Enzymes

Enzymes are essential proteins that act as biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being used up in the process. Every living cell contains thousands of enzymes, each controlling a specific reaction. The sum of all these reactions is called metabolism. Enzymes can be involved in anabolic reactions (building larger molecules from smaller ones, e.g., building starch from glucose) or catabolic reactions (breaking down large molecules into smaller ones, e.g., digesting protein into amino acids). Without enzymes, these reactions would occur too slowly to sustain life.

The action of an enzyme is explained by the 'lock and key' model. Each enzyme has a specific, three-dimensional shape which includes a small depression called the active site. The molecule the enzyme acts upon is called the substrate. The shape of the substrate is complementary to the shape of the active site, like a key fitting into a lock. The substrate binds to the active site, forming an enzyme-substrate complex. Inside the complex, the reaction is catalysed, and the substrate is converted into products. The products then detach from the active site, leaving the enzyme free to catalyse another reaction. This specificity is why amylase only digests starch and not protein.

Temperature has a significant effect on the rate of enzyme-catalysed reactions. As temperature increases from a low level, the enzyme and substrate molecules gain more kinetic energy. This leads to more frequent collisions between the substrate and the active site, increasing the rate of reaction. This continues until the enzyme reaches its optimum temperature, which is the temperature at which it works most efficiently. For most human enzymes, this is around 37°C. If the temperature increases much beyond the optimum, the enzyme starts to denature. The high temperature breaks the weak bonds holding the enzyme in its specific 3D shape. The active site changes shape and is no longer complementary to the substrate. The substrate cannot bind, and the reaction stops. This change is irreversible.

Like temperature, pH also affects an enzyme's activity. Each enzyme has an optimum pH at which its rate of reaction is maximal. For example, pepsin, a protease in the stomach, works best at an acidic pH of around 2. In contrast, trypsin, which works in the small intestine, has an optimum pH of about 8. If the pH is too far from the optimum (either too acidic or too alkaline), the enzyme will denature. The pH affects the ionic bonds that help maintain the enzyme's specific 3D shape. A change in pH disrupts these bonds, causing the shape of the active site to change. The substrate can no longer fit, and the enzyme's activity decreases or stops completely. Unlike heat denaturation, slight pH changes can sometimes be reversed, but extreme pH changes cause irreversible denaturation.

Enzymes are not just vital in cells; humans have harnessed their power for various industrial and commercial purposes. In digestion, our bodies produce extracellular enzymes like amylase (breaks down starch to maltose), protease (breaks down protein to amino acids), and lipase (breaks down lipids to fatty acids and glycerol). Industrially, these same principles are used. Biological washing powders contain proteases and lipases to break down protein and fat stains (like blood and grease) at lower wash temperatures, saving energy. In food production, pectinase is used to break down pectin in fruit cell walls, which clarifies fruit juice and increases the yield. Lactase is used to break down lactose into glucose and galactose, creating lactose-free milk for intolerant individuals. Finally, in the production of cheese, the enzyme chymosin (rennet) is used to curdle milk.

In industrial processes, it is often beneficial to use immobilised enzymes. This means the enzymes are attached to an insoluble material or trapped inside small beads or fibres. For example, enzymes can be encapsulated in a gel (like alginate beads). The substrate solution is then passed over these beads. The substrate molecules are small enough to diffuse into the beads and bind with the active sites of the enzymes. The products, also small, diffuse back out. This technique has several key advantages: 1) The enzyme is not mixed with the product, so no costly separation is needed, resulting in a purer product. 2) The enzyme can be easily reused for subsequent batches, which is cost-effective. 3) Immobilised enzymes are often more stable and resistant to changes in temperature and pH.