Structure of protein

FOUR LEVELS OF PROTEINS

1. s Structure of protein R.HARISHMA, I MSC MICROBIOLOGY, 18PY07.

2. PROTEIN Derived from Greek word “ protos ” – meaning primary. It plays a variety of activities. Each protein is a polymer of aminoacid. Amino acids linked by a peptide bond- polypeptide. Each amino acid consists of a carbon atom, an amino group, a carboxyl group as a side chain.

3. STRUCTURE OF PROTEIN Four levels of protein structure, Primary structure Secondary structure Tertiary structure Quaternary structure

4. PRIMARY STRUCTURE

5. PRIMARY STRUCTURE Linear arrangement of aminoacids present in the polypeptide chain. Peptide bond. Its starts amino N terminal end and ends with carboxyl C terminal end. Peptide bond is rigid and planer. All peptide bond in proteins occurs in trans configuration.

6. The rotation is permitted about the bond between nitrogen and alpha carbon atoms and between the alpha carbon and carbonyl carbon atoms. Two rotatory angles, phi ( φ) Psi ( ψ) Phi and psi have a value between −180º to +180º

7. RAMACHANDRAN PLOT To identify secondary structure of proteins depending upon the rotation (or) degree of freedom of dihedral angles. CHARACTERISTICS OF RAMACHANDRAN PLOT: Four quadrants. Y axis – ψ and X axis – φ . Definite position in this plot indicate definite secondary structure of protein.

8. RAMACHANDRAN PLOT

9. Both allowed and disallowed region. Glysine – Most allowed region. Proline – Least allowed region. Two phi and psi angles are 180º .

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11. SECONDARY STRUCTURE

12. SECONDARY STRUCTURE Regular folding of primary structure is called secondary structure. Folding and hydrogen bonding between neighbouring aminoacids – results to form rigid and tubular structure called helix . Pauling and corey identified , Alpha helix Beta pleated sheet

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14. ALPHA - HELIX Coiled structure. Peptide bond would form a right handed helical structure by simple twist about the α-carbon-to-nitrogen and the α-carbon-to-carboxyl carbon bonds. It can be right handed (or) left handed. Stabilized by hydrogen bonded to the NH and CO group of main chain.

15. Pitch of 5.4 Å . Per turn of helix 3.6 aminoacids are present. One residue occupies 1.5 Å . Conformations with φ = − 60º and ψ = − 45º to − 50º . Alpha helix found in protein alpha keratin .

16. BETA PLEATED SHEET Pauling and corey identified a stable conformation – Beta pleated sheet. Stabilized by hydrogen bond between NH and CO groups in different polypeptide strands. One residue occupies 3.5 Å . Two types, Parallel β pleated sheet Antiparallel β pleated sheet

17. Parallel beta pleated sheet: The polypeptide strands run in the same direction. N-C terminus .

18. Antiparallel beta pleated sheet: The polypeptide strands run in the opposite direction. The N- is opposite to the C terminus.

19. Both parallel and antiparallel beta sheets have similar structure. Random coil: It is the third type of secondary structure.

20. Other secondary structures: β turn Collagen triple helix

21. TERTIARY STRUCTURE

22. TERTIARY STRUCTURE The tertiary structure of protein is the overall three Dimensional shape that arises from all of the secondary structures of its polypeptide chain. functions are depends… Globular (or) fibrous tertiary structure. Do not occur randomly.

23. Forces involved in stabilizing tertiary structure, including Electrostatic forces Vander Waals Hydrogen bond Disulfide bonds For example, Myoglobin

24. Myoglobin Relatively small. Compact macromolecule. Oxygen binding hemi protein. Found in muscle cells. Molecules contain single polypeptide chain of 153 aminoacid residues.

25. MYOGLOBIN

26. QUATERNARY STRUCTURE

27. QUATERNARY STRUCTURE Connectivity between two (or) more polypeptide chain. Complexity – very high Some globular proteins consists of two (or) more interacting peptide chains. These chains may be identical (or) different in primary structure.

28. Forces are involved, Disulfide Hydrogen Hydrophobic Ionic bonds Example, Hemoglobin

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