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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Enzymes as Biological Catalysts
Enzymes are amazing protein molecules that speed up chemical reactions in living organisms without being used up themselves. They're essential for nearly all processes in your body - from digesting your lunch to copying your DNA. Without enzymes, these reactions would happen too slowly to sustain life!
Key Definitions:
Enzymes are vital because they can speed up reactions by millions of times! For example, the decomposition of hydrogen peroxide into water and oxygen would take years without the enzyme catalase, but happens in seconds with it. Your body contains thousands of different enzymes, each designed for a specific job.
Enzymes are proteins made up of amino acid chains folded into a specific 3D shape. This precise shape creates a special pocket called the active site, which has a shape that perfectly fits the substrate - like a key fitting into a lock.
Enzymes work by lowering the activation energy needed for a reaction to occur. This means reactions can happen much more easily and quickly. They do this by bringing substrate molecules together in the correct orientation, or by stressing chemical bonds to make them easier to break.
The most basic model of enzyme action is the 'lock and key' model. This explains how enzymes work:
The substrate molecule (the key) approaches the enzyme's active site (the lock).
The substrate fits into the active site, forming an enzyme-substrate complex.
The reaction occurs and products are formed. The enzyme is unchanged and released to work again.
Enzymes are highly specific - they usually catalyse only one reaction or a group of closely related reactions. This specificity comes from the unique shape of each enzyme's active site, which only allows certain substrate molecules to fit correctly.
People with lactose intolerance lack sufficient amounts of the enzyme lactase, which breaks down lactose (milk sugar) into simpler sugars. Without enough lactase, lactose passes undigested into the large intestine where bacteria ferment it, causing uncomfortable symptoms like bloating and gas. This shows how the absence of just one specific enzyme can affect the body!
Several factors can change how quickly enzymes work. Understanding these helps us control enzyme reactions in laboratories and industry.
As temperature increases, enzyme activity increases because molecules move faster and collide more frequently. However, if it gets too hot (usually above 40-50°C for human enzymes), the enzyme denatures - its shape changes, the active site is disrupted and it stops working.
Each enzyme works best at a specific pH called its optimum pH. Changes in pH can alter the shape of the active site. For example, digestive enzymes in the stomach work best in acidic conditions (low pH), while those in the small intestine prefer alkaline conditions (high pH).
Increasing enzyme or substrate concentration generally increases the rate of reaction up to a point. Eventually, adding more substrate won't help if all enzyme molecules are already working at maximum capacity.
Scientists often use graphs to show how enzymes behave under different conditions. These are important to understand for your exams!
A typical temperature-enzyme activity graph shows:
A typical pH-enzyme activity graph shows:
Enzymes aren't just important in our bodies - they're widely used in industry and everyday products too!
Enzymes are used in many industries:
Enzymes have important medical uses:
Biological washing powders contain enzymes like proteases, lipases and amylases that break down protein, fat and starch stains. They work best at lower temperatures (30-40°C), saving energy compared to non-biological powders that need hotter water. However, some people with sensitive skin may develop allergic reactions to these enzyme-containing detergents. This real-world application shows how understanding enzyme properties (temperature sensitivity, specificity) has practical benefits!
Sometimes, other molecules can interfere with enzymes and stop them working properly. These are called inhibitors and come in two main types:
These molecules have a similar shape to the substrate and compete for the active site. They temporarily block the enzyme but can be overcome by adding more substrate.
These attach to the enzyme away from the active site, changing its shape so the substrate no longer fits properly. Adding more substrate doesn't help overcome this type of inhibition.
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