In this lesson, we will explore metabolism, which encompasses the biochemical reactions that occur in living organisms to sustain life. These processes enable organisms to grow, reproduce, maintain their structures, and respond to their environments. Metabolism consists of two main parts: anabolism, where larger molecules are built, and catabolism, where larger molecules are broken down. Typically, catabolism releases energy that is then used in anabolism, making these reactions a form of energy transfer.

Enzymes play a vital role in metabolism by acting as biocatalysts, speeding up and regulating these biochemical pathways. The term “metabolism” comes from a Greek word meaning “change.” The concept was first introduced by Ibn-e-Nafees, who noted that “the body and its parts are always undergoing change.”

Enzymes are proteins that facilitate biochemical reactions without being altered themselves. They act on specific molecules called substrates, transforming them into products. All chemical reactions require a certain amount of energy to start, known as activation energy. This energy requirement can slow down reactions, but enzymes lower this barrier, allowing reactions to occur more quickly.

Enzymes reduce activation energy in several ways: they may change the shape of the substrate, disrupt the charge distribution on substrates, or position substrates correctly to promote reactions. As a result, enzymatic reactions proceed at a faster rate.

Characteristics of Enzymes

The term “enzyme” was first introduced by German physiologist Wilhelm Kühne in 1878. Enzymes are globular proteins made up of long chains of amino acids that fold into three-dimensional shapes. Almost all enzymes are proteins, and they catalyze reactions at rates millions of times faster than similar uncatalyzed reactions. Like all catalysts, enzymes are not consumed in the reactions they facilitate.

Enzymes are highly specific to the types of reactions they catalyze and the substrates they act upon. A small part of the enzyme, known as the active site, is directly involved in the reaction; it recognizes and binds to the substrate. Cells can adjust enzyme production based on their needs, and enzyme activity can be influenced by inhibitors (which slow down reactions) and activators (which speed them up).

Some enzymes require additional non-protein molecules called cofactors to function. These can be inorganic (like metal ions) or organic (like vitamins). When organic cofactors are tightly bound to the enzyme, they are called prosthetic groups, while loosely attached organic cofactors are referred to as coenzymes, which help transport chemical groups between enzymes.

Enzymes often work together in sequences known as metabolic pathways, where the product of one enzyme serves as the substrate for the next.

Uses of Enzymes

Enzymes are widely used in various industries to speed up chemical reactions. For example:

1. Food Industry: Enzymes break down starch into simple sugars for making bread and buns.
2. Brewing Industry: Enzymes help break down starch and proteins for yeast fermentation to produce alcohol.
3. Paper Industry: Enzymes reduce starch viscosity, aiding in paper production.
4. Biological Detergents: Protease enzymes remove protein stains from clothes, while amylase enzymes help eliminate stubborn starch residues.

Factors Affecting the Rate of Enzyme Action

Enzymes are sensitive to their environment, and any changes that affect their chemistry or shape can influence their activity. Various factors can impact the rate at which enzymes work.

Temperature

Temperature significantly affects the rate of enzyme-catalyzed reactions. As temperature increases, the rate of these reactions speeds up until it reaches a specific point known as the optimum temperature for that enzyme. For human enzymes, the optimum temperature is around 37°C. At this temperature, heat increases the activation energy and kinetic energy, accelerating reactions. However, if the temperature rises too high, the increased vibrations can cause the enzyme’s structure to break down, a process called denaturation. This leads to a rapid decrease in enzyme activity, and it may stop completely.

Substrate Concentration

The concentration of substrates also influences enzyme activity. When there are enough enzyme molecules available, increasing the substrate concentration will increase the reaction rate. However, there comes a point when all the active sites on the enzymes are occupied, known as saturation of active sites. At this stage, adding more substrate will not further increase the reaction rate.

pH

Each enzyme operates at its maximum efficiency within a narrow range of pH levels called the optimum pH. A slight change in pH can slow down or completely block enzyme activity. Different enzymes have specific optimum pH values; for example, pepsin, which works in the stomach, is active in acidic conditions, while trypsin, which functions in the small intestine, works better in alkaline conditions. Changes in pH can affect the ionization of amino acids at the enzyme’s active site, impacting its function.

Mechanism of Enzyme Action

When an enzyme binds to its substrate, a temporary enzyme-substrate (ES) complex is formed. The enzyme then catalyzes the reaction, transforming the substrate into a product. After the reaction, the ES complex breaks apart, releasing the enzyme and the product.

To explain this process, German chemist Emil Fischer proposed the lock and key model in 1894. According to this model, both the enzyme and substrate have specific shapes that fit perfectly together, which accounts for enzyme specificity. In 1958, American biologist Daniel Koshland introduced the induced-fit model, suggesting that the active site of the enzyme is flexible and can change shape to better fit the substrate. This model is now more widely accepted than the lock and key model.

Specificity of Enzymes

There are over 2,000 known enzymes, each involved in a specific chemical reaction. Enzymes are substrate-specific; for example, protease breaks down proteins, while amylase works on starch. Similarly, lipase only acts on lipids, breaking them down into fatty acids and glycerol. The specificity of each enzyme is determined by the shape of its active site, which has a specific geometric structure that fits only certain substrates.

Practical Work

To observe enzyme activity in vitro (outside a living organism), we can design an experiment using meat proteins as the substrate and pepsin as the enzyme. The question is: Can pepsin digest the proteins in meat?

Apparatus Required: Meat, test tube, pepsin solution, HCl, and Biuret reagent.

Background Information:

• In vitro means outside a living body, while in vivo refers to reactions occurring inside a living body.
• Animal flesh, or meat, contains a lot of proteins.
• Pepsin is produced in the stomach in its inactive form, pepsinogen, and acts on proteins.

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