Enzymes

Enzymes are proteins that act as biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being used up in the process. Within every living cell, thousands of chemical reactions, collectively known as metabolism, are taking place. These reactions are either anabolic (building up large molecules, e.g., protein synthesis) or catabolic (breaking down large molecules, e.g., digestion). Enzymes control the rate of these reactions, ensuring they happen fast enough to sustain life. Most enzymes work inside the cells where they are made (intracellular enzymes, like catalase which breaks down harmful hydrogen peroxide), while others are secreted to work outside the cell (extracellular enzymes, like the digestive enzyme amylase).

The way an enzyme works is explained by the 'lock and key' model. Each enzyme has a specific three-dimensional shape which includes a small depression or pocket called the active site. The molecule that the enzyme acts upon is called the substrate. The shape of the substrate is complementary to the shape of the active site, much like a specific key fits into a specific lock. The substrate binds to the active site to form an enzyme-substrate complex. Inside the complex, the reaction is catalysed, converting the substrate into products. The products then leave the active site, leaving the enzyme unchanged and free to catalyse another reaction. This specificity explains why an enzyme that digests starch (amylase) cannot digest protein.

Temperature significantly affects the rate of enzyme-catalysed reactions. As temperature increases from a low value, both enzyme and substrate molecules gain kinetic energy. This leads to more frequent collisions between them, and these collisions have more energy, increasing the rate of reaction. This continues until the enzyme reaches its optimum temperature, which is the temperature at which it has maximum activity. For most human enzymes, this is around 37°C. If the temperature increases beyond the optimum, the enzyme starts to denature. The high temperature causes the bonds holding the protein's specific 3D shape to vibrate and break. This changes the shape of the active site, so the substrate can no longer bind. The enzyme is denatured and the reaction rate falls rapidly. This change is irreversible.

Just like temperature, pH also has a profound effect on enzyme activity. Each enzyme has an optimum pH at which it functions most effectively. For example, pepsin, an enzyme in the acidic environment of the stomach, has an optimum pH of around 2, while trypsin, in the alkaline small intestine, works best at a pH of about 8. If the pH moves away from the optimum (either more acidic or more alkaline), the rate of reaction decreases. This is because pH affects the ionic and hydrogen bonds that maintain the enzyme's specific 3D shape. A change in pH alters the shape of the active site, making it less complementary to the substrate, thus slowing the reaction. Extreme changes in pH can cause the enzyme to denature permanently, just as high temperatures do.

Enzymes are widely used in industry because they are highly specific and can work under mild conditions (lower temperatures and pressures), which saves energy and reduces costs. Most industrial enzymes are sourced from microorganisms like bacteria and fungi, which can be grown in large quantities. Key examples include:

1. Biological Washing Powders: Contain proteases to break down protein-based stains (e.g., blood, grass) and lipases to break down fat/oil-based stains.
2. Food Processing: 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 in milk to produce lactose-free milk for intolerant individuals.
3. Baby Food Production: Proteases are used to pre-digest the protein in baby foods, making them easier for infants to digest.

In many industrial processes, it is beneficial to use immobilised enzymes. Immobilisation is a technique where enzyme molecules are attached to or trapped within an inert, insoluble support material, such as small beads of alginate gel or synthetic fibres. This has several major advantages over using free enzymes in a solution. Firstly, the product is not contaminated with the enzyme, which simplifies the purification process and reduces costs. Secondly, the immobilised enzymes can be easily recovered from the mixture and reused, making the process more economical. Thirdly, the process can be run continuously by passing the substrate over the immobilised enzymes in a column, which is more efficient than batch production. Finally, immobilisation often increases the stability of the enzyme, allowing it to withstand higher temperatures or non-optimal pH conditions.