BIOCHEM LECT

• Enzymes


  Catalytic proteins

  
• Enzymes are best known for their catalytic role

  
• Almost every chemical reaction in the body is driven by an enzyme

  
• Enzymes

  • Globular proteins
  • Tertiary structure important (bent and folded into spherical or globular shape)
  • Soluble in water
  • Most enzymes are globular proteins
• Enzymes


  Biological catalysts that speed up the rate of the biochemical reaction
• Enzymes


  • Lower a reaction's activation energy
  • Enzymes are NOT CONSUMED (reusable)

  
• Activation energy


  An input of energy before molecules will react together
• Catalysts


  • Substances that speed up the rate of a chemical reaction

  
• Enzymes cause cellular reactions to occur millions of times faster than corresponding uncatalyzed reactions
• Enzymes can increase the rate of a reaction by a factor of up to 1020 over uncatalyzed reactions
• An enzyme alters the rate of a reaction, but not its free energy change or position of equilibrium

  
• G


  Gfinal -Ginitial
• Active site


  Special pocket or cleft in enzyme molecules where the substrate/s in biochemical reactions attach

  
• Substrate


  Reactant/s in biochemical reactions

  
• Enzyme-substrate complex


  Forms when the substrate binds to the enzyme's active site

  
• Holoenzymes


  Enzymes that consist of a protein part (apoenzyme) and a non-protein part (cofactor/coenzyme)

  
• Apoenzymes


  The protein part of an enzyme
• Cofactor


  Non-protein, inorganic substances (metals or ions) bound within the enzyme molecule
• Coenzyme


  Small organic molecules acting as carrier molecules for enzymes
• Cosubstrate


  Coenzymes that bind transiently/loosely to enzymes near the active site

  
• Prosthetic group


  Coenzymes that bind tightly/permanently to enzymes
• Collagen contains hydroxylysine and hydroxyproline, and their hydroxylation is catalyzed by enzymes that require ascorbic acid (Vit. C)

  
• In Vit. C deficiency, hydroxylation is impaired, and the triple helix of the collagen is not assembled properly
• Enzyme nomenclature/naming

  • Retains old traditional names
  • Names from the species of origin
  • Suffix "-ase" identifies a substance as an enzyme
  • Names provide information about the function (rather than the structure) of the enzyme
  • Systematic naming by the International Union of Biochemistry and Molecular Biology using the Enzyme Commission (EC) number
• Enzyme classes


  • Oxidoreductases
  • Transferases
  • Hydrolases
  • Lyases
  • Isomerases
  • Ligases/Synthetases

  
• Enzyme-substrate complex


  • Provides an alternative pathway with lower activation energy through which a reaction can occur
  • Can lower activation energy by bending substrate molecules in a way that facilitates bond-breaking, helping to reach the transition state

  
• Enzymes

  • Catalytic power (ratio of enzyme catalyzed rate of a reaction to the uncatalyzed rate)
  • Specificity (selectivity of enzyme to their substrate)
  • Regulation (Control of enzymatic reaction)

  
• Lock and key theory

  Postulated in 1894 by Emil Fischer, states that only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme)
• Induced fit theory


  Suggested by Daniel Koshland in 1958, allows for small changes in the shape or geometry of the active site of an enzyme to accommodate a substrate

  
• The forces that draw the substrate into the active site are the same forces that maintain tertiary structure in the folding of peptide chains (hydrogen bond, hydrophobic interaction, ionic/electrostatic interaction or salt bridges)

  
• Specificity


  The ability of an enzyme to choose the exact substrate from a group of similar chemical molecules, through structural and conformational complementarity between enzymes and substrates

  
• Induced fit model


  More thorough explanation for the active-site properties of an enzyme because it includes the specificity of the lock-and-key model coupled with the flexibility of the enzyme protein
• Induced fit model suggested by Daniel Koshland


  1958
• Induced fit model


  Allows for small changes in the shape or geometry of the active site of an enzyme to accommodate a substrate
• Forces that draw the substrate into the active site

  • Hydrogen bond
  • Hydrophobic interaction
  • Ionic/electrostatic interaction or salt bridges
• Specificity


  Ability of an enzyme to choose exact substrate from a group of similar chemical molecules

  
• Types of enzyme specificity

  • Stereo (Stereochemical) specificity
  • Substrate (Absolute) specificity
  • Group specificity
  • Bond (Relative) specificity
  • Geometrical specificity

  
• Stereo (Stereochemical) specificity


  Specific to substrate and its optical conformation
• Substrate (Absolute) specificity


  Specific to only one substrate and reaction
• Group specificity


  Specific to bonds and groups surrounding the bonds