enzymes

• Enzymes


  • Regulate biochemical pathways acting on substrate molecules (reactants) to form a final product
  • Interaction between enzyme and substrate molecule involves the formation of enzyme-substrate complexes
  • Organic catalysts (contain carbon, hydrogen, oxygen and nitrogen, and initiates/speeds up reactions)
• Specificity


  Different enzymes act as catalysts for different biochemical reactions by binding to specific substrates
• Enzyme specificity


  • Many enzymes are highly specific and can only catalyse one specific reaction
  • Others can act on multiple substrates and catalyse multiple reactions
• Catalytic power


  Enzymes DO NOT make reactions occur, they ONLY speed up reactions
• Active site


  Pocket or groove-like part of the enzyme formed by tertiary folding of the protein, complex 3D shape interacts with specific substrate to catalyse specific reaction
• Enzyme-substrate complex


  When active site binds to substrate
• Substrate


  Compound that is acted on by an enzyme
• Products


  Compound that is produced due to enzyme activity
• Enzyme-substrate interaction


  • Multiple hydrogen bonds and hydrophobic reactions form between the substrate and active site to stabilise the substrate
• Enzyme-substrate interaction models


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


  'Key' = substrate, 'lock' = active site
• Induced-fit-model


  When substrate binds to active site, a change in shape (conformational change) of the active site occurs, more accurate representation of enzyme-substrate interactions
• Enzyme catalytic power


  • Reactions are often reversible, although, not always
  • Usually different enzymes catalyse a reaction in each direction
  • Direction of reaction depends on concentration of substrates and products + energy requirements
  • Enzyme is not used up in reaction thus can be reused over and over again
• Activation energy


  Input of energy required for a reaction to start
• Catalytic power of enzymes


  Ability to reduce the level of activation energy, less energy required for reaction to occur
• How enzymes reduce activation energy


  • Influencing proximity and orientation, the micro-environment, ion exchange
• Anabolic reaction


  Building up
• Catabolic reaction


  Breaking down
• Substrates bind to the R groups lining the active site
• Too much/not enough of a particular substance


  Negative effect on the whole organism or unable to properly break some substances down
• Excess substance


  Waste energy and resources
• Cells


  • Have mechanisms that regulate biochemical reactions to not be over or under
• Enzymes


  Control metabolism, regulation of biochemical reactions, control photosynthesis and cellular respiration
• Factors that regulate enzyme activity


  • Amount of final products and speed at which are produced
  • Temperature, pH and concentration of substrate + enzyme affect rate of enzymatic reactions
  • Presence of inhibitor, enzyme function is reduced or stopped altogether
• If conditions are optimal

  Highest enzyme activity, fastest rate of reaction
• If conditions are sub-optimal


  Reduced enzyme activity, reduced energy production
• Temperature


  • Rate of reaction will usually increase as temp increases
  • Can denature at high temps
  • Denaturation of enzymes causes permanent change in shape of the active site, active site no longer complementary to substrate, leading to reduced reaction rate
  • Humans optimum temp 36-38C, normal body temp 37C
  • Many mammalian enzymes begin to denature at temps above 40C
  • Some enzymes have much higher optimum temps, e.g. Taq polymerase at 70C
  • If enzymes are cooled below optimum temp, rate of reaction will slow down
• pH


  • Measure of acidity
  • Enzymes have specific pH range for optimal functioning
  • Tertiary structure is affected (becomes unfolded) with pH levels too above or below this range
  • Micro-environment of enzyme may provide different pH suitable for enzyme to function
• Enzyme and substrate concentration


  • Single enzyme molecule can be continuously reused, active site occupied for just milliseconds at a time
  • More enzyme molecules present, shorter wait time for substrates, increased enzyme concentration = increased rate of reaction (in presence of substrate)
  • Raising concentration of substrate = sometimes increased reaction rate, greater chance of substrate molecule binding to active site, saturation point: maximum rate of reaction when every possible active site is filled
• Types of enzyme inhibition


  • Reversible inhibition
  • Irreversible inhibition
  • Competitive inhibition
  • Non-competitive inhibition
  • Feedback inhibition
• Reversible inhibition


  Inhibitors bind to the enzyme via H-bonds and other weak inter-molecular bonds, these weak bonds can be easily broken, and the inhibitor can dissociate from the enzyme, if bonded to active site, increasing substrate concentration will reduce effect
• Irreversible inhibition

  Inhibitors bind covalently to the enzyme's active site, this blocks the enzyme's ability to bind to its normal substrate, the strong covalent bond makes this blockage permanent
• Competitive inhibition


  When shape of inhibitor is similar to shape of substrate, similarity in shape allows inhibitor to bind to active site instead of substrate, blocking the substrate yet does not trigger catalytic reaction
• Non-competitive inhibition


  When inhibitor binds to enzyme site other than active site (allosteric site), binding to this site changes shape (conformation) of enzyme, substrate can not bind to active site, prevents catalytic reaction even if substrate is bound