Enzymes Lowering Activation Energy of Chemical Reactions

Section 1: Enzymes Lowering Activation Energy of Chemical Reactions

I. Function and Essence of Enzymes

1. Enzyme Action in Cellular Metabolism

Cellular metabolism is the foundation of cellular life activities, involving the release, storage, and utilization of energy through chemical reactions. Many chemical reactions occur continuously within cells, collectively known as cellular metabolism. Cellular metabolism is indispensable for life, but it also produces metabolic waste and potentially harmful substances, such as hydrogen peroxide. Fortunately, cells contain substances that can promptly decompose hydrogen peroxide into oxygen and water, and this substance is called catalase. Let's explore the action of catalase using the decomposition of hydrogen peroxide under different conditions as an example.

1. Heating Facilitates the Decomposition of Hydrogen Peroxide

Heating promotes the decomposition of hydrogen peroxide because it provides energy to hydrogen peroxide molecules, converting them from a stable state to a more readily decomposable active state. The energy required for molecules to transition from a stable state to an active state capable of undergoing chemical reactions is called activation energy.

Iron and catalase facilitate the decomposition of hydrogen peroxide by lowering the activation energy required for this reaction. To illustrate, if we liken a chemical reaction to driving a car over a high mountain, "heating" would be akin to increasing the throttle, while using a catalyst would be akin to finding a tunnel through the mountain for the driver. Compared to inorganic catalysts, enzymes significantly reduce activation energy and exhibit higher catalytic efficiency.

It is precisely due to the catalytic action of enzymes that cellular metabolism can proceed rapidly and orderly under mild conditions.

1. The Essence of Enzymes

What exactly are enzymes? Before the 19th century, people did not understand the nature of enzymes. Through in-depth studies on fermentation processes in brewing in the 19th century, scientists gradually uncovered the "veil" of enzymes.

"In science, there is no easy path; only those who climb steep mountains without fear of hardship can hope to reach the glorious summit." Combining the study of this section, discuss your understanding of Marx's quote.

II. Characteristics of Enzymes

1. Chemical Nature of Enzymes Different from Inorganic Catalysts

Generally, enzymes are organic substances with catalytic properties produced by living cells, with the majority being proteins.

How is the catalytic action of enzymes different from that of inorganic catalysts?

Enzymes are Efficient

From your previous lesson, you already know that enzymes are highly efficient. Extensive experimental data show that enzymes can catalyze reactions with efficiencies 10^3 to 10^6 times greater than inorganic catalysts. What significance does this have for cells? Imagine if the catalytic efficiency of catalase in cells were low. When your muscle cells need a large supply of energy for activities such as soccer or running, if the catalytic efficiency of the enzymes involved were low, the chemical reactions supplying energy would proceed very slowly. Could you still run so fast?

Inorganic catalysts can catalyze a wide range of chemical reactions. For example, acids can catalyze the hydrolysis of proteins, fats, and starch.

Can enzymes catalyze multiple chemical reactions like inorganic catalysts?

Enzymes are Specific

Catalase can only catalyze the decomposition of hydrogen peroxide and cannot catalyze other chemical reactions. Similarly, urease catalyzes only the decomposition of urea and is inactive in other chemical reactions. Each enzyme can only catalyze one or a class of chemical reactions. The orderly progression of cellular metabolism is inseparable from the specificity of enzymes.

Many inorganic catalysts can catalyze chemical reactions under conditions of high temperature, high pressure, strong acids, or bases.

What Conditions are Required for Enzymatic Catalysis?

Enzymatic Activity Conditions are Moderate

Experiments show that the temperature and pH of a solution affect enzyme activity. Compared to inorganic catalysts, enzymatic reactions generally occur under relatively mild conditions. Scientists use quantitative analysis methods to measure the activity of the same enzyme under different temperatures and pH conditions and plot curves based on the data obtained (Figure 5-2, Figure 5-3). Analyzing these two curves reveals that enzyme activity is highest at optimal temperatures and pH levels. Enzyme activity significantly decreases at temperatures or pH levels that are too high or too low.

Excessive acidity, alkalinity, or high temperatures can damage the spatial structure of enzymes, leading to permanent deactivation. At around 0°C, enzyme activity is very low, but the spatial structure of enzymes is stable, and enzyme activity increases at suitable temperatures. Therefore, enzyme preparations should be stored at low temperatures.

The orderly progression of various chemical reactions in cells is also related to the distribution of enzymes within cells. For example, enzymes related to photosynthesis are distributed in chloroplasts in plant leaf mesophyll cells, while enzymes related to respiration are distributed in the cytoplasmic matrix and mitochondria, allowing photosynthesis and respiration to proceed simultaneously in different cellular compartments without interference.