Proteins

• The monomers of proteins are

  Amino acids
• How is a dipeptide formed?

  When the amine and carboxyl group on 2 amino acids join together ( condensation reaction) to form a peptide bond and loss of water - creating a dipeptide
• How’s a polypeptide formed ?

  When more than 2 amino acids join together (condensation reaction) - joining their amine and carboxyl group to form a peptide bond and lose water
• Proteins are made up of …

  one or more polypeptides
• 3 components of an amino acids general structure

  • carboxyl group (-COOH)
  • amine/amino group (-NH2)
  • R group (variable side group)
• All living things share a bank of how many amino acids ?

  20 amino acids
• what’s the only difference between each amino acid?

  They have different variable groups - what makes up their R group
• Amino acids are linked together by what reaction to form polypeptides ?

  condensation reaction
• What’s released during the condensation reaction ?

  a molecule of water
• what are the bo ds formed between amino acids in a condensation reaction called ?

  peptide bonds
• What are a proteins 4 structural levels called ?

  • primary
  • secondary
  • tertiary
  • quaternary
• Primary structure

  The sequence of amino acids in the polypeptide chain
• Secondary Structure

  1. Hydrogen bonds form between the amino acids in the chain
  2. This makes is coil into an alpha helix or fold into a beta pleated sheet ( secondary structure)
  3. The type of secondary structure formed depends on the primary structure in that region - certain amino acids more likely in each
• Tertiary Structure

  1. the coiled or folded chain is often coiled or folded further
  2. More bonds form between different parts of the polypeptide chain , including hydrogen and ionic bonds( attractions between - & + charges on different parts of the molecule)
  3. Disulfide bridges also form whenever 2 molecules of the amino acid cysteine come close together ( sulfur atom in one cysteine bonds to the sulfur atom in the other)
  4. For proteins made from a single polypeptide chain- the tertiary structure forms their final 3D structure
• Quaternary Structure

  1. some proteins are made of several different polypeptide chains held together by bonds eg. haemoglobin, insulin, collagen
  2. The quaternary structure is the way these polypeptide chains are assembled together
  3. This is their final 3D structure
• Protein Functions : Enzymes

  1. roughly spherical due to tight folding of polypeptide chains.
  2. soluble and often have roles in metabolism (e.g.. digestive enzymes and enzymes that synthesize large molecules )
  3. if heated the tertiary structure can can shape changing active site shape and causing it to denature
• Protein Functions : Antibodies

  1. involved in the immune response.
  2. made up of two light (short) polypeptide chains and two heavy (long) polypeptide chains bonded together. Antibodies have variable regions and the amino acid sequences in these regions vary greatly.
• Protein Functions : Transport proteins

  e.g. channel proteins are present in cell membranes. Channel proteins contain hydrophobic and hydrophilic amino acid, which cause the protein to fold up and form a channel . These proteins transport molecules and ions across membranes
• Protein functions : Structural Proteins

  1. Physically strong
  2. Consist of long polypeptide chains lying parallel to each other with cross links between them.
  3. examples = keratin and collagen
• Biuret Test for Proteins

  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) sulfate solution
  3. If the protein is present the solution turns purple
  4. If there's no protein, the solution will stay blue

  
• Equation for polypeptide formed by condensation reaction:

  =

  
• example amino acid general structure :

  =

  
• What's the difference between a polypeptide and a protein?

  • In order to be classed as a protein a polypeptide has to fold into a complex 3D shape.
  • once the polypeptide has folded into the correct shape it can then carry out its function e.g. as an enzyme or a hormone
  • At this point we would refer to it as a protein molecule (many proteins actually consist of several different polypeptides - forming a large and complex molecule)
• Tertiary structure of a protein image :

  =

  A) alpha helix

  B) beta pleated sheet

  
• Quaternary structure of a protein image :

  =

  
• Hydrogen bonding in proteins :

  • due to slight positive and negative charges on each R group containing a hydroxyl a hydrogen bond can form
  • hydrogen bonds are weak bonds and are easily broken by high temperature or pH changes

  
• Ionic bonds in proteins:

  • found between amino acids with charged R groups
  • oppositely charged R group attract each other and form an ionic bond
  • this bond holds different part of the polypeptide chain together and contributes to the structure of the protein
  • Ionic bonds are broken by changes in pH ( a reason why enzymes can denature under acidic or alkaline conditions)
• Disulfide bonds in proteins:

  • occurs when each R group contain a sulfur atom and they covalently bond - disulfide bond
  • Disulfide bonds are relatively strong and are not broken by high temperatures or pH changes

  A) cysteine (R group contains sulfur)

  
• What are the 2 groups of proteins called ?

  1. Globular proteins
  2. Fibrous proteins
• Features of Globular Proteins :

  1. usually spherical shape caused by tightly folded polypeptide chains
  2. Soluble in water -non-polar Hydrophobic amino acid groups on the inside and polar hydrophyllic amino acids on the outside -
  3. play important physiological roles as they can be easily transported around organisms and be involved in metabolic reactions. examples =
  4. transport proteins = haemoglobin and myoglobin
  5. Enzymes = DNA polymerase
  6. Hormones = insulin

  
• Features of Fibrous Proteins :

  1. Shape - long strands formed from parallel polypeptide chains held by crosslinks due to hydrogen bonds
  2. often play a structural role e.g. in bones, tendons and arteries
  3. Insoluble in water - have a large proportions of amino acids with hydrophobic R groups
  4. examples - keratin, collagen, fibrin, myosin, actin