Secondary structure refers to regular, recurring arrangements in space of adjacent amino acid residues in a polypeptide chain. It is maintained by hydrogen bonds between amide hydrogens and carbonyl oxygens of the peptide backbone. The major secondary structures are α-helices and β-structures.

Differences between Alpha Helix and beta-pleated sheet (beta sheet)

Alpha Helix

beta-pleated sheet (beta sheet)

In alpha helix; amino acid chain is in a  right-handed spiral conformation or clockwise

 

In beta sheets; amino acid chain is in an almost fully extended conformation, linear or ‘sheet like’

Forms a rod like structure

Sheet like structure

It is held together by hydrogen bonds between the C=O (carbonyl Oxygen) of residue 1 and the NH (amide H) of residue 4.

It is held by hydrogen bond between the N−H groups in the backbone of one strand and C=O groups in the backbone of the adjacent strands.

Intramolecular hydrogen bonding within the polypeptide chain

intermolecular hydrogen bonding, between two separate chains or beta strands. 

Alpha helix: Only one type

Two types: Parallel beta sheets: The chains or beta strands  run parallel (all N terminals on one end)

anti-parallel beta sheets: Chains runs in opposite direction (N terminal and C terminal ends alternate)

The R groups of the amino acids stick outward from the α helix, where they are free to interact

 R groups extend above and below the plane of the sheet

It can be a single chain

Cannot exist as a single beta strand

Dimensions

An average alpha-helix is 10 residues long (15 Â0 in length), generally alpha-helices can range between 4 to 40 residues in length.

It completes one turn every 3.6 residues

A β-strand is a polypeptide chain typically 3 to 10 amino acids long with backbone in an extended conformation.

 

 

Most preferred residues: as Ala, Glu, Leu and Met Least preferred residues: Pro and Gly

Proline, which cannot fit into a helix, and introduces a kink often called as helix terminator

amino acids such as tryptophan, tyrosine, and phenylalanine, which have large ring structures in their R groups, are often found in β pleated sheets, as this structure provides plenty of space for the side chains

Many proteins contain both α helices and β pleated sheets

Occurrence in  Keratin, hemoglobin, myoglobin has high proportion of alpha helix structure

Silk fibres or fibroin

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