Question

For enzyme-catalyzed reactions that follow the mechanism $$\mathbf{E}+\mathbf{S} \rightleftharpoons \mathbf{E} \cdot \mathbf{S}$$ $$\mathrm{E} \cdot \mathrm{S} \rightleftharpoons \mathrm{E}+\mathrm{P}$$ a graph of the rate as a function of \([\mathrm{S}]\), the concentration of the substrate, has the following appearance: Note that at higher substrate concentrations the rate no longer changes with [S]. Suggest a reason for this.

Short answer

At higher substrate concentrations, the rate of the enzyme-catalyzed reaction no longer changes with [S] because the enzyme molecules are saturated with the substrate. In this situation, all enzyme molecules are occupied with substrate and forming enzyme-substrate complexes. Adding more substrate does not result in any increase in the reaction rate since there are no unoccupied enzyme molecules available for additional substrates to interact with.

Step-by-step solution

Understand the reaction mechanism

First, we need to understand the reaction mechanism. The enzyme (E) interacts with the substrate (S) to form an enzyme-substrate complex (E·S). This step is reversible, meaning the complex can either dissociate back into enzyme and substrate or proceed to the second step. In the second step, the enzyme-substrate complex (E·S) breaks down into enzyme (E) and product (P). This step is also reversible. The rate of the enzyme-catalyzed reaction depends on the concentration of the substrate [S].

Investigate the enzyme saturation

At low substrate concentrations, the reaction rate increases as the substrate concentration increases. This is because there are more substrate molecules available for the enzymes to interact with, increasing the probability of the enzyme-substrate complex forming. However, at higher substrate concentrations, the rate no longer changes with [S]. This is because in high concentration of substrate, most enzyme molecules have already formed complexes with substrate molecules. This means that the enzyme is saturated, and the enzymes cannot take up any additional substrate molecules to react.

Understand the reason behind the constant reaction rate

As we increase [S], there is a point where the enzymes are working at their maximum capacity and cannot work any faster. This plateauing of reaction rate is due to the saturation of enzyme with the substrate. Since all the enzyme molecules are occupied with substrate and forming enzyme-substrate complexes, adding more substrate does not result in any increase in the reaction rate because there are no free enzyme molecules available for additional substrates to interact with. To conclude, at higher substrate concentrations, the rate of the enzyme-catalyzed reaction no longer changes with [S] because the enzyme molecules are saturated with the substrate, and further increases in substrate concentration does not result in any increase in the reaction rate.

Key concepts

Enzyme-Substrate Complex

The enzyme-substrate complex is formed when an enzyme binds to its specific substrate. Imagine this as a lock-and-key model, where the enzyme (E) is the lock, and the substrate (S) is the key. This binding leads to the formation of an enzyme-substrate complex (E·S). This complex is crucial because it's the initial step in the catalytic process.

When the enzyme and substrate come together, it's not a permanent bond. They can separate back into the original enzyme and substrate, or the reaction can proceed, leading to a product. This reversible binding allows the enzyme to be used repeatedly and makes the process efficient. The formation of this complex is essential for lowering the activation energy, making the reaction quicker and more efficient.

• The enzyme-substrate complex is specific and temporary.
• It's the starting point of the enzyme-catalyzed reaction.

Reaction Rate

The reaction rate for enzyme-catalyzed reactions is influenced by various factors, with substrate concentration being one of the most significant. Initially, as the substrate concentration increases, the reaction rate also increases. This is because more substrate molecules are available to form enzyme-substrate complexes.

However, the rate doesn't increase indefinitely. At some point, a balance is achieved, and adding more substrates won't speed up the reaction. This occurs because the enzymes have reached their capacity to process the substrate. Thus, understanding the reaction rate helps us know how efficiently an enzyme catalyzes a reaction at different concentrations.

• Initial increase in rate with more substrates.
• Rate levels off when enzymes are saturated.

Substrate Concentration

Substrate concentration plays a pivotal role in the dynamics of enzyme activity. At low concentrations, the increase in substrate leads to a higher chance of enzyme-substrate complex formation. This means that the reaction rate will rise, as more enzymes are occupied transforming substrate into product.

But what happens when substrate concentration keeps increasing? Initially, this is beneficial, but beyond a certain point, even with extra substrates, the reaction speed won't change. This is because all active sites of the enzyme are occupied, and no more substrate can be processed at once. This explains why substrate concentration is only beneficial up to a certain level.

• Low substrate concentration leads to more complex formation.
• Excessive substrate results in enzyme saturation.

Enzyme Saturation

Enzyme saturation is a crucial concept that explains the limit to how fast an enzyme-catalyzed reaction can go. When more substrate is added, the reaction speed increases, but only up to a point. Why does this happen? It's because the enzymes have limited active sites available for binding.

Once all the active sites are occupied, the enzyme can't work any harder or faster. This level is known as saturation. At this point, no matter how much more substrate you add, the reaction rate remains constant. This saturation ensures that the enzyme is working at its maximum capacity, highlighting the importance of enzyme availability in bio-reactions.

• Enzymes have a maximum speed when saturated.
• Extra substrate won't increase the reaction rate.