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Proteins, amino_acid_structure, Amino acids are linked to one another a…

- - - - Primary (1º) Structure
        
        The first level of structural organisation in a protein is the order / sequence of amino acids which comprise the polypeptide chain
        
        The primary structure is formed by covalent peptide bonds between the amine and carboxyl groups of adjacent amino acids
        
        Primary structure controls all subsequent levels of protein organisation because it determines the nature of the interactions between R groups of different amino acids
      - Secondary (2º) Structure
        
        The secondary structure is the way a polypeptide folds in a repeating arrangement to form α-helices and β-pleated sheets
        
        This folding is a result of hydrogen bonding between the amine and carboxyl groups of non-adjacent amino acids
        
        Sequences that do not form either an alpha helix or beta-pleated sheet will exist as a random coil
        
        Secondary structure provides the polypeptide chain with a level of mechanical stability (due to the presence of hydrogen bonds)
        
        In pictures, alpha helices are represented as spirals (purple ; left) and beta-pleated sheets as arrows (blue ; right)
      - Tertiary (3º) Structure
        
        The tertiary structure is the way the polypeptide chain coils and turns to form a complex molecular shape (i.e. the 3D shape)
        
        It is caused by interactions between R groups; including H-bonds, disulfide bridges, ionic bonds and hydrophobic interactions
        
        Relative amino acid positions are important (e.g. non-polar amino acids usually avoid exposure to aqueous solutions)
        
        Tertiary structure may be important for the function of the protein (e.g. specificity of active site in enzymes)
      - Quaternary (4º) Structure
        
        Multiple polypeptides or prosthetic groups may interact to form a single, larger, biologically active protein (quaternary structure)
        
        A prosthetic group is an inorganic compound involved in protein structure or function (e.g. the heme group in haemoglobin)
        
        A protein containing a prosthetic group is called a conjugated protein
        
        Quaternary structures may be held together by a variety of bonds (similar to tertiary structure)
    - - Dipeptides & Peptide bonds