1.4 Proteins and Enzymes

• Proteins

  proteins are polymers made of amino acids joined by peptide bonds. they contain carbon, hydrogen, oxygen, nitrogen and sometimes sulfur.
• structure of amino acids

  • a central C atom
  • an amine group NH2
  • a carboxylic group COOH
  • a hydrogen H atom
  • A variable R group, which determines the properties of amino acid

  
• How are peptide bonds formed?

  peptide bonds form via a condensation reaction, where the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule.

  
• what are four levels of protein structure

  1. primary structure - the sequence of amino acids in a polypeptide chin.
  2. secondary structure - hydrogen bonding forms alpha helices or beta-pleated sheets
  3. tertiary structure - further folding due to hydrogen bonds, ionic bonding, disulfide bridges and hydrophobic interactions forming a unique 3 d shape.
  4. quaternary structure - multiple polypeptides linked together, eg haemoglobin has 4 polypeptide chains.
• what are the two main types of proteins how do they differ?

  1. globular proteins- soluble, compact, and spherical eg enzymes haemoglobin and insulin.
  2. fibrous proteins - insoluble, long and strong eg keratin
• what bonds help maintain a proteins strcture?

  • peptide bonds between amino acids
  • hydrogen bonds between c=o and N=H in secondary structure.
  • ionic bonds between charged Rgroup
  • disulfide bridges strong covalent bond
  • hydrophobic interactions non-polar R groups
• What causes proteins to denature?

  • high temperature breaks the hydrogen and ionic bonds
  • high pH affects the ionic bonds and charges on amino acids
  • organic solvents or heavy metals - disrupt interactions
• How do you test for proteins?

  • add Biuret regent
  • Positive result purple/lilac colour indicates the peptide bonds
  • negative remains blue
• compare lipids and proteins

  • proteins are polymers of amino acids whereas lipids are not polymers.
  • proteins contain nitrogen but lipids don't
  • lipids are mainly for energy storage and membrane structure while proteins have diverse roles like enzymes, hormones ad structural components.
• What are enzymes, and why are they important?

  enzymes are biological catalysts that speed up reactions by lowering the activation energy. they remain unchanged after the reaction and are essential for metabolic processes.
• what type of biological molecule are enzymes made of?

  enzymes are globular proteins with a specific tertiary structure which determines the shape of their active site.
• how does an enzyme's active site determine its specificity?

  the active site has a specific shape that is complementary to the substrate. only the correct substrate can bind forming an enzyme substrate complex.
• what are two models of enzyme action?

  Induced fit is an accepted model because it explains how enzymes lower activation energy.

  • lock and key model - the active site is fixed and fits only to one specific substrate. the active site is complementary to the substrate.
  • induced fit model - the active site is flexible and changes shape slightly to fit the substrate more closely.
• what factors affect enzyme activity?

  • temperature - increase kinetic energy, but too high = denaturation
  • pH - alters ionic bonds, changing the active site shape
  • substrate concentration - more substrate = more reaction until saturation point
  • enzymes concentration - more enzyme = faster reaction, if enough substrate is available
  • inhibitors - slow down or stop the enzyme activity.
• what are two types of inhibitors?

  • competitive inhibitors - similar shape to the substrate, bind to the active site, prevent the substrate binding. can be overcome by increasing the substrate concentration.
  • non-competitive inhibitors - bind to an allosteric site, changing the active site shape, and cannot be overcome by adding more substrate.
• what happens when an enzyme is denatured?

  the tertiary structure changes, altering the active site shape so the substrate can no longer bind. this is caused by high temp or extreme pH changes.
• how can enzyme activity be measured?

  by measuring :

  • the rate of substrate disappearance
  • the rate of product formation eg oxygen, and bubbles in a catalase experiment.