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Biology LibreTexts

Mechanisms

Kinetics of an un-catalyzed chemical reaction vs. a catalyzed chemical reaction

Gibbs Free Energy (G) is used to describe the useful energy in a reaction or the energy capable of doing work.  In Figure 1, energy refers to the free energy of the reaction (G).  ΔG is the overall energy released during the reaction and accounts for the equilibrium of the reaction.  Equilibrium is reached when substrate is being converted into product at the same rate as product is being converted into substrate.  Enzymes do not affect ΔG or ΔGo between the substrate and the product.  Enzymes do affect the activation energy.  The activation energy is the difference in free energy between the substrate and the transition state.  The transitions state is the intermediary state of the reaction, when the molecule is neither a substrate or product.  The transition state has the highest free energy, making it a rare and un-stable intermediate.  An enzyme helps catalyze a reaction by decreasing the free energy of the transition state.  As a result, more product will be made because more molecules will have the energy necessary for the reaction to occur and the reaction will occur at a faster rate.  

Un-Catalyzed Chemical Reaction:  

S↔S*↔P

Substrate is converted into product when the substrate has enough energy to overcome the activation energy and be converted into product.  

Catalyzed Chemical Reaction: 

S+E↔ES↔ES*↔EP ↔E+P

Once an enzyme is introduced into the reaction, the enzyme binds to the substrate forming an enzyme/substrate complex (ES).  As a result this complex decreases the activation energy, allowing the reaction to occur at a faster rate and form the enzyme/product complex (EP).  This complex then dissociates, into the product and the enzyme.  The enzyme is then free to catalyze another reaction.  

Figure 1

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Quantitatively, what is the effect of reducing Ea?

For reaction A↔B, V = k[A]

k=(hT/kb)exp(-Ea/RT)

h= Plank’s constant; kb = Boltzman’s constant,

So k and thus V are inversely and exponentially related to Ea and directly related to T:

A 6 kJ/mol reduction in Ea gives ca 10x increase in k and V

∆h ~ exp(+6000/8.3*300) ~ 11 (reduction in Ea is an increase from –Ea)

V(catalyzed)/V(uncatalyzed) for various enzymes varies from 104 to 1021, meaning Ea is reduced by ca 23 to 126 kJ/mol

How do enzymes reduce Ea?

These effects raise G(ES):cage effect, orientation, steric straining of bonds (stress from H-, Vanderwaal’s, ionic bonds), dislocation of bonding electrons through +/- charges

These effects reduce G(ES*): covalent bonds, acid- base catalysis, low-barrier hydrogen bonds, and metal ion catalysis

Different classes of enzymes may use different mechanisms:

  1. Oxidoreductases (oxidation-reduction reactions)
  2. Transferases (transfer of functional groups)
  3. Hydrolases (hydrolysis reactions)
  4. Lyases (addition to double bonds)
  5. Isomerases (isomerization reactions)
  6. Ligases (formation of bonds with ATP cleavage)

Enzyme Mechanisms

binding of substrate.png

formation of es complex.png

proton donation by his.png

cleavage of peptide bond.png

release of amino product.png

nu attack by water.png

collapse of intermediate.png

carboxyl product release.png
substrate release.png