How do enzymes control the chemical reactions of life?
Explain how enzymes work as biological catalysts and how their activity is affected by conditions.
How enzymes lower activation energy, the lock-and-key and induced-fit models, and the effects of temperature, pH, concentration, and inhibitors on enzyme activity, for TCE Biology Unit 2.
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What enzymes do
Almost every chemical reaction in a cell is controlled by an enzyme. Enzymes are biological catalysts: they speed up reactions without being changed or used up, so a small amount can catalyse many reactions. They make life possible by allowing reactions to happen fast enough at body temperature, when otherwise they would be far too slow.
Enzymes work by lowering the activation energy of a reaction, the energy barrier that must be overcome for the reaction to proceed. By bringing reactants together and straining their bonds, the enzyme provides an easier pathway, so the reaction happens at a much lower energy cost.
The active site and specificity
Each enzyme has a region called the active site, a precisely shaped pocket where the substrate (the reactant) binds. The shape of the active site matches the substrate, which is why each enzyme catalyses only one type of reaction. Two models describe the fit:
- The lock-and-key model: the substrate fits the active site exactly, like a key in a lock.
- The induced-fit model: the active site changes shape slightly as the substrate binds, moulding around it to grip it more tightly. This is the more accurate modern view.
Once bound, the enzyme and substrate form an enzyme-substrate complex, the reaction occurs, and the products are released, leaving the enzyme free to work again.
Effect of temperature
As temperature rises, molecules move faster and collide more often, so enzyme activity increases up to an optimum. Beyond the optimum, the heat breaks the bonds holding the enzyme's shape, and the active site changes so the substrate no longer fits. This is denaturation, and it is usually permanent. Human enzymes typically work best near body temperature.
Effect of pH
Each enzyme has an optimum pH at which its active site holds the correct shape. Moving away from this pH changes the charges on the enzyme and disrupts the active site, lowering activity, and extreme pH denatures the enzyme. Different enzymes have different optima: the stomach enzyme pepsin works best in acid conditions, while many others prefer near-neutral pH.
Effect of concentration
- Increasing substrate concentration speeds up the reaction until all active sites are busy. After that the enzyme is the limiting factor and the rate levels off (saturation).
- Increasing enzyme concentration speeds up the reaction as long as there is enough substrate, because more active sites are available.
Inhibitors
Inhibitors are molecules that reduce enzyme activity:
- Competitive inhibitors have a shape similar to the substrate and block the active site by competing for it. Adding more substrate can overcome them.
- Non-competitive inhibitors bind elsewhere on the enzyme and change the active site's shape, so the substrate no longer fits. Adding more substrate does not help.
Many drugs and poisons work as enzyme inhibitors, and cells use inhibition to control their own metabolism.
Metabolism and pathways
Enzymes rarely act alone. Cells run metabolic pathways, sequences of reactions in which the product of one enzyme is the substrate of the next. Controlling one enzyme can switch a whole pathway on or off, often by feedback inhibition, where the final product inhibits an enzyme early in the pathway. This keeps the cell from overproducing substances and links enzyme control to the wider regulation of cell chemistry.