Question

Protection of an Enzyme against Denaturation by Heat When enzyme solutions are heated, there is a progressive loss of catalytic activity over time due to denaturation of the enzyme. A solution of the enzyme hexokinase incubated at \(45{ }^{\circ} \mathrm{C}\) lost \(50 \%\) of its activity in \(12 \mathrm{~min}\), but when incubated at \(45^{\circ} \mathrm{C}\) in the presence of a very large concentration of one of its substrates, it lost only \(3 \%\) of its activity in \(12 \mathrm{~min}\). Suggest why thermal denaturation of hexokinase was retarded in the presence of one of its substrates.

Short answer

The substrate stabilized hexokinase, reducing its denaturation rate at high temperatures.

Step-by-step solution

Understand Enzyme Denaturation

Enzymes are proteins that can lose their function when exposed to high temperatures due to the denaturation process. Denaturation involves the unfolding of the enzyme's structure, which is crucial for its activity.

Role of Substrates in Stabilization

When an enzyme's active site is occupied by a substrate, it can help stabilize the enzyme's structure, making it less prone to denaturation. This stabilization effect occurs because the binding of the substrate can reinforce the enzyme's native conformation.

Apply to Exercise Scenario

In the exercise, when hexokinase was incubated with a high concentration of substrate, its denaturation rate decreased. The substrate likely acted as a stabilizer, maintaining the enzyme's active three-dimensional structure and preventing rapid denaturation at elevated temperatures.

Key concepts

Enzyme Stability

Enzyme stability refers to the ability of an enzyme to maintain its structure and function under different conditions. Enzymes are proteins that play critical roles in biological processes by acting as catalysts. However, their stability can be affected by environmental factors such as temperature, pH, and the presence of inhibitors or activators.
Here are some key points regarding enzyme stability:

• Intrinsic Factors: These include the amino acid sequence and the structural configuration of the enzyme, which dictate its inherent stability.
• Extrinsic Factors: Conditions such as temperature and pH can dramatically influence how stable an enzyme is. For instance, extreme temperatures can cause enzymes to unfold, leading to loss of activity through denaturation.
• Stabilizing Agents: Some molecules, like substrates or co-factors, can bind to the enzyme, enhancing its stability and functionality. This interaction often helps maintain the enzyme's three-dimensional structure.

Understanding enzyme stability is crucial in many applied sciences, including biochemistry and pharmaceuticals, where maintaining enzymatic activity is necessary for desired outcomes.

Substrate-Enzyme Interaction

The interaction between a substrate and an enzyme is fundamental to the enzyme's ability to catalyze reactions. This interaction is highly specific, often compared to a "lock and key" mechanism, where the substrate fits precisely into the enzyme's active site.
Substrate-enzyme interactions involve several important principles:

• Specificity: Each enzyme is specific to a particular substrate or group of substrates, due to the unique shape and chemical environment of its active site.
• Binding: When a substrate binds to the enzyme, it can induce a change in the enzyme's structure, known as an "induced fit," that enhances the catalytic efficiency.
• Stabilization: Binding of the substrate can stabilize the enzyme, protecting it from adverse environmental conditions such as high temperatures.

In the case of hexokinase, the binding of a substrate likely helped mitigate the effects of heat-induced denaturation by reinforcing the enzyme's stability and maintaining its functional shape.

Thermal Stability of Enzymes

The thermal stability of enzymes determines how well an enzyme can function at elevated temperatures without losing activity due to denaturation. Exposing enzymes to high heat can lead to the unfolding of their structural proteins, rendering them inactive. However, certain factors can enhance thermal stability.
Key factors affecting thermal stability of enzymes include:

• Protein Structure: Robust structural features, like disulfide bridges and tight hydrophobic cores, can enhance thermal stability.
• Presence of Substrates: As observed with hexokinase, a high concentration of substrate can provide additional stability, allowing the enzyme to resist denaturation by holding its active conformation in place.
• Adaptations: Enzymes from thermophiles, organisms that thrive in hot environments, show inherent thermal stability due to evolutionary adaptations in their protein structure.

Understanding and improving the thermal stability of enzymes is crucial in industrial applications, where enzymes are often required to function at higher temperatures for efficiency.