Enzymes

Enzymes

1. characteristics:

   • LOWERS ACTIVATION ENERGY
     • Stabilizes or alters the transition state
   • Reverse entropy (decrease entropy) and bind to substrate (increase enthalpy)
   • NOT permanently altered by a reaction
   • DOES NOT change thermodynamic conditions
     • No change in G, H, S, Keq

2. Increases the probability and rate of which a reaction will occur

3. Acid-Base Catalysis

   • Glycolysis
     • DHAP:
     • GAP (G3P)
     • Reaction 5: DHAP converts to GAP
       • Mechanism of TIM (triose phosphate isomerase)

4. Nucleophilic Attack: Trypsin

   • HIS57, ASP102, SER195
     • Catalytic triad:
       • HIS57 N: steals H from SER195
         • ASP102 COO- stabilizes NH+ of HIS57
           • SER195 O- attacks C=O on substrate polypeptide backbone
             • H2O attacks tetrahedral C, breaking peptide bond
       • Oxyanion transition state: LOWERS FREE ENERGY
   • Two Substrates: protein and water
   • Two Products: Two polypeptides

5. Regulation:

   • Competitive inhibition
     • Inhibitor noncovalently binds to the same site as the substrate
       • Inhibitor is structurally similar to the substrate of an enzyme
     • AZT; reverse transcriptase
   • Ki= Inhibitory dissociation
   • Ks= substrate dissociation
   • Noncompetitive (MIXED) inhibition
     • Inhibitor and substrate bind at different sites
       • Can bind simultaneously
       • ACTase:
         • End product CTP bunds to and inhibits the first enzyme in reaction (ACTase)
           • Allosteric inhibitor
             • CTP weakens substrate binding
               • Tense state
                 • Low affinity
               • Relaxed state
                 • High affinity

6. Kinetics

   • Michaelis-Menton Graph
     • Catalysis is rate-limiting step
       • Vmax = Kcat x Etot
       • Km = CONCENTRATION (x-axis) at HALF V-max
       • Vmaz = SLOPE IS ZERO; Horizontal line; Highest substrate concentration
     • Units:
       • V's= M*sec^-1
       • K-cat: sec^-1
       • Ks and Km: M
       • Efficiency: sec^-1*M^-1
         • E=K-cat/Km
     • Competitive inhibition
       • Binds to active site and substrate cannot bind
         • NO chante in Vmaz or K-cat
         • INCREASE in Km
     • Noncompetitive Inhibition
       • LOWERS Vmax
         • INCREASES Km
           • Decreases substrate binding