Protein Structure. Protein Structure Tertiary & Quaternary

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1 Lecture 4 Protein Structure Protein Structure Tertiary & Quaternary Dr. Sameh Sarray Hlaoui

2 Primary structure: The linear sequence of amino acids held together by peptide bonds. Secondary structure: The peptides bonds allow some rotation that produces the secondary structures: examples: α-helix and β-pleated sheet α-helix: - helical (spiral) arrangement of a single polypeptide chain stabilized by intrachain hydrogen bonding between H attached to peptide N and O atom attached to peptide C) - Residue n and n+4 β-pleated sheet : - Unlike α helix, β-sheet is formed by interchain hydrogen bonds between 2 or more neighbouring polypeptides. - b-strands are often visualized as broad arrows: When the adjacent chains run in: Opposite directions (N C and C N): anti-parallel b-pleated sheet Same direction (N C and N C): parallel b-pleated sheet

3 Tertiary Structure it s the three dimensional (3D) structure of protein formed when the secondary structures further fold up on themselves (interaction between secondary structure elements yields to tertiary structure). This folding determine its function These interactions include the peptide backbone as well as the side chain. So there is interactions between: -Side chains -Side chains and polypeptide backbone In the tertiary structure, the hydrophobic side chains held interior while the hydropholic and ionized groups are on the surface (outside) of the protein.

4 Tertiary Structure The tertiary structure is stabilized by: The Hydrogen bonds. Disulphide bonds (S-S) Ionic interactions: between negative and positive group if the chains folded in such a way that they were close to each other. Hydrophobic interactions Covalent bond.

5 Exemple1: Pyruvate Kinase Tertiary Structure β-pleated Sheets, α-helix, Random Coils There may be more than one type of secondary structure within the tertiary structure of a protein

6 Exemple 2: Myoglobin Organization -Monomeric protein present in heart and skeletal muscles. - It consists of 8 separate -helix (A through H) connected by short non helical regions

7 Domains Domain: a distinct region (sequence of amino acids) of a protein. Structural domain: an independently-folded part of a protein that folds into a stable structure. Domains are separated by a loosely folded region. A protein may consist of only a single, or may have many domains. Larger proteins generally consist of connected structural domains. Structural domains are often functional units: - Ligand binding (e.g. a peptide) - Transmembrane: spanning the plasma membrane. - Contains the catalytic site (in enzymes) - DNA-binding (transcription factors)

8 Quaternary Structure Proteins possess a quaternary structure if they consist of two or more polypeptide chains joined by forces other than peptide bonds. The forces that stabilize these aggregates are hydrogen and ionic bonds formed between residues on the surface of the polypeptide chains Such proteins are termed oligomers or multimers, and the individual polypeptide chains of which they are composed are termed protomers, monomers, or subunits Individual chains may be identical, somewhat similar, or totally different

9 Quaternary Structure: Hemoglobin Tetramer

10 Hierarchical nature of protein structure Primary structure (Amino acid sequences) Secondary structure (α-helix, β-sheet ) Tertiary structure (Three-dimensional structure formed by assembly of secondary structures) Quaternary structure (Structure formed by more than one polypeptide chains)

11 Protein Denaturation Results in the loss of the native conformation (shape) of the protein due to destruction of quaternary, tertiary, and/or secondary structures Leads to a loss of protein function Denaturing agents include: High temperature Large changes of ph, Others: alcohols, urea, heavy metals, etc

12 THE END!