3.2.3. How Enzymes work?

• Many reactions in biochemistry are spontaneous, i.e. they are thermodynamically favorable (ΔG < 0). 
• The oxidation of glucose to produce carbon dioxide and water is thermodynamically favorable (ΔG°= -2870 kJ/mol).   However, a jar of sugar in water is highly stable in the absence of microbial contamination.  
• Biological reactions are almost always under kinetic control. Thus, a given amount of energy must be put into the system (energy of activation) in order for energy to be released from biochemical reactions.  The rate of a reaction is related exponentially to the energy of activation.
• Two ways to effect the reaction are- i) raising the free energy of the substrates, or ii) decreasing the energy of activation for the reaction.
• Enzymes catalyze reactions by lowering the activation energy barrier.

Figure 3.3: Energetics of stabilization of T.S. by an enzyme.

3.3. Advantages and Important Stereochemical Features of Enzyme Catalysis:

1. Enzymes are natural proteins, available from renewable resources.
2. Enzyme catalysis need: Mild Reaction Conditions – 37° C, near neutral pH and std. pressure.
3. Enzymes are Biodegradable – waste management problems reduced.
4. They are highly specific.
5. Enzymes display three major types of selectivities:
   • Chemoselectivity: Enzyme acts on a single type of functional group, other sensitive functionalities, which would normally react to a certain extent under chemical catalysis, survive. As a result, enzyme catalyzed reactions tend to be cleaner.
   • Regioselectivity and diastereoselectivity: Enzyme, out of their complex three-dimensional structure, can distinguish between functional groups situated in different regions of the substrate molecule.
   • Enantioselectivity: Enzymes are protein, made up of L-amino acids →, so, enzymes are chiral catalysts. So, any type of chirality present in the substrate molecule is recognized in the enzyme-substrate complex. Thus a prochiral substrate may be transformed into an optically active product and both enantiomers of a racemic substrate may react at different rates.
6. Because of enantioselective nature, synthetic chemists have become interested in enzyme catalyzed enantioselective transformation.
7. They are environmentally acceptable (‘GREEN’ ), as they are completely degraded in the environment.