Catalysts & Binding
- Catalysts are classified into two categories depending on whether they are in the same phase as the reactants or not
- Homogeneous
- Heterogeneous
- Heterogeneous catalysts are in a different phase to the reaction
- For example, the use of platinum or palladium metals (solid) in organic reactions such as hydrogenation reactions
- Homogeneous catalysts exist in the same phase as the reactants
- Homogeneous catalysts most often catalyze gaseous and aqueous reactions
- For example, aqueous zinc chloride, ZnCl2, is used to catalyze the organic reaction between hydrogen chloride and alcohol
Enzyme Catalysis
- Enzymes are examples of biological catalysts that can speed up the rate of biological reactions in living organisms
- Most enzymes are large protein molecules with molecular weights ranging from about 10,000 to about 1 million amu
- Enzymes are very selective in the reactions they catalyze
- Some are absolutely specific, operating for only one substance in only one reaction
- Enzyme molecules possess an active site
- This is part of the molecule with a shape that allows a specific reactant molecule (substrate) to bind
- The binding between the substrate and active site involves dipole-dipole attractions, hydrogen bonds, and dispersion forces
- Two models currently exist to explain how an enzyme and its substrate interact
- Model 1:
- The substrate molecule fits into the active site on the enzyme molecule, somewhat in the way a key fits into a lock
- This results in the formation of an enzyme-substrate complex as a reaction intermediate
- On binding to the enzyme, the substrate may have bonds weakened or new bonds formed that help yield the products
- Model 2:
- This is often called the induced fit model
- This suggests that the active site of an enzyme changes its shape to fit its substrate
- Model 1:
Enzyme Catalysis Models
Diagram A shows the lock and key model relationship between the enzyme’s active site and the reactant molecules (substrate) while diagram B shows the enzyme’s active site changing shape to allow the substrates to bind.