Enzymes: Structure, Function, and Regulation

Parts of an Enzyme

Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are typically proteins with specific structural components that determine their function.

• Apoenzyme: The protein portion of an enzyme, which is inactive without its cofactor.

• Co-factor: A non-protein chemical compound (such as a metal ion or organic molecule) required for the enzyme's activity. Cofactors can be inorganic (e.g., Mg2+, Zn2+) or organic (called coenzymes, e.g., NAD+).

• Active Site: The region on the enzyme where substrate molecules bind and undergo a chemical reaction. The active site has a specific shape that fits the substrate.

• Holoenzyme: The complete, active enzyme consisting of the apoenzyme plus its cofactor. Difference from Apoenzyme: The apoenzyme alone is inactive; the holoenzyme is catalytically active because it includes the necessary cofactor.

Enzyme-Catalyzed Reaction Energy Diagram

Enzyme-catalyzed reactions can be visualized using energy diagrams, which show the energy changes during the progress of a reaction.

• Reaction Catalyzed by Enzyme: The reaction with the lower activation energy (smaller peak) is catalyzed by the enzyme. In the diagram, this is typically labeled as '2'.

• Label 3: Represents the activation energy required for the reaction to proceed without an enzyme.

• Label 4: Represents the overall energy released during the reaction (difference between reactants and products).

• Exergonic vs. Endergonic: If the products have lower energy than the reactants, the reaction is exergonic (energy-releasing).

Equation:

$\Delta G = G_{products} - G_{reactants}$

If $\Delta G < 0$, the reaction is exergonic.

How Enzymes Catalyze Reactions

Enzymes catalyze reactions by lowering the activation energy required for the reaction to occur. They do this through specific interactions with their substrates.

• Substrate: The molecule upon which the enzyme acts.

• Active Site: The region of the enzyme where the substrate binds.

• Product: The molecule(s) produced from the enzymatic reaction.

• Conformational Change: When the substrate binds to the active site, the enzyme may undergo a change in shape (induced fit), which facilitates the conversion of substrate to product.

Example: The enzyme hexokinase binds glucose (substrate) at its active site, undergoes a conformational change, and converts glucose to glucose-6-phosphate (product).

Factors Affecting Enzyme Activity

Enzyme activity is influenced by environmental factors such as pH, temperature, and substrate concentration. These factors can be visualized using rate of reaction graphs.

• pH: Enzymes have an optimal pH at which their activity is highest. Area A: Low activity due to suboptimal pH. Area B: Activity decreases as pH moves away from optimum.

• Temperature: Enzyme activity increases with temperature up to an optimum, then decreases due to denaturation. Area A: Low activity at low temperatures. Area B: Activity drops at high temperatures due to enzyme denaturation.

• Substrate Concentration: Activity increases with substrate concentration until the enzyme is saturated. Area A: Low activity at low substrate concentration. Area B: Maximum activity when all active sites are occupied (enzyme saturation).

Enzyme Inhibition

Enzyme inhibitors are molecules that decrease or prevent enzyme activity. There are two main types: competitive and non-competitive inhibitors.

Example: Cyanide is a non-competitive inhibitor of cytochrome c oxidase, blocking cellular respiration.

Additional info: Enzyme kinetics and inhibition are foundational concepts in biochemistry and molecular biology, relevant for understanding metabolic regulation and drug design.