Proteins Structure: Secondary Structure

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Proteins Secondary Structural Organizational assumption points are

  1. No rotation occurs round the peptide bond (as it is partly double bonded in nature).
  2. The chain of amino acids form a rhythmical structure – forming a repeating pattern.
  3. That the maximum number of interactions from Hydrogen bonding possible are occurring, independent of the type of residue (amino acid).

Proteins Structure: Secondary Structure

Now to explain these points:

1. As mentioned, the C-N bond is partly double bonded and so does not rotate. The bond length of a normal C-N bond is 1.49Å (angstroms, click here for more info), while the length of a normal C=N bond is 1.28Å. The length of the peptide bond is between these, at 1.28Å.

Proteins Structure: Secondary StructureThis is due to the C-N bond resonating between single and double bonded forms, as shown above.

2. Two different folding points exist. These are called phi and psi. A perfect helix structure (covered later) needs both phi (Φ) and psi (Ψ) to be at an angle of about -60 degrees.
Proteins Structure: Secondary Structure3. Hydrogen bonds occur between the C=O and H-N of other amino acids. In α helixes, the C=O: would form a hydrogen bond to the N-H  of 4th residues ahead in the spiral (directly above).

Proteins Structure: Secondary Structure

The Secondary Structure of the Proteins can be classified into 3 types based on the Number of Polypeptide chains present in polypeptide molecule. They are,

a) The α helix – Having One Polypeptide Chain

b) The β-pleated sheet – Having Two Polypeptide chains

c) The Triple helical structure – Having Three Polypeptide chains

1. The α helix:

  • The α helix consists with one polypeptide chain.
  • It is present both in the fibrous proteins that in globular proteins .
  • It characterizes the keratin α as well as fibrinogen and myosin.
  • This helical shape is the result of intramolecular chemical bonds leading to the structure of spiral staircase.
  • The right handed α helix is more stable than left handed helix.
  • This helical shape has no right of 0.54 nm.
  • Certain amino acids (particularly PROLINE) disrupts the α helix. Larger number of acidic (Asp, Glu) or Basic (Lys, Arg and His) amino acids also interfere with α helix structure.
  • Stability is ensured by:
    • hydrogen bonds which are formed between the groups C = O and NH 4 amino acids apart. these links have an inclination of 30 °.
    • the Van der Waals interactions.
    • its length plus a helix α is, the more stable it is a very strong cooperativity.
  • In a turn of the helix is 3.66 amino acids, and travels a distance of 0.54nm. The spacing of each amino acid is o.15nm. 18 amino acids there are 5 rounds.
  • In general, an α helix consists of 5 to more than 40 amino acids.
    • Amino acids promote the formation of α helix are Ala , Glu , Leu , Met .
    • Amino acids are bad trainers Pro , Gly , Tyr , Ser .
  • Α helices can be hydrophilic , amphipathic or hydrophobic .
  • It depends on the amino acid composition of the propeller.
  • Indeed, the amino acid radicals are turning out the axis of the helix, standard terms their response to their environment. Thus, if the α helix contains only hydrophobic amino acids , so it is put in contact with hydrophobic surfaces, such as the lipid bilayer.
  • If the hydrophobic residues are positioned on one side and hydrophilic residues on the other side, the α helix is amphipathic (or amphiphilic ).
  • That is to say, we will find the interface of hydrophilic and hydrophobic regions .

2. The β-pleated sheet:

  • The β-pleated sheet structure proposed by Pauling and Corey.
  • The β-pleated sheet structure have two Polypeptide chains.
  • It consists of the juxtaposition of β strands, chain conformation very stretched.
  • Chains are presented in “Pleated sheet “(to take the first topographical sense- succession of “roofs”) .
  • Involved in the peptide bonds that cross-linking and there are many bends.
  • Less hydrogen bonds between the strands,
  • The beta pleated sheet structure can be divided into two types based on the orientation of peptide chains. in a sheet, may be parallel or antiparallel .
  • In Parallel sheet structure, the orientation of the two polypeptide chains are in the same direction. The Amino groups (-NH2) in the two polypeptide chains are in the same direction. Eg: β-Keratin
  • In Anti-Parallel sheet structure, the orientation of the two polypeptide chains are in the opposite direction. The Amino groups (-NH2) in the two polypeptide chains are in the opposite direction. Eg: Silk Fibroin

Proteins Structure: Secondary Structure

  • This type of sheet is more stable because the hydrogen bonds are in perfect alignment.
  • After the α helices, they dominate in the secondary structures of proteins are very often present in β sheets: Gly , Val , Ile (3 nonpolar amino acids.)
  • In vivo , the β sheets do not have a really flat 3D structure.They are subjected to a torsion in proteins.
  • They are represented by a bunch of arrows indicating the direction of strand: the amino terminus to the carboxy terminus, and torsion.
  • They are frequently found buried in the tertiary structure of globular proteins.

3. Triple Helical Structure:

  • Collagen is the Rod-shaped molecule and most abundant protein of  mammals.
  • The structure of Collagen is in Triple helical in structure.
  • It is the principal structural element of the human body and make up 25% o 33% of all the body protein.
  • It is found in the connective tissues such as tendons, cartilages, the organic matrix of bones and the cornea of the eye.
  • Every third amino acid is Glycine in the Collagen.
  • In triple helical structure, three polypeptide chains are twisted around each other itself.

4.Other Secondary structures:

a) Loops and bends (or towers):

Around one third of the amino acids in a protein are part of loops or elbows to U-turns of the peptide chain. Elbows usually bind two antiparallel β strands. They include 2-4 amino acids are short. The shorter they are, the fewer possible spatial conformations. For stabilized, there may be hydrogen bonding between the first and fourth amino acid. Amino acids are good trainers elbows Gly and Pro.

loops are longer and therefore comportment more than 4 amino acids. They therefore allow more possible conformations. All amino acid loops do not participate in intramolecular hydrogen bonds. This allows for an easier interaction with the solvent.  Generally, there are loops between: helices α, α helices and β strands, strands or parallel β sheets of different transcription factors have a very special reason: helix-loop-helix .

b) Poly-gly, pro-poly left helix collagen:

Poly-pro and poly-Gly are synthetic polymers of proline and glycine . aqueous solvent in poly-pro has a left helix conformation ,poly-gly oscillates between left propeller and β sheet . chains naturally containing these two amino acids can therefore have a left-handed helix structure as collagen . The left propeller is smaller than α helices, they account for only three amino acids per turn of helix.

c. Super-secondary structures:

  • They concern optionally as globular proteins . These are flexible and have a wide variety of activities. The structures (or patterns) super-secondary represent typical associations of frequent flyers and propellers.
  • The pattern of ” EF hand “of calmodulin super secondary is a helix-loop-helix. The loop will accommodate a calcium atom. Some biological properties are related to these structures.
  • Quote also: the Greek key motif (4 β strands), the leucine zipper (two α helices), the reason zinc finger (1 loop, a zinc atom, two Cys + 2 His or 4 Cys ), the strand pattern βα helixβ strand .
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