biochem

• Enzyme

  Biological catalyst
• Active site

  Part of enzyme to which the substrate binds and the reaction takes place
• Substrate

  A reactant in an enzyme-catalyzed reaction
• ES complex

  The intermediate formed when the substrate is bound at the active site of an enzyme
• Protein Functions


  • Catalysis
  • Structure
  • Defence
  • Transport and storage
  • Movement
  • Regulation
• Enzyme


  A biological catalyst
• Enzymes can increase the rate of a reaction by a factor of up to 1020 over an uncatalyzed reaction
• Catalysis


  The process of increasing the rate of chemical reactions
• Catalyst


  The substance that facilitates catalysis
• Enzymes as catalysts


  • Increase the rate of a reaction
  • Not consumed by the reaction
  • Act repeatedly to increase the rate of reaction
  • Often "specific" - promote only 1 particular reaction, others catalyze a family of similar reactions
• Enzyme catalyzed reactions


  • Higher reaction rates (106 to 1020 > than uncatalyzed reaction)
  • Milder reaction rates (< 100°C, atmospheric pressure, nearly neutral pH)
  • Greater reaction specificity (rarely have side products)
• Enzyme catalysis reaction


  1. Enzyme binds to substrate to form ES complex

  2. Formation of transition state complex ES*

  3. Formation of enzyme-product complex EP

  4. Release of product
• Reaction velocity (V)


  The rate of enzyme reaction measured by the rate of appearance of products or disappearance of substrates
• Enzyme activity


  1 unit (U) is the amount of enzyme that catalyses the reaction of 1 μmol of substrate per minute under specified conditions
• Activation energy (ΔG°‡)


  The energy required to start a reaction/the amount of energy needed to get the reactants to transition state
• Transition state


  The intermediate stage in a reaction in which the old bonds break and new bonds are formed
• Transition state theory


  1. Enzyme (E) must approach substrate (S), substrate attaches to active site through non-covalent bonds

  2. Formed the high energy (unstable) ES complex

  3. In ES complex, the covalent bond in substrate is in the process of breaking while the EP complex is forming
• Enzyme-substrate interaction models


  • Lock-and-key model
  • Induced fit model
• Active site


  Relatively small 3D region within the enzyme where substrates bind by weak non-covalent interactions
• Holoenzyme


  An enzyme in its complete form including polypeptide(s) and cofactor
• Apoenzyme


  Enzyme in its polypeptide form without any necessary prosthetic groups or cofactors
• Cofactor


  Nonprotein molecule that assists in an enzyme's catalytic reaction
• Coenzyme


  Smaller organic or organometallic molecule derived from a vitamin that assists an enzyme
• Cosubstrate


  Weakly bound to enzyme, temporarily associated with enzymes, e.g. NAD+, FAD+
• Prosthetic group


  Coenzymes that are covalently bound, tightly bound to enzyme and always present, e.g. heme in catalase
• Allosteric enzyme


  Enzymes that can be regulated by the binding of an effector molecule at a site other than the active site
• Effectors


  Molecules that can bind to an allosteric enzyme and either increase (positive effectors) or decrease (negative effectors) its activity
• Heterotropic allosterism


  Allosteric regulation where the effector is different from the substrate
• Homotropic allosterism


  Allosteric regulation where the effector is the same as the substrate
• Isoenzyme


  Multiple forms of the same enzyme that catalyze the same reaction but differ in their kinetic properties or regulation
• Multienzyme


  Enzymes that consist of multiple polypeptide chains
• Cofactor


  Nonprotein molecule that assists in an enzyme catalytic reaction
• Categories of cofactors


  • Metal ions
  • Coenzymes
• Coenzyme


  Smaller organic or organometallic molecule derived from vitamin
• Key words for today

  • 6 classes of enzymes
  • Factors affecting enzyme reaction rate
  • Allosteric enzyme, effectors (positive and negative), heterotropic and homotropic allosterism
  • Isoenzyme and multienzyme
• Enzymes


  • Biological catalyst
• 6 categories of enzymes


  • Each enzyme has an official international name ending with -ase and a classification number
  • Number consists of 4 digits (referred to a class and subclass of reaction)
• Classification of enzymes - examples


  • CH3CH2OH + NAD+ → CH3CH=O + NADH + H+
• Factors influencing enzyme reaction rate


  • Substrate concentration
  • Temperature
  • pH
  • Enzyme concentration
  • Inhibitor
• Substrate saturation

  • Increasing the [substrate] increases the rate of reaction (enzyme activity)
  • Enzyme saturation limits reaction rates
  • At the saturation point, the reaction will not speed up, no matter how much additional substrate is added