Enzymes and Cofactors Definition

Enzymes: Definition and Characteristics

• Enzymes are proteins that act as catalysts, increasing the rate of biochemical reactions without being consumed in the process.
• Most enzymes catalyze reactions that either make or break covalent bonds.
• Some enzymes act as transporters and use chemical energy, for example, breaking down ATP.
• Each enzyme catalyzes only specific reactions.
• There are six major classes of enzymes, grouped based on the type of reaction they catalyze.

Cofactors, Coenzymes, and Prosthetic Groups

• Cofactors are non-protein chemical compounds or metallic ions required for the activity of an enzyme as a catalyst.
• There are two types of cofactors: inorganic ions and complex organic molecules.
• Inorganic ions are obtained through the diet, and many organic cofactors are derived from vitamins as they cannot be synthesized by humans.
• Coenzymes are further divided into two types: prosthetic groups and co-substrates.
• Prosthetic groups are coenzymes permanently bound to the enzyme, while co-substrates bind transiently. They are released from the enzyme at some point but then rebind for another reaction to occur.
• Apoenzyme + cofactor (or coenzyme/prosthetic group) = holoenzyme.

Enzyme Catalysis: How do enzymes achieve catalysis?

• Enzymes bind to their substrate(s) at the active site, which is made up of a few amino acids within the polypeptide chain called catalytic amino acids.
• Non-covalent interactions facilitate this binding, based on the shape and chemical complementarity between the substrate(s) and the active site.
• For a reaction to occur, energy is needed to reach the transition state, which is the intermediate stage between reactants and products. This energy is called activation energy.
• The higher the activation energy, the slower the reaction proceeds.
• Enzymes speed up reactions by stabilizing the transition state, making it easier to reach. This is done by binding the transition state better than either the substrates or the products.
• If the enzyme binds too strongly to the substrates, the reaction would not proceed.
• Similarly, if the enzyme binds too strongly to the products, they would not be released.
• Stabilization occurs through multiple non-covalent interactions between the transition state and the enzyme. The active site is chemically and sterically complementary to the transition state.
• The induce fit model describes how enzymes change shape to allow optimal binding of the transition state. As the enzyme binds the substrate(s), catalyzes the reaction, and releases products, it adopts different conformations.

Thermodynamics of Enzymes

• Enzymes accelerate reactions but do not alter the equilibrium of a reaction, which is determined by the concentrations of substrates and products.
• Enzymes do not change the standard free energy change (∆G°) of a reaction.
• ∆G = ∆H - T∆S
• ∆G is the free energy change, ∆H is the enthalpy change, T is the temperature, and ∆S is the entropy change.
• If ∆G is less than zero, the reaction is spontaneous.
• If ∆G is greater than zero, the reaction is not favored and requires energy.
• If ∆G is equal to zero, the reaction has reached equilibrium.

Specificity of Enzymes

• Multiple weak non-covalent interactions between the substrate(s) and enzyme provide the binding energy that contributes to catalysis and specificity.
• These interactions are complementary to a specific substrate, meaning other molecules lack these interactions, resulting in substrate specificity.

Summary

• Interactions between substrates and enzymes are optimized for a specific transition state.
• The binding energy contributes to catalysis and specificity.