Enzymes (Chapter 4)

• Enzymes


  • Large, globular proteins
  • Specific three-dimensional conformation
  • Active site is a very specific area of the protein which recognises and binds to the substrate
  • Shape determines function
  • Folding of polypeptide and R-groups exposure determines the active site properties
• Active Site


  1. Substrates have specific shapes to fit into the active sites

  2. Upon binding, the active site undergoes a conformational change to accommodate for the substrate, and the substrate undergoes a small change in turn. This allows for a stronger connection.

  3. Chemical bonds (i.e. Hydrogen bonds, hydrophobic interactions) hold the substrate and active site together in the enzyme-substrate complex

  4. Enzyme-substrate complex formed then dissociates

  5. Product is formed and enzyme is unaltered

  6. The substrate must be a COMPLEMENTARY shape to the active site. NOT the SAME shape
• Enzyme Activity


  1. When a reaction takes place the atoms in the substrate are rearranged. Existing bonds are broken and new bonds are formed

  2. For this to happen the substrates absorb energy (activation energy) so that the bonds can break. When the new bonds form energy is either released or more is absorbed
• Cofactors


  1. Essential non-protein parts are called cofactors – these could be metallic ions such as calcium, copper, zinc, potassium or magnesium

  2. If the cofactor is an organic non-protein it is called a COENZYME
• Co-enzymes


  1. A coenzyme is defined as an organic molecule that binds to the active sites of certain enzymes to assist in the catalysis of a reaction

  2. Coenzymes can function as intermediate carriers of factors (electrons, hydrogen, energy) during these reactions or be transferred between enzymes as functional groups
• ADP (unloaded); ATP (loaded); Energy (carries)
• Activation energy: The MINIMUM AMOUNT of ENERGY REQUIRED for a REACTION to PROCEED
• Allosteric site: A BINDING SITE on an ENZYME, where MOLECULES OTHER THAN THE SUBSTRATE may BIND
• Anabolic: DESCRIBES a TYPE of CHEMICAL REACTION that REQUIRES ENERGY and involves CONSTRUCTING MOLECULES FROM SIMPLER COMPONENTS
• Biochemical pathway: A series of reactions, catalysed by enzymes, where the product of one reaction is the substrate for the next reaction
• Catabolic: DESCRIBES a TYPE of CHEMICAL REACTION that RELEASES ENERGY and involves the BREAKING DOWN MOLECULES INTO SIMPLER COMPONENTS
• Catalyst: A SUBSTRATE that INCREASES the RATE OF REACTION by LOWERING the ACTIVATION ENERGY and providing an ALTERNATIVE REACTION PATHWAY
• Coenzyme: An ORGANIC MOLECULE that CONTAINS CARBON and BINDS TO ENZYMES to HELP them FUNCTION; examples are NADP, NAD and FAD
• Endergonic: Energy is absorbed when the reaction occurs
• Exergonic: Energy is released when the reaction occurs
• Induced-Fit Model: The substrate is not exactly complimentary to the active site of the enzyme. However, upon binding with the substrate, the active site is able to alter its shape to mould around the substrate.
• Lock-and-Key Model: When the SHAPE of the SUBSTRATE is a PERFECT FIT for the UNIQUE SHAPE of the ACTIVE SITE of the ENZYME
• Substrate: A MOLECULE that BINDS to the ACTIVE SITE of an ENZYME and then takes PART in a REACTION; also referred to as a REACTANT