solvent to maximise their H-bonding chapters, ie tend to form H-bonds to solvent too as N-H groups. This gives ascension to a bend in the helix axis.

   

4. iii(10)-Helices. Strictly, these grade a distinct class of helix but they are always short and frequently occur at the termini of regular blastoff-helices. The proper name 3(10) arises because there are three residues per turn and ten atoms enclosed in a band formed by each hydrogen bond (note the hydrogen atom is included in this count). There are main chain hydrogen bonds between residues separated by three residues along the chain (ie O(i) to N(i+3)). In this classification the Pauling-Corey alpha-helix is a 3.six(13)-helix. The dipoles of the 3(10)-helix are not so well aligned as in the alpha-helix, ie information technology is a less stable structure and side chain packing is less favourable.

   

The Beta-Canvas.

Pauling and Corey derived a model for the conformation of fibrous proteins known equally beta-keratins. In this conformation the polypeptide does not form a roll. Instead, it zig-zags in a more extended conformation than the blastoff-helix. Amino acid residues in the beta-conformation accept negative phi angles and the psi angles are positive. Typical values are phi = -140 degrees and psi = 130 degrees. In dissimilarity, blastoff-helical residues take both phi and psi negative. A section of polypeptide with residues in the beta-conformation is refered to every bit a beta-strand and these strands can associate by master chain hydrogen bonding interactions to class a beta sheet.

In a beta-canvass two or more polypeptide chains run alongside each other and are linked in a regular manner past hydrogen bonds betwixt the main concatenation C=O and N-H groups. Therefore all hydrogen bonds in a beta-sheet are between different segments of polypeptide. This contrasts with the alpha-helix where all hydrogen bonds involve the aforementioned element of secondary structure. The R-groups (side bondage) of neighbouring residues in a beta-strand indicate in reverse directions.

Imagining two strands parallel to this, one above the plane of the screen and i behind, it is possible to grasp how the pleated appearance of the beta-canvas arises. Annotation that peptide groups of side by side residues betoken in contrary directions whereas with alpha-helices the peptide bonds all point i way.

The axial distance between side by side residues is three.5 Angstroms. In that location are ii residues per echo unit which gives the beta-strand a 7 Angstrom pitch. This compares with the alpha-helix where the centric distance between side by side residues is merely one.5 Angstroms. Conspicuously, polypeptides in the beta-conformation are far more extended than those in the alpha-helical conformation.

Parallel, Antiparallel and Mixed Beta-Sheets.

In parallel beta-sheets the strands all run in one direction, whereas in antiparallel sheets they all run in contrary directions. In mixed sheets some strands are parallel and others are antiparallel.

Below is a diagram of a three-stranded antiparallel beta-sheet. Information technology emphasises the highly regular design of hydrogen bonds between the main chain NH and CO groups of the constituent strands.

In the classical Pauling-Corey models the parallel beta-sheet has somewhat more distorted and consequently weaker hydrogen bonds between the strands.

Beta-sheets are very common in globular proteins and most comprise less than 6 strands. The width of a six-stranded beta-sheet is approximately 25 Angstroms. No preference for parallel or antiparallel beta-sheets is observed, but parallel sheets with less than four strands are rare, perhaps reflecting their lower stability. Sheets tend to be either all parallel or all antiparallel, merely mixed sheets do occur.

The Pauling-Corey model of the beta-canvass is planar. However, about beta-sheets plant in globular protein X-ray structures are twisted. This twist is left-handed equally shown below. The overall twisting of the sheet results from a relative rotation of each residue in the strands by thirty degrees per amino acid in a correct-handed sense.

Parallel sheets are less twisted than antiparallel and are always cached. In contrast, antiparallel sheets can withstand greater distortions (twisting and beta-bulges) and greater exposure to solvent. This implies that antiparallel sheets are more stable than parallel ones which is consistent both with the hydrogen bail geometry and the fact that small parallel sheets rarely occur (encounter higher up).

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Dorsum to the Top
j.cooper 2/ane/95

Source: http://www.cryst.bbk.ac.uk/PPS95/course/9_quaternary/3_geometry/torsion.html

Posted by: reisingermonce1983.blogspot.com

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