Proteins

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  • Proteins
    • Amino acids
      • Primary structure
        • A  condensation  reaction forms a covalent peptide bond and water is eliminated
          • A  polypeptide chain is formed.
            • Amino acids in a polypeptide chain are often called amino acid residues as part of the molecule is lost
            • To calculate the number of different possibilities we need to times 20 for every amino acid
        • The primary structure of the protein is given by the specific sequence of amino acids that make up the protein
          • This is determined by the mRNA
          • Protease enzymes catalyse the hydrolysis reactions
            • Hormones have to be broken down so their effects are not permanent. The target cell usually contains an enzyme to do so.
            • Older skin is less able to rebuild the protein collagen
        • Secondary structure
          • This refers to the coiling and pleating of parts of the polypeptide molecule
          • Tertiary structure
            • This refers to the overall 3D structure of the final polypeptide or protein molecule
              • Globular proteins
                • Roll up into a compact globe with hydrophobic R-groups on the inside and hydrophilic on the outside - water-soluble
                • Have metabolic roles e.g. enzymes and antibodies
              • Fibrous proteins
                • Form fibres and are usually insoluble
                • Have structural roles e.g. collagen and keratin
            • Held in place by a number of different bonds and interactions
              • Disulfide bridges happen between two CYSTEINE molecules (R group is sulfur) - only broken by reducing agents
              • Ionic bonds between two oppositely charged amino acids
              • Hydrogen bonds
              • Hydrophobic and hydrophilic interactions
            • Quaternary structure
              • Haemoglobin
                • Globular transport protein
                • Made of four polypeptide subunts - each are held together by tertiary bonds
                • Each subunit  contains a prosthetic haem group which contains a Fe2+ ion which binds with the oxygen molecule
              • Collagen
                • Fibrous structural protein
                • Made up of three polypeptide chains with hydrogen bonds in between
                • The collagen molecules then form covalent bonds called cross-links with another collagen molecule. This is called a collagen fibril. Mnay fibrils = a fibre
                • Functions
                  • In artery walls to prevent bursting at high pressure
                  • Tendons between muscles and bones
                  • Bones, cartilage and connective tissue are also made of caollagen
      • Excess amino acids are toxic due to the amino group
    • Are structural components e.g. of muscle and bone
    • Are enzymes, antibodies and many hormones
    • Are membrane carriers and pores e.g. for active transport and facilitated diffusion

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