enzymes

• Enzyme


  A biological catalyst
• Enzymes


  • With the exception of some RNAs that catalyze their own self-cleavage, all enzymes are proteins
  • Enzymes can increase the rate of a reaction by a factor of 10^9 to 10^20 over an uncatalyzed reaction
  • Some catalyze the reaction of only one compound
  • Others are stereoselective; for example, enzymes that catalyze the reactions of only L-amino acids
  • Others catalyze reactions of specific types of compounds or bonds; for example, trypsin catalyzes hydrolysis of peptide bonds formed by the carboxyl groups of Lys and Arg
• Trypsin catalyzes the hydrolysis of peptide bonds formed by the carboxyl group of lysine and arginine

  
• Classification of enzymes


  • Oxidoreductases: Oxidation-reduction reactions
  • Transferases: Group transfer reactions
  • Hydrolases: Hydrolysis reactions
  • Lyases: Addition of two groups to a C-C double bond, or removal of two groups to create a C-C double bond
  • Isomerases: Isomerization reactions
  • Ligases: The joining to two molecules
• Oxidoreductase


  Catalyzes oxidation-reduction reactions
• Transferase


  Catalyzes group transfer reactions
• Hydrolase


  Catalyzes hydrolysis reactions
• Lyase


  Catalyzes the addition of two groups to a C-C double bond, or removal of two groups to create a C-C double bond
• Isomerase


  Catalyzes isomerization reactions
• Ligase


  Catalyzes the joining of two molecules
• Apoenzyme


  The protein part of an enzyme
• Cofactor


  A nonprotein portion of an enzyme that is necessary for catalytic function; examples are metallic ions such as Zn2+ and Mg2+
• Coenzyme


  A nonprotein organic molecule, frequently a B vitamin, that acts as a cofactor
• Substrate


  The compound or compounds whose reaction an enzyme catalyzes
• Active site


  The specific portion of the enzyme to which a substrate binds during reaction
• Activation


  Any process that initiates or increases the activity of an enzyme
• Inhibition


  Any process that makes an active enzyme less active or inactive
• Competitive inhibitor


  A substance that binds to the active site of an enzyme thereby preventing binding of substrate
• Noncompetitive inhibitor


  Any substance that binds to a portion of the enzyme other than the active site and thereby inhibits the activity of the enzyme
• Enzyme activity


  A measure of how much a reaction rate is increased
• Enzyme concentration increases


  Reaction rate increases
• Substrate concentration increases


  Reaction rate increases
• Temperature increases


  Reaction rate increases
• pH changes


  Reaction rate changes
• Lock-and-key model


  The enzyme is a rigid three-dimensional body, and the enzyme surface contains the active site
• Induced-fit model


  The active site becomes modified to accommodate the substrate
• Competitive inhibitor binds to active site


  Substrate cannot bind
• Noncompetitive inhibitor binds to site other than active site


  Substrate can still bind but no catalysis occurs
• Both the lock-and-key model and the induced-fit model emphasize the shape of the active site, but the chemistry of the active site is the most important
• Five amino acids participate in the active site in more than 65% of the enzymes studied to date: His, Cys, Asp, Arg, Glu

  
• Four of these five amino acids have either acidic or basic side chains; the fifth has a sulfhydryl group (-SH)
• Feedback control


  An enzyme-regulation process where the product of a series of enzyme-catalyzed reactions inhibits an earlier reaction in the sequence
• Proenzyme (zymogen)


  An inactive form of an enzyme that must have part of its polypeptide chain hydrolyzed and removed before it becomes active
• Allosterism


  Enzyme regulation based on an event occurring at a place other than the active site but that creates a change in the active site
• Allosteric enzyme


  An enzyme regulated by allosterism
• Negative modulation


  Inhibition of an allosteric enzyme
• Positive modulation


  Stimulation of an allosteric enzyme
• Regulator


  A substance that binds to an allosteric enzyme
• Protein modification, especially phosphorylation/dephosphorylation, can affect enzyme activity
• Isoenzyme (Isozymes)


  An enzyme that occurs in multiple forms; each catalyzes the same reaction