Protein and Enzymes

• BIOLOGY IS THE STUDY OF: PROTEINS & SHAPES FITTING TOGETHER
• Amino acids


  Contain the elements Nitrogen (N), Carbon (C), Hydrogen (H) and Oxygen (O). Some also contain Sulphur (S).
• Variety of proteins and their functions

  • Haemoglobin - Allows Oxygen to bind and be transported
  • Antibody - Binds to specific antigens, used in immune response
  • Enzymes - Reduces activation energy in metabolic reactions
  • Actin and Myosin - Structural proteins involved in muscle contraction
  • Keratin - Structural protein found in nails, hooves, talons
  • Collagen - Structural protein found in tendons
• Monomers


  Amino acids
• Essential amino acids


  Obtained from diet
• Non-essential amino acids


  Can be synthesised by the body
• General structure of an amino acid

  Central (alpha) carbon atom to which an amine group (H2N) and a carboxyl group (COOH) are attached. The different 'R' group is what makes amino acids different.

  
• R groups


  Can be positively charged, negatively charged, hydrophilic (attracted to water), or hydrophobic (repelled by water)
• Formation of a dipeptide

  Two amino acids join by a condensation reaction, producing a water molecule
• Polypeptide chain


  Many (hundreds of) amino acids (monomers) join to form a polymer

  
• A peptide chain will always have an amine group at one end (N terminal end) and a carboxyl group at the opposite end (C terminal end)

  
• The number of peptide bonds in the chain will be one less than the total number of amino acids originally joined together
• Levels of protein structure

  • Primary (1°)
  • Secondary (2°)
  • Tertiary (3°)
  • Quaternary (4°)
• Primary structure


  The number and sequence of amino acids in a polypeptide chain

  
• Secondary structure


  The way the polypeptide chain folds or coils into alpha helixes and/or beta pleated sheets, held together by weak hydrogen bonds

  
• Tertiary structure


  The further folding of the polypeptide chain into a specific complex 3D shape, held together by hydrogen bonds, ionic bonds, and disulfide bridges between R groups

  
• Quaternary structure


  Some proteins consist of two or more polypeptide chains joined together

  
• The specific shape of a protein is essential to its function
• Proteins only denature at high temperatures (NOT AT LOW TEMPERATURES!) and changes to the pH
• Increasing temperature increases the kinetic energy of the molecules making them vibrate more, this can break the weak H bonds in the secondary & tertiary structure
• Changing the pH of the environment breaks the IONIC bonds between the R groups in the tertiary structure
• As the bonds break, the SPECIFIC TERTIARY shape of the molecule is lost, this is DENATURATION
• Globular protein & function examples


  • Haemoglobin - 4 polypeptide chains, globular, functional protein
• Structural protein & function examples

  • Collagen - 3 polypeptide chains, fibrous, structural protein

  
• Describe the structure of proteins

  1. Polymer of amino acids

  2. Joined by peptide bonds

  3. Formed by condensation

  4. Primary structure is order and sequence of amino acids

  5. Secondary structure is folding of polypeptide chain due to weak hydrogen bonding forming α-helices & β-pleated sheets

  6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic/di-sulfide bonds

  7. Quaternary structure is two or more polypeptide chains joined together

  
• The Biuret test detects the presence of peptide bonds in proteins
• Enzymes


  Globular proteins that act as biological catalysts, increasing the rate of chemical reactions by lowering the activation energy
• Enzyme action


  1. Reactants

  2. Products

  3. Activation energy (Ea) is the minimum energy required for a successful chemical reaction

  4. Enzymes lower the activation energy needed for a reaction by stressing/distorting/weakening the bonds in the substrate during the formation of an enzyme-substrate complex

  
• Active site


  The specific complementary shaped region on an enzyme where the substrate binds

  
• Lock and Key model


  The active site is specifically complementary to its substrate, fitting it like a key in a lock
• Lock and key model

  The active site of the enzyme is the lock, the substrate is the key. Only one substrate will fit into the active site of a particular enzyme.

  
• Stages of enzyme action in lock and key model

  1. The active site is rigid and does not change shape

  2. The substrate enters/binds to the enzymes active site

  3. The substrate fits exactly into the active site – they are complementary

  4. Products are formed and no longer fit into the active site, so is released

  5. The enzyme is free to take part in another reaction

  
• Sucrase does not hydrolyse lactose
• Why sucrase does not hydrolyse lactose


  • Lactose has a different shape/structure
  • Does not fit/bind to active site of enzyme/sucrase
  • The substrate and the enzyme active site are not complementary
  • No Enzyme Substate Complexes form
• Enzyme names usually end with 'ase' e.g. catalase, anhydrase, Dehydrogenase, ATP synthase and ATP Hydrolase
• The examiner will often ask why one enzyme will only catalyse only one type of reaction, for example, why protease hydrolyses a protein and not a carbohydrate
• No matter what the enzymes are, the underpinning theory of Lock and Key applies
• Induced fit model


  The active site is not fixed/rigid. The active site can change its shape (slightly flexible). When the substrate binds to the active site (when it is aligned correctly), the substrate 'induces' a change in the shape of the active site making it more complimentary.
• Steps in induced fit model

  1. The substrate enters the enzymes active site and binds to it forming the Enzyme substrate complex

  2. The binding of the substrate molecule/s induces the change in the shape of the active site

  3. The 'slight' change in shape of the specific 3D tertiary structure of the active site, applies stress or distorts the bonds within the substrate(s) molecule(s) which lowers the Ea of the reaction

  4. When substrate leaves, the active site returns to its original shape

  
• A protein's tertiary structure alters when it interacts with other molecules