Digestive Enzymes and the Mechanism of Enzyme Action

Enzymes as Biological Catalysts

Metabolic processes within cells consist of chemical reactions that require catalysts to increase their rate. Without enzymes, these reactions would occur far too slowly to support life. Enzymes are involved in almost every essential process, including respiration, protein synthesis, photosynthesis, and digestion. Because they are not consumed in the reactions they facilitate, they are referred to as biological catalysts. The specific set of enzymes present within a cell determines which metabolic pathways can take place.

The Active Site and Enzyme Specificity

Enzymes are primarily proteins that possess a unique three-dimensional area called the active site. This site is where the chemical reaction occurs.

Enzymes exhibit specificity, meaning they only interact with a particular substrate or a specific type of molecule. The substrate fits into the active site much like a key fits into a lock. For instance, the enzyme amylase binds specifically with starch, while proteases bind with proteins. This is known as the lock and key hypothesis.

During a reaction, enzymes convert substrate molecules into different molecules called products. The combination of the enzyme and substrate is known as the enzyme substrate complex ($ESC$). After the reaction, the enzyme remains unchanged and is free to catalyse further reactions.

An enzyme may break one substrate into two products or join two substrates into a single product.

Hydrolysis and Condensation

Substrates can be broken down through the addition of water, a process called a hydrolysis reaction. Conversely, products can be formed by the removal of water, known as a condensation reaction.

Induced Fit Theory

While the lock and key model emphasizes fixed shapes, the induced fit theory suggests that the active site is flexible. When the correct substrate enters the active site, the enzyme changes shape slightly to fit more snugly around the substrate. This adjustment allows the reaction to proceed more efficiently.

Factors Affecting Enzyme Action

Every enzyme has an optimum temperature and $\text{pH}$ at which it functions most effectively. When conditions deviate from these optima, the rate of reaction decreases.

Temperature

As temperature increases, enzymes and substrates gain more kinetic energy, leading to more frequent and energetic collisions. This increases the reaction rate until the optimum temperature is reached. However, if the temperature rises too far above the optimum, the bonds maintaining the enzyme's 3D structure break. The enzyme becomes denatured, and the active site loses its functional shape, meaning the substrate can no longer fit.

pH

Different enzymes are adapted to different $\text{pH}$ levels. For example, pepsin in the stomach has an optimum $\text{pH}$ of approximately $2$, while enzymes in the small intestine typically work best at $\text{pH } 8$. Significant deviations from the optimum $\text{pH}$ cause the enzyme to denature.

Digestive Enzymes: Amylases, Proteases, and Lipases

In the human digestive system, specific enzymes are responsible for breaking down macronutrients into smaller, absorbable molecules:

1. Amylases: These enzymes break down carbohydrates, such as starch, into simple sugars like maltose. They are produced in the salivary glands and the pancreas.
2. Proteases: These enzymes digest proteins into amino acids. Pepsin is a well known protease in the stomach, while others operate in the small intestine.
3. Lipases: These enzymes break down fats (lipids) into fatty acids and glycerol. They are primarily secreted by the pancreas into the small intestine.

Worked Examples

Exercise 26: Why are catalysts necessary in order for reactions to take place in organisms? Answer: Without catalysts like enzymes, metabolic reactions would happen too slowly to sustain life processes.

Exercise 27: Some enzymes were mixed with a substrate. Explain why only enzyme Y catalyses the reaction with this substrate. Answer: Enzyme Y has an active site with a shape that is complementary to the substrate. Enzymes X and Z have different shapes that do not allow the substrate to bind, demonstrating enzyme specificity.