Enzymes and Metabolism - TCE Biology (Tasmania)
“Enzymes: structure and function (globular protein, active site, denaturation); factors affecting rate of reaction (temperature, pH, enzyme concentration, substrate concentration, inhibitors, coenzymes and cofactors); competitive and non-competitive inhibitors; coenzymes and cofactors (general understanding only)”
Enzymes are protein catalysts that speed up chemical reactions by lowering activation energy, without being used up. Each enzyme has an active site shaped to fit a specific substrate, described by the lock-and-key and induced-fit models. Enzyme activity depends on temperature, pH, substrate and enzyme concentration, and the presence of inhibitors. Extreme heat or pH denatures the enzyme and stops it working.
What this dot point is asking
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.
An enzyme is a globular protein that acts as a biological catalyst, speeding up a specific chemical reaction by lowering its activation energy, without being permanently changed or consumed in the process.
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.
High temperature or extreme pH disrupts the bonds that hold an enzyme in its precise shape. The active site loses its form, the substrate can no longer bind, and activity stops. Denaturation is usually irreversible, so a boiled enzyme does not recover when cooled.
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.
A graph of reaction rate against temperature rises to a peak at about 37 degrees, then falls sharply. The rising part shows faster molecular collisions increasing activity. The peak is the optimum temperature. The sharp fall shows denaturation as heat destroys the active site. The asymmetry, a gentle rise but a steep fall, is a clue that denaturation is occurring rather than simply slower reactions.
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.
Coenzymes and cofactors
Some enzymes only work when a non-protein helper is attached. Cofactors are inorganic ions, often obtained from minerals in the diet, and coenzymes are organic molecules, many made from vitamins. They help the substrate bind or take part in the reaction, so a shortage can slow an enzyme-controlled reaction. BIO315124 needs only this general understanding, not specific examples.
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.
A frequent error is to treat enzymes as reactants that are consumed. Enzymes are catalysts: they are released unchanged after each reaction and can be reused many times. They are not used up, which is why a tiny amount of enzyme can process a large amount of substrate.
Enzymes are part of criterion 5 (analyse the processes and mechanisms by which biological systems are regulated), examined in Section B of the TASC exam. The course names the globular protein structure, the active site, denaturation, the factors affecting rate, competitive and non-competitive inhibitors, and a general understanding of coenzymes and cofactors. Enzymes then reappear in photosynthesis, cellular respiration and DNA replication.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original7 marksA student measures the rate of an enzyme-controlled reaction at temperatures from to . The rate rises to a peak at and then falls sharply to almost zero by . Explain the shape of this curve, including why the rate rises, why it peaks, and why it falls at high temperature.Show worked answer →
A 7 mark answer explains the three regions of the curve in molecular terms.
- Rise (0 to 40 degrees)
- As temperature increases, enzyme and substrate molecules gain kinetic energy and move faster, so they collide more frequently and with more energy. More successful collisions form enzyme-substrate complexes, so the reaction rate rises.
- Peak (40 degrees)
- This is the optimum temperature, where the rate is highest because collision frequency is high but the enzyme is still correctly folded.
- Fall (above 40 degrees)
- High temperature breaks the bonds holding the enzyme's tertiary structure, so the active site changes shape (denaturation). The substrate no longer fits, fewer enzyme-substrate complexes form, and the rate drops to near zero. Denaturation is permanent.
Markers reward the kinetic-energy/collision reasoning for the rise, naming the optimum, and denaturation of the active site for the fall.
Original5 marksUsing the induced-fit model, explain why enzymes are specific, and explain how a competitive inhibitor reduces the rate of an enzyme-controlled reaction.Show worked answer →
A 5 mark answer applies induced fit to specificity and explains competitive inhibition.
Specificity (induced fit). Each enzyme has an active site with a shape complementary to its specific substrate. In the induced-fit model the active site moulds slightly around the substrate as it binds. Only a substrate of the correct shape can bind and be moulded, so each enzyme acts on one substrate or reaction type.
Competitive inhibitor. A competitive inhibitor has a shape similar to the substrate, so it binds to the active site and blocks it. While it occupies the active site, substrate molecules cannot bind, so fewer enzyme-substrate complexes form and the rate falls. The effect can be reduced by raising substrate concentration.
Markers reward the complementary-shape/induced-fit reasoning for specificity and the active-site-blocking mechanism for competitive inhibition.