Question

Consider a biochemical reaction \(\mathrm{A} \rightarrow \mathrm{B}\), which is catalyzed by A-B dehydrogenase. Which of the following statements is true? A. The reaction will proceed until the enzyme concentration decreases. B. The reaction will be more favorable at \(0^{\circ} \mathrm{C}\). C. A component of the enzyme is transferred from A to B. D. The free energy change \((\Delta G)\) of the catalyzed reaction is the same as for the uncatalyzed reaction.

Short answer

D. The free energy change \((\Delta G)\) of the catalyzed reaction is the same as for the uncatalyzed reaction.

Step-by-step solution

Understanding the Role of Enzymes

An enzyme, such as A-B dehydrogenase in this case, catalyzes reactions by lowering the activation energy but does not change the overall free energy change \((\Delta G)\) of the reaction.

Considering Enzyme Concentration

Enzyme concentration can affect the rate of the reaction but does not determine how far the reaction will proceed. So statement A is incorrect.

Considering Temperature Effects

Enzyme activity can be temperature-dependent, but typically enzymes do not function optimally at extreme temperatures such as \(0^{\circ} \mathrm{C}\). Thus, statement B is not correct in most practical scenarios.

Analyzing the Transfer of Components

For statement C, transfer of a component would imply a change in the enzyme itself or a change in the molecular structure involving the enzyme directly, which does not align with the typical role of enzymes as catalysts.

Free Energy Change in Catalyzed vs. Uncatalyzed Reactions

The free energy change \((\Delta G)\) of a reaction is a property of the reactants and products, and it remains the same whether the reaction is catalyzed or not. Therefore, statement D is correct.

Key concepts

Activation Energy

Activation energy is the minimum energy required to start a chemical reaction. Enzymes, such as A-B dehydrogenase, play a crucial role in lowering this activation energy, making it easier for the reaction to proceed. When activation energy is reduced, more molecules have the energy needed to react, which increases the reaction rate. This is one reason why enzymes are so vital in biochemical processes.

For instance, consider the reaction \(\text{A} \rightarrow \text{B}\). Without the enzyme, the activation energy is high, and fewer molecules can transform from A to B. However, with the enzyme, the activation energy is lowered, facilitating a quicker and more efficient reaction.

It's important to remember that enzymes only lower the activation energy; they do not alter the overall energy balance of the reaction.

Free Energy Change

The free energy change \((\text{ΔG})\) of a reaction indicates whether a reaction is spontaneous. It is the difference in energy between the reactants and the products. For the reaction \(\text{A} \rightarrow \text{B}\), if \(\text{ΔG}\) is negative, the reaction occurs spontaneously.

Enzymes do not change the \(\text{ΔG}\) of a reaction. This means that the overall energy required or released in a reaction remains the same, regardless of whether the reaction is catalyzed by an enzyme or not. The \(\text{ΔG}\) is a property of the reactants and products and reflects the energy landscape of the reaction.

So, even though an enzyme might speed up the reaction, the free energy change stays consistent. This is why statement D from the exercise is accurate.

Enzyme Activity

Enzyme activity refers to the rate at which an enzyme catalyzes a reaction. Several factors, including enzyme concentration, substrate concentration, temperature, and pH, influence enzyme activity.

Although increasing enzyme concentration generally increases the reaction rate, there is a limit to this effect. At a certain point, adding more enzyme does not increase the reaction rate. This is because all the available substrate molecules are already being acted on by the enzyme.

Similarly, temperature affects how enzymes function. Most enzymes have an optimal temperature range where their activity is maximized. Extreme temperatures, like \(0^{\text{o}}\text{C}\), usually decrease enzyme activity because the molecular movements necessary for the reaction are slowed or the enzyme becomes denatured. This is why statement B is incorrect.

Reaction Rate

The reaction rate is the speed at which reactants are converted into products. In enzyme-catalyzed reactions, several factors influence this rate:

• Substrate Concentration: Higher concentrations of substrate generally increase the reaction rate until the enzyme becomes saturated.
• Temperature: Reaction rate typically increases with temperature up to an enzyme's optimal temperature. Beyond that, the enzyme may denature, leading to a decrease in reaction rate.
• pH Levels: Each enzyme works best at a specific pH range. Deviating from this optimal pH can reduce enzyme activity.
• Enzyme Concentration: More enzymes generally mean more active sites available for the reaction, increasing the rate until substrate becomes the limiting factor.

It's also essential to note that while enzymes speed up reactions, they do not affect the equilibrium position of the reaction. They make it easier to reach equilibrium faster by lowering the activation energy.