Problem 24 - Biochemistry: A Short Course

Question

The reaction of NADH with oxygen to produce \(\mathrm{NAD}^{+}\) and \(\mathrm{H}_{2} \mathrm{O}\) is very exergonic, yet the reaction of NADH and oxygen takes place very slowly. Why does a thermodynamically favorable reaction not take place rapidly?

Step-by-Step Solution

Answer

Slow reaction rate is due to high activation energy despite being favorable.

Step 1: Understanding Thermodynamic Favorability

A reaction being exergonic means that it releases energy, making it thermodynamically favorable. For the reaction \( ext{NADH} + ext{O}_2 \rightarrow ext{NAD}^+ + ext{H}_2 ext{O}\), the products have lower free energy than the reactants, indicating spontaneity.

Step 2: Exploring Reaction Kinetics

Despite being thermodynamically favorable, the rate of a reaction depends on its kinetics, which is influenced by the activation energy. High activation energy can slow down a reaction.

Step 3: Role of Activation Energy

Activation energy is the energy barrier that must be overcome for a reaction to proceed. Even if a reaction is exergonic, it might have a high activation energy that slows down its rate.

Step 4: Biological Influence on Reaction Rate

In biological systems, enzymes often catalyze reactions by lowering the activation energy, speeding up the reaction. In the absence of catalytic enzymes, the reaction might proceed slowly.

Key Concepts

thermodynamics

Thermodynamics helps us understand whether a reaction can happen based on energy changes. When we say a reaction is exergonic, it means the process releases energy. This energy release signifies that the reaction is thermodynamically favorable.
In the case of the reaction involving NADH and oxygen turning into \( \mathrm{NAD}^{+} \) and \( \mathrm{H}_{2} \mathrm{O} \), it is exergonic because the energy of the products is lower than that of the reactants. This means that once the reaction starts, it tends to go all the way to completion, releasing energy along the way.

• Exergonic = Energy-releasing reaction
• Thermodynamics = Focus on energy changes
• Spontaneous = Reaction can occur under the right conditions

The concept of spontaneity in thermodynamics doesn't mean fast. It simply means that, under optimal circumstances, the reaction is natural and does not require extra energy input to proceed.

reaction kinetics

While thermodynamics tells us that a reaction can happen, reaction kinetics details how quickly it proceeds. The rate at which the reaction of NADH with oxygen occurs depends largely on reaction kinetics.
Kinetics is influenced by different factors such as temperature, concentration of reactants, and notably, the activation energy of the reaction. The kinetic aspect explains why some reactions, even though possible energetically, don't proceed rapidly.

• Reaction kinetics = Study of reaction rates
• Factors = Temperature, reactant concentration, and more

In this context, the kinetics of the reaction primarily contributes to its slow progression. Despite the thermodynamic favorability, the rate is slow because it might require a certain amount of energy to get started.

activation energy

Activation energy is like a hurdle a reaction must get over to proceed. Even when reactions are '-thermodynamically favorable, they can still face kinetic challenges due to high activation energy. This energy is basically the minimum amount needed to initiate a reaction.
For the NADH and oxygen reaction, it can be understood that despite being exergonic, it has a steep energy hurdle to clear. Hence, it doesn’t progress quickly without help.

• Activation energy = Minimum energy to start a reaction
• High activation energy = Slow reaction rate without catalysts

It's important to visualize activation energy as the initial energy push needed. Once this energy barrier is overcome, the reaction can proceed faster and release the intended energy.

enzymatic catalysis

Enzymes play a crucial role in biological systems by speeding up reactions that are otherwise slow due to high activation energy. Enzymatic catalysis involves the enzymes lowering the activation energy required for a reaction.
In our specific reaction, without enzymes, the conversion of NADH and oxygen happens very slowly. When enzymes are present, they facilitate this reaction by making it easier and quicker, significantly reducing the time taken to overcome the activation energy barrier.

• Enzymes = Biological catalysts
• Lower activation energy = Faster reactions

Without enzymatic assistance, vital biological reactions might occur too slowly to sustain life. That’s why enzymes are key players in maintaining efficient cellular processes.