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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: pH Effects on Enzymes
Enzymes are biological catalysts that speed up chemical reactions in living organisms. They're incredibly important for all life processes, from digestion to respiration. However, enzymes are quite fussy about their working conditions and pH is one of the most critical factors affecting how well they work.
Key Definitions:
pH is a measure of how acidic or alkaline a solution is. It's based on the concentration of hydrogen ions (H⁺) in the solution. The pH scale runs from 0 to 14:
Each step on the pH scale represents a 10-fold change in H⁺ concentration. So pH 4 is 10 times more acidic than pH 5 and 100 times more acidic than pH 6!
Enzymes are proteins with a specific 3D shape. The shape is crucial because it creates an 'active site' where substrates (the molecules the enzyme works on) fit in - like a key in a lock.
This 3D shape is held together by several types of chemical bonds and interactions, including hydrogen bonds and ionic bonds. These bonds can be affected by changes in pH, which is why pH has such a big impact on enzyme function.
Enzymes have an optimum pH - the pH value at which they work best. At this pH, the enzyme's active site has the perfect shape to bind to its substrate. If the pH moves away from this optimum, the enzyme becomes less effective and eventually stops working altogether.
When the pH is too low (too acidic), there are too many H⁺ ions in the solution. These can:
Result: The substrate no longer fits properly and the enzyme's activity decreases.
At the optimum pH, the enzyme has the perfect shape for its job. The active site is exactly the right shape to bind to the substrate, allowing the enzyme to work at maximum efficiency.
Different enzymes have different optimum pH values, depending on where in the body or cell they normally work.
When the pH is too high (too alkaline), there are too few H⁺ ions in the solution. This can:
Result: Again, the substrate doesn't fit properly and enzyme activity decreases.
If the pH becomes extremely different from an enzyme's optimum, the enzyme can become denatured. Denaturation is when the enzyme's 3D structure completely falls apart, like unravelling a ball of wool. Once an enzyme is denatured, it can't function at all and this change is usually permanent.
You can see protein denaturation when you cook an egg. The clear egg white (which contains a protein called albumin) turns white and solid when heated. This is because the heat denatures the protein, changing its structure permanently. Strong acids can have a similar effect - this is why lemon juice can be used to 'cook' fish in dishes like ceviche!
Not all enzymes work best at the same pH. In fact, enzymes have evolved to work best in the conditions where they're normally found in the body.
Digestive enzymes work in different parts of the digestive system, each with its own pH:
These different optima make sense when you think about where these enzymes work. The stomach is very acidic (pH 1.5-3.5), while the small intestine is slightly alkaline (pH 7.5-8.5).
Most enzymes that work inside our cells have an optimum pH close to 7, which matches the neutral pH inside most cells. However, some cellular compartments have different pH values:
The enzymes in these compartments have evolved to work best at these specific pH values.
Understanding how pH affects enzymes has many practical applications in everyday life and industry:
These contain enzymes that break down protein stains (like blood or egg). They work best at the slightly alkaline pH of washing solutions.
Some modern detergents contain enzymes that have been engineered to work across a wider pH range.
Enzymes are used in food production, from making cheese to clarifying fruit juices. The pH must be carefully controlled to ensure the enzymes work efficiently.
For example, rennet (containing the enzyme chymosin) is used to curdle milk in cheese-making and works best at a slightly acidic pH.
Some medical conditions involve pH changes that affect enzyme function. For example, acidosis (when the blood becomes too acidic) can disrupt many enzymes in the body.
Understanding enzyme pH optima also helps in developing medicines that work in specific parts of the body with different pH values.
People with lactose intolerance lack the enzyme lactase, which breaks down the milk sugar lactose. They can take lactase supplements to help digest dairy products. These supplements are designed to work at the acidic pH of the stomach, where they need to start breaking down lactose before the food moves into the intestines. This is a great example of how understanding enzyme pH optima helps in developing effective treatments!
In the lab, you might investigate how pH affects enzyme activity. A common experiment uses the enzyme catalase, which is found in potato or liver tissue and breaks down hydrogen peroxide into water and oxygen.
By measuring the rate of oxygen production at different pH values, you can determine the optimum pH for catalase. Typically, you would:
The graph would typically show a bell-shaped curve, with the highest point representing the optimum pH.
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