Proteins perform important functions like binding, catalysis, and acting as structural elements. There is a linear relationship between a gene's DNA sequence and the amino acid sequence of its encoded protein. Proteins have several levels of structure including primary, secondary, tertiary, and quaternary. The secondary structure includes alpha helices and beta sheets formed via hydrogen bonds between backbone peptide groups. Tertiary structure involves folding of these secondary elements into compact 3D structures stabilized by various interactions. Quaternary structure refers to complexes of multiple polypeptide chains held together by intermolecular forces.

1. BCH 803:
Protein Structure And Function

2. Some of biochemical functions performed by proteins:
 Binding – specific recognition of other molecules
 Catalysis – most of the catalysts are proteins
 Switching – molecular switches to control cellular processes
 Structural proteins – major structural elements of living systems
There is a linear relationship between the DNA base sequence of
a gene and the amino acid sequence of the protein.

3. Levels of organization of protein

4. The Amide/Peptide Bond
Amide/peptide bonds are stable in water at neutral pH and the properties of the peptide bond have
important effects on the stability and flexibility of polypeptide chains in water.

5. The Polypeptide Chain

6. The Alpha Helix

7. The turning of polypeptide chain to form alpha helix

8. The Forms of Alpha
helix

11. Chemical interactions that Stabilize
Proteins
 Hydrogen Bond
 Ionic Bond
 Hydrophobic interactions
 van der waals interactions
 Disulphide Bond/Bridge
 Coulombic interactions

12. Proteins showing mixture of α-helix and
Sheet

13. Connections between adjacent polypeptide strands in beta pleated
sheet

14. The Triple helix of Collagen

15. The Tertiary structure of
protein
Structure of sperm whale myoglobin – 153 residues folded into 8 alpha
helix
It is the folding of its secondary structural element

16. Structure of the Jack Bean protein – Concanavalin A
Structure of the Human Carbonic anhyydrase
Globular proteins consists of different proportions of α-helix and ß-sheets
The location of side chains varies with polarity: Nonpolar residues in the interior,
Charged polar residues on the surface and Uncharged polar residues on the surface.
Globular proteins are efficiently packed adopting relax conformation.

17. The Super secondary structure or Motif

18. Domains
Immunoglobulin fold
Retinol Binding protein
Domains are two or more globular clusters of approximately 200 residues
Domains are structurally independent units that each have the characteristics of a small globular protein
Domains are classified as α domains (exclusively helices), ß domains (exclusively ß sheets)and α/ ß domains
(containing α helices and ß sheets)

23. Quaternary structure
 Many proteins are composed of more than one polypeptide chains called
oligomers
 Each polypeptide chain is monomer; dimers-2, trimer-3, tetramer-4,
pentamers-5, hexamers-6
Oligomers composed of only one type of monomers e.g keratin is homotrimer
Oligomers composed of monomers encoded by different genes-heterodimers

24.  Intermolecular interactions dependon complementarity
Hydrogen-bond donors are opposite acceptors
Non-polar groups are opposite other non-polar groups
Positive charges are opposite negative charges
Protein –stabilizing interactions contribute to the formation of intermolecular
interfaces e.g hydrophobic interaction, hydrogen bond, salt bridges, disulphide
bond and metal ion ligation occur at the interfaces of protein-protein or
protein-peptidecomplexes.

25.  Identical subunits are usually symmetric because in order to interact, each
subunit must possess binding region and its complement
Non-identical subunits form asymmetric complex