Proteins

• Proteins are made from long chains of amino acids
• The monomers of proteins are amino acids
• A dipeptide is formed when two amino acids join together
• A polypeptide is formed when more than two amino acids join together
• proteins are made up of one or more polypeptide
• Amino acids have the same general structure - a carboxyl group, an amine or amino group and an R group
• Carboxyl group - COOH
• An amine or amino group = NH2
• All living things share a bank of only 20 amino acids. The only difference between them is what makes up their R group.
• Polypeptides are formed by condensation reactions
• Amino acids are linked together by condensation reactions to form polypeptides. A molecule of water is released during the reaction. The bonds formed between amino acids are called peptide bonds. The reverse happens during digestion
• Proteins have four structural levels:

  1. Primary structure
  2. Secondary structure
  3. Tertiary structure
  4. Quaternary Structure
• Primary Structure - This is the sequence of amino acids in the polypeptide chain
• Secondary structure - the polypeptide chain doesn't remain flat and straight hydrogen bonds form between the amino acids in the chain this makes it automatically coil into an alpha helix or fold into a beta pleated sheet
• Tertiary structure - the coiled or folded chain of amino acids is often coiled and folded further. More bonds form between different parts of the polypeptide chain, including hydrogen bonds and iconic bonds (attractions between negative and positive charges on different parts of the molecule)
• Disulphide bridges also form whenever two molecules of the amino acid cysteine come close together - the sulfur atom in one cysteine bonds to the shofar atom in the other. For proteins made from a single polypeptide chain, the tertiary structure forms their final 3d structure
• Quaternary structure - Some proteins are made of several different polypeptide chains held together by bonds. The quaternary structure is the way these polypeptide chains are assembled together. For proteins made from more than one polypeptide chain (e.g. haemoglobin, insulin, collagen) the quaternary structure is the proteins final 3D structure
• Proteins have a function in enzymes by them being usually roughly spherical due to the tight folding of the polypeptide chains. They're soluble and often have roles in metabolism and other enzymes help to synthesise large molecules.
• Another function of proteins is antibodies by being involved in immune response. They're made ip of two light polypeptide chains and two heavy polypeptide chains bonded together. Antibodies have variable regions - the amino acid sequences in these regions vary greatly
• Another function of proteins is transport proteins by channel proteins being present in cell membranes and these channel proteins contain hydrophobic and hydrophilic amino acids which cause the proteins to fold up and form a channel. These proteins transport molecules and ions across membranes.
• The final function of proteins is structural proteins by them being physically strong. These proteins consist of long polypeptide chains lying parallel to each other with cross links between them. Structural proteins include keratin and collagen
• Keratin is found in hair and nails
• Collagen is found in connective tissue
• You need a biuret test to test for proteins
• They are two stages towards a biuret test:

  1. The test solution needs to be alkaline so first you add a few drops of sodium hydroxide solution.
  2. Then you add some copper(II) sulphate solution.
• If there is protein present the solution turns purple
• If there is no protein in the solution then the solution will stay blue