Modern Biology offers students a hands-on introduction to the role of enzymes in plant development with IND-22: Characterization of Peroxidases in Plants. As with every experiment with Modern Biology, you will find everything you need for each experiment, other than general lab supplies with the complete kits we provide. But how do you introduce this topic to your biology class?

If we were substitutes teaching your class, our presentation would go something like this:

Temperature has a significant effect on enzymatic reactions. Enzymes are proteins that catalyze biochemical reactions by lowering the activation energy required for the reaction to occur. However, like all proteins, enzymes are sensitive to changes in temperature.

At low temperatures, enzymatic reactions occur more slowly. That’s because the enzymes have less kinetic energy. 

As temperature increases, the rate of the enzymatic reaction also increases. This is because the molecules move faster, resulting in more frequent collisions between the enzyme and substrate.

Higher temperatures don’t always increase the speed of the chemical reaction.

If the temperature becomes too high, the enzyme may denature. It loses its structure, leading to a loss of enzymatic activity. Denaturing can occur because high temperatures cause the protein structure to vibrate more vigorously, disrupting the weak bonds that hold the protein in its specific conformation. Once the enzyme has denatured, it cannot bind to its substrate, and the reaction will not occur.

Each enzyme has a temperature optimum at which it functions best. The temperature optimum for enzymes varies between species, but it is typically between 30 and 40°C for enzymes in human cells. Enzymes from extremophilic organisms, such as those that live in hot springs or deep-sea hydrothermal vents, can have temperature optima that exceed 100°C.

Explaining Peroxidases in Plants

We will be looking especially closely at peroxidases in plants.

Peroxidases are important enzymes in plants because they play several roles in growth, development, and stress responses. Some of the important functions of peroxidases in plants include:

  1. Lignin biosynthesis: Peroxidases are involved in the biosynthesis of lignin, the complex polymer that provides structural support to plant cells. Peroxidases catalyze the polymerization of monolignols, which are precursors of lignin, into lignin polymers. This process helps plants to form rigid cell walls. It helps them resist environmental stress.
  2. Defense against pathogens: Peroxidases play a role in the plant’s defense against infections by pathogens. When a plant is infected, peroxidases are produced and secreted into the extracellular matrix. They crosslink the cell wall polymers. This strengthens the cell wall and slows or stops the spread of the pathogen.
  3. Reactive oxygen species (ROS) scavenging: Peroxidases neutralize reactive oxygen species (ROS) generated as byproducts of various metabolic processes.
  4. Abiotic stress response: Plants face various environmental stressors, such as drought, salinity, and extreme temperatures. Peroxidases are part of the way the plant responds to such stresses. Under drought stress, for example, peroxidases regulate water transport by modifying the cell wall composition and maintaining cell turgor pressure.
  5. Seed germination: Peroxidases play a role in seed germination. They break down the seed coat and release stored nutrients for seedling growth.

The bottom line is that peroxidases are important in plant growth, development, and stress responses. They are involved in lignin biosynthesis, defense against pathogens, ROS scavenging, abiotic stress response, and seed germination. The functions of peroxidases in plants highlight their importance in plant survival and adaptation to changing environmental conditions.

What is the effect of temperature on peroxidases?

Peroxidases are an enzyme that catalyzes a substrate’s oxidation by hydrogen peroxide, using a heme group as a cofactor. The activity of peroxidases is influenced by temperature; like other enzymes, they have an optimal temperature range.

At low temperatures, the activity of peroxidases is slow due to a lack of kinetic energy; at very low temperatures, the enzyme may become completely inactive. As the temperature increases within the optimal range, the rate of reaction increases due to more frequent collisions between the enzyme and the substrate, and the enzyme-catalyzed reaction becomes more efficient.

At temperatures above the optimal range, the enzyme activity begins to decline, and above a certain temperature, the enzyme may denature, resulting in a complete loss of activity. This temperature at which the enzyme activity starts to decline and denaturation begins is known as the denaturation temperature.

The optimal temperature range for peroxidases varies depending on the source of the enzyme. For example, horseradish peroxidase has an optimal temperature range between 20-30°C, while peroxidase from the fungus Phanerochaete chrysosporium has an optimal temperature range between 30-45°C.

Where are peroxidases found in plants?

Different types of peroxidases can be found in different locations. For example, lignin peroxidases are mainly found in the cell wall. That’s where they are involved in the polymerization of lignin. Cytosolic peroxidases are involved in ROS scavenging and oxidative stress responses.

Some examples of plant peroxidases and their locations are:

  • Horseradish peroxidase (HRP) – found in the cell wall and apoplast of horseradish roots.
  • Catalase-peroxidase (KatG) – found in the cytosol and peroxisomes of plant cells.
  • Ascorbate peroxidase (APX) – found in the cytosol, chloroplasts, and mitochondria of plant cells.
  • Guaiacol peroxidase (GPX) – found in the cell wall, apoplast, and vacuole of plant cells.

Do all plants contain peroxidases?

Peroxidases are ubiquitous enzymes found in all plant tissues, including leaves, stems, roots, and seeds. Different types of peroxidases are present in various plant species and are involved in diverse biochemical pathways.

What happens to the hydrogen peroxide released by peroxidases in plants?

Hydrogen peroxide (H2O2) is a toxic byproduct of many cellular processes, including those catalyzed by peroxidases in plants. Peroxidases are enzymes involved in various processes, such as lignin biosynthesis, suberin formation, and defense against pathogens.

Plants have several mechanisms to detoxify hydrogen peroxide released by peroxidases. One of the primary mechanisms is the activity of enzymes such as catalase and peroxiredoxins that decompose hydrogen peroxide into water and oxygen. This process helps to reduce the toxicity of hydrogen peroxide and prevent damage to the plant’s cells.

Another mechanism involves the transport of hydrogen peroxide to specific sites within the plant cell where it is required for signaling purposes. Hydrogen peroxide has been shown to act as a signaling molecule in various plant processes, such as defense responses, stomatal closure, and programmed cell death.

But isn’t peroxide harmful to plants?

Refer your students to the above. Point out that peroxidases regulate reactions that take thousandths of a second in tiny, tiny amounts.

When your students master these concepts, they are ready to formulate their own hypotheses to test in the lab!

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