proteins

• the monomers of proteins are amino acids
• • a dipeptide is formed when 2 amino acids join together
  • a polypeptide is formed when more than 2 amino acids join together
  • proteins are made up of one or more polypeptides
• amino acids have the same general structure:

  • a carboxyl group -COOH
  • an amine group -NH2
  • and R group (variable side group)
  • all attached to a carbon atom
• R groups generally contain carbon - only exception to this rule is glycine - R group consists of a single H atom
• all living things share a bank of 20 amino acids - only difference is what makes up their R group
• • amino acids are linked together by condensation reactions
  • to form dipeptides and polypeptides
  • molecule of water is released
  • bonds that form between amino acids = peptide bonds
  • reverse reaction (hydrolysis) happens when dipeptides and polypeptides are broken down
• primary structure - the sequence of amino acids in a polypeptide chain
• secondary structure:

  • polypeptide does not remain flat and straight
  • hydrogen bonds form between the amino acids in the chain
  • makes it automatically coil up into an alpha helix or 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 - hydrogen bonds (attractions between negative and positive charges on different parts of the molecule)
  • disulfide bridges also form whenever 2 molecules of the amino acid cysteine come close together
  • sulfur atom in one cysteine binds to the sulfur atom in the other
  • for proteins made from a single polypeptide chain - tertiary structure forms their final 3D structure
• Quaternary structure:

  • some proteins are made of several different polypeptide chains held together by bonds
  • quaternary structure is the way that these polypeptide chains are assembled together
  • for proteins made from more than one polypeptide chain (e.g. haemoglobin, insulin, collagen) - this is the proteins final 3D structure
• protein's shape determines its function - e.g. haemoglobin is a soluble compact protein which makes it easy to transport - great for carrying oxygen around the body
• enzymes - roughly spherical shape due to the tight folding of the polypeptide chains. Soluble and often have roles in metabolism e..g some break down large food molecules and help synthesise large molecules
• antibodies - involved in immune system response and are found in the blood - made up of 2 light (short) polypeptide chains and 2 heavy (long) polypeptide chains bonded together

  have variable regions - the amino acid sequences in these regions vary greatly
• transport proteins:

  • e.g. channel proteins present in a cell membrane
  • channel proteins contain hydrophobic and hydrophilic amino acids, which cause the protein to fold up and form a channel
  • these protein transport molecules transport molecules and ions across membranes
• structural proteins:

  • physically strong
  • consist of long polypeptide chains lying parallel to each other with cross-links between them
  • e.g. keratin (nails and hair) and collagen (connective tissue)
  • collagen has 3 polypeptide chains tightly coiled together, makes it strong - good supportive tissue in animals
• to test for protein - Biuret test:

  • solution needs to be alkaline - add a few drops of sodium hydroxide
  • add some copper (II) sulfate solution
  • if protein present - solution turns purple
  • no protein - remains blue