Non-repetitive structures:

Other than helix and sheets, which form major portion of proteins structure, there are other non repetitive folds or structures. They are referred as coil or loop conformations. The straight runs of secondary structures are reversed by loops or coil structure. -turns connect the ends of two adjacent segments of anti-parallel b sheets.This turn is 180o involving 4 amino acids.

Type I and type II b turns: differ by 180o flip of peptide unit linking residue 2 and 3.

Type II always have glycine as the third residue

Protein Tertiary and Quaternary structure:

•Overall three dimensional arrangement of all the atoms in a protein (including folding and spatial organization of different secondary structure) is referred as tertiary structure.

•Many proteins contain more than one polypeptide chains and their three dimensional structure is dependent on the subunit organization. The spatial organization of different subunits in protein is reffered as quaternary structure.

•Protein are classified in two types based on higher order structure;

Fibrius proteins: Polypeptide chains arranged in long stands or sheets.

Globular proteins: Polypeptide chains folded into a spherical or globular shape

Fibrous proteins: keratin, collagen and silk fibroin •Structural function to provide strength and flexibility

•The fundamental structural unit is simple repeat of an element of secondary structure•Mostly water insoluble

Priciple of permanent waving of hair:

Chemical manipulation of keratin structure

Collagen: Found in connective tissue, tendons, cortilage

Structure left handed helix with three AA residue per turn

Three separate polypeptide chains are supertwisted about each other35% Gly, 11% Ala, 21% Pro or hydroxyproline

Food product Gelatin (denatured collagen) is derived from collagen.

Closely packed because of Gly and sharp twisting by proline residue

Intr and inter-molecular crosslinking through Lys and His

Fibroin: A b pleated sheet

Insects, silkworm, spiders produce fibroin to fabricate cocoon, web, nests.

Rich in Gly and Ala residues, permiting close packing with interlocking side chain of AA residues.

The overall structure is stabilised by extensive hydrogen bonding

Silk does not stretch much because the b sheets are already in extended form. The structure is flexible because of week interaction between the sheets.

Comparison of compact globular structure of BSA with its extended forms ( helics and sheets)

Globular proteins: Enzymes and regulatory proteins

Different segments of polypeptide chains fold back on each otherResulting in a compact form.

Tertiary structure of Myoglobin