Amino Acids, Proteins and Porphyrins
The Coiling & folding of protein requires
Structural organization of Proteins:
a. Primary structure: → is the Linear sequence of amino acids held together by peptide bonds (is a covalent bonds)
i. Secondary structure: The peptide chain thus formed assumes a three —dimensional secondary structure by way of folding or coiling consisting of a helically coiled, zig-zag linear or mixed form. The linkage or bonds involved in secondary structure formation is Hydrogen bonds
a. Regular helical coils established by H2 bonds (in α-helix — Intra chain hydrogen bonding is present) may be right or left handed but only right α-helix is found in protein.
b. n = 3.6 amino acid residues are present in one complete turn p (pitch) = 0.54 nm or 5.4 A°, is distance per turn along its axis
Each amino acid residues advances by 0.15 nm along the helix
c. Proline is never found in a Keratin rich α-helical structure
B. β-pleated sheat structure —
1. the hydrogen in β-pleated sheet structure is Interchain (but intra chain in a-helix)
2. may be parallel or Antiparallel
3. silk fibroin is rich in 3-pleated sheets (rich in glycine, serine, and alanine)
C. Triple helix →eg. collagen, is rich in proline and hydroxyproline and cannot form α-helix or β-pleated sheet
3. Tertiary structure : Bonds responsible are: -
a. Hydrophobic interactions (non-covalent) → major stabilizing forces for tertiary structure forming a
b. compact three-dimensional structure. Tertiary structure is biologically active structure]
c. Hydrogen bonds (non-covalent)
d. Disulfide bonds (covalent)
e. Ionic bonds.
f. Quaternary structure: Proteins that contains two or more Functional polypeptide chains associated by non- covalent forces are said to exhibit quaternary structure.
The non-covalent bonds responsible for this, are → Hydrogen bonds, hydrophobic interactions, and Ionic bonds (Electrostatic or salt bridge)