enzyme part 1 week 5

Introduction to Enzymes

Enzymes are biological catalysts that play a vital role in facilitating chemical reactions in biological systems. They work by lowering the activation energy required to initiate these reactions, thereby increasing the rate at which they occur. This lesson will delve into the workings of enzymes, the concept of activation energy, and how substrate concentration affects enzymatic reactions.

Activation Energy

• Definition: Activation energy is the minimum energy needed for a chemical reaction to occur, representing the energy barrier that must be surmounted.

• Illustrative Example: Consider a ball resting in a hole. To roll the ball down, energy must be applied to lift it from the hole; this energy is analogous to activation energy in a chemical reaction.

• Combustion Example: In combustion, such as burning wood, a flame is required to provide the activation energy necessary for ignition. Once the wood is ignited, it continues to burn without additional energy input.

Role of Catalysts

• Catalysts are substances that increase the rate of a chemical reaction by lowering the activation energy barrier.

• Enzymes as Catalysts: In biological systems, enzymes serve as catalysts that lower the activation energy of biochemical reactions, allowing these reactions to proceed at a faster rate.

• Importantly, while enzymes lower the activation energy, they do not alter the overall change in free energy (ΔG) of the reaction.

Mechanism of Enzyme Action

• Enzymes bind to substrates at specific regions known as the active site, forming an enzyme-substrate complex. This complex facilitates the conversion of substrates into products.

• Specificity: Each enzyme is highly specific to its substrates due to the unique shape of its active site, which enables effective binding and catalysis.

• Upon substrate binding, the enzyme undergoes a structural change that enhances catalysis, creating a transition state where the activation energy is significantly lowered, enabling the formation of products.

Outcomes of Enzyme-Substrate Interaction

• The products formed from the reaction have low affinity for the active site, causing them to dissociate. The enzyme remains unchanged and ready to catalyze new substrate molecules again.

Need for Cofactors and Coenzymes

• Some enzymes require additional non-protein molecules to function effectively:

  • Cofactors: These are inorganic ions (e.g., magnesium ions) that assist in catalytic activity.

  • Coenzymes: Organic molecules that serve as cofactors, which are often involved in the reaction and may change form (e.g., NAD+ which acts as an electron acceptor in various reactions).

  • Prosthetic Groups: These are permanently attached organic or inorganic components essential for enzyme function; their removal can denature the enzyme (e.g., metalloenzymes which contain tightly bound metal ions).

Effect of Substrate Concentration on Reaction Rate

• The reaction rate of enzymes can be influenced by substrate concentration:

  • Low Substrate Concentration: The reaction rate increases linearly as more substrate is available.

  • Intermediate Substrate Concentration: The rate of reaction begins to slow as the active sites become occupied, preventing further increases in reaction rate.

  • High Substrate Concentration: The reaction rate plateaus as all active sites are saturated, reaching a maximum velocity (V_max).

• This behavior is typical in enzyme kinetics and reflects the fundamental relationship between substrate concentration and enzymatic reaction efficiency.