Enzymes: Biological Catalysts – Structure, Function, and Mechanism

Enzymes: Biological Catalysts

Definition and General Properties

Enzymes are globular proteins that act as biological catalysts, accelerating the rate of chemical reactions without being consumed in the process. Some enzymes are RNA molecules known as ribozymes. Enzymes specifically bind to their substrates (reactants) at the active site.

• Catalyst: A substance that increases the rate of a chemical reaction without undergoing permanent change.

• Substrate: The reactant molecule upon which an enzyme acts.

• Active Site: The region of the enzyme where substrate binding and catalysis occur.

Example: Hexokinase catalyzes the phosphorylation of glucose to glucose-6-phosphate in glycolysis.

Enzyme Mechanism and Reaction Rates

Enzymes function by lowering the activation energy required for a reaction to proceed, thereby increasing the reaction rate. They do not alter the equilibrium constant (Keq) or the thermodynamic favorability (ΔG) of a reaction.

• Transition State Stabilization: Enzymes stabilize the transition state, reducing the activation energy ($E_a$).

• Reaction Rate: Enzymes increase the rate at which equilibrium is reached, but do not change the position of equilibrium.

• Specificity: Enzymes are highly specific for their substrates due to the precise structure of their active sites.

Equation:

$E_a$ (with enzyme) < $E_a$ (without enzyme)

Example: Hexokinase binds glucose and ATP, facilitating the transfer of a phosphate group to glucose.

Practice Questions and Key Concepts

• Hexokinase is a fibrous protein? False. Hexokinase is a globular protein.

• Enzymes and Equilibrium: Enzymes do not change the equilibrium constant (Keq) or the thermodynamic favorability (ΔG) of a reaction.

• Enzyme Catalysis: Enzymes catalyze reactions by stabilizing the transition state, not by making reactions thermodynamically favorable or forcing nonspontaneous reactions to occur.

• Effect on Reaction Rate: Enzymes allow a chemical reaction to proceed extremely fast by lowering the activation energy.

How Enzymes Catalyze Reactions

• Do not alter equilibrium constant (Keq): The ratio of products to reactants at equilibrium remains unchanged.

• Do not change ΔG: The overall free energy change of the reaction is unaffected.

• Stabilize the transition state: Enzymes lower the activation energy by stabilizing the transition state.

• Do not force reactions in one direction: Enzymes accelerate both the forward and reverse reactions equally.

• Do not change nonspontaneous reactions into spontaneous ones: Enzymes cannot make a thermodynamically unfavorable reaction proceed spontaneously.

Summary Table: Enzyme Effects on Chemical Reactions

Property	Effect of Enzyme
Reaction Rate	Increases (reaction proceeds faster)
Equilibrium Constant (Keq)	No change
Free Energy Change (ΔG)	No change
Transition State	Stabilized (lower activation energy)
Direction of Reaction	No change (accelerates both directions)

Additional info:

• Enzymes are essential for life because they allow biochemical reactions to occur at rates compatible with cellular processes.

• Most enzymes are proteins, but some RNA molecules (ribozymes) also have catalytic activity.