Unit 7 Enzymes
Key unit competence: Explain the mode of action of enzymes, factors that affect their activity, and why they are essential for life.
Key ideas
- Enzymes are globular proteins that act as biological catalysts: they speed up metabolic reactions by lowering the activation energy, without being used up. Anabolic reactions build molecules; catabolic reactions break them down.
- Naming: usually the substrate + "-ase" (maltase, lipase, protease) or the reaction (oxidoreductases, hydrolases, transferases, ligases). Some old names remain (pepsin, trypsin, rennin).
- Properties: specific; work in small amounts and are reused; reversible reactions; affected by temperature, pH, concentrations and inhibitors; have a high turnover number (molecules of substrate converted per second).
- Lock and key model: the active site has a rigid shape complementary to the substrate. Induced fit model: the active site changes shape slightly when the substrate binds, fitting it more closely and putting strain on its bonds; this better explains how enzymes lower activation energy.
- Temperature: rate doubles for about every 10 °C rise (Q₁₀ ≈ 2) up to the optimum (about 37–40 °C in humans); above it, bonds holding tertiary structure break and the enzyme is denatured. Some bacteria in hot springs have enzymes with optima above 70 °C.
- pH: changes in H⁺ concentration alter ionic and hydrogen bonds in the active site; each enzyme has an optimum pH (pepsin ~2, salivary amylase ~7, trypsin ~8).
- Substrate concentration: rate increases until all active sites are occupied (Vmax); then enzyme concentration limits the rate. Enzyme concentration: rate is proportional to enzyme concentration when substrate is in excess.
- Inhibitors: competitive inhibitors resemble the substrate and compete for the active site (effect reduced by adding more substrate; e.g. malonate inhibits succinate dehydrogenase). Non-competitive inhibitors bind elsewhere (allosteric site), changing the shape of the active site (effect not reversed by more substrate; e.g. heavy metals such as mercury and lead, cyanide on cytochrome oxidase). End-product inhibition regulates metabolic pathways.
- Cofactors help enzymes: inorganic ions (Cl⁻ activates amylase, Mg²⁺), coenzymes (organic, often from vitamins: NAD from vitamin B3, FAD from B2), and prosthetic groups (permanently bound, e.g. haem).
- Importance: enzymes make life possible at body temperature; control metabolic pathways (respiration, photosynthesis, digestion, DNA replication); uses in industry and medicine (washing powders, food processing, diagnostic test strips, biosensors).
Common misconceptions
- "Enzymes provide energy for reactions." No: enzymes lower the activation energy; they do not supply energy.
- "Competitive and non-competitive inhibitors both bind to the active site." No: non-competitive inhibitors bind to another site and change the active site's shape.
- "Enzymes are destroyed at low temperature." No: they are inactive at low temperature and work again when warmed.
- "At Vmax, adding more substrate increases the rate." No: all active sites are full; only more enzyme would raise the rate.
Worked examples
Example 1. Explain how a competitive inhibitor and a non-competitive inhibitor can be distinguished experimentally.
Solution
Increase the substrate concentration. With a competitive inhibitor the rate rises back towards normal, because substrate molecules outcompete the inhibitor for active sites. With a non-competitive inhibitor the maximum rate stays lower, because the inhibitor changes the active site shape regardless of substrate.
Example 2. The rate of an enzyme reaction is 2 units at 20 °C. Use Q₁₀ = 2 to estimate the rate at 30 °C and 40 °C, and explain why the rate at 60 °C may be zero.
Solution
Each 10 °C rise doubles the rate: 4 units at 30 °C and 8 units at 40 °C. At 60 °C the enzyme is likely denatured: bonds holding the tertiary structure break, the active site loses its shape, so no enzyme-substrate complexes form.
Practice questions
Define enzyme and explain how enzymes speed up reactions.
Show answer
An enzyme is a globular protein acting as a biological catalyst. It binds the substrate at the active site, forming an enzyme-substrate complex that lowers the activation energy, so the reaction proceeds faster at body temperature.
Common mistake: Saying enzymes add energy.
Compare the lock and key and induced fit models.
Show answer
Lock and key: rigid active site exactly complementary to the substrate. Induced fit: active site changes shape slightly on binding, enclosing the substrate and straining its bonds; explains activation energy lowering better.
Common mistake: Saying induced fit means the enzyme fits any substrate.
Explain the effect of pH on enzyme activity.
Show answer
Each enzyme has an optimum pH; changes in H⁺ ions alter ionic and hydrogen bonds in the active site, changing its shape and charge; far from the optimum the enzyme is denatured.
Common mistake: Saying pH changes the substrate only.
Distinguish between competitive and non-competitive inhibition with examples.
Show answer
Competitive: inhibitor resembles substrate and binds the active site; reversed by more substrate (malonate, ethanol-methanol). Non-competitive: binds elsewhere and changes active site shape; not reversed by substrate (lead, mercury, cyanide).
Common mistake: Saying both bind the active site.
Name three types of cofactors with examples.
Show answer
Inorganic ions (Cl⁻ for amylase, Mg²⁺); coenzymes (NAD, FAD, coenzyme A from vitamins); prosthetic groups (haem in catalase).
Common mistake: Calling substrates cofactors.
Explain the effect of substrate concentration on reaction rate.
Show answer
At low substrate concentration, rate rises as more active sites are filled; at high concentration all active sites are occupied (Vmax), so the rate levels off and enzyme concentration becomes limiting.
Common mistake: Saying rate rises forever.
State four importances of enzymes in living organisms.
Show answer
Speed up metabolic reactions at body temperature; control metabolic pathways; digestion of food; respiration and photosynthesis; DNA replication and protein synthesis; defence (lysozyme).
Common mistake: Saying enzymes are a source of energy.
Explain end-product inhibition.
Show answer
The final product of a metabolic pathway inhibits (often non-competitively) the first enzyme of the pathway, so when the product accumulates, the pathway slows; when product falls, inhibition is removed: a negative feedback control.
Common mistake: Saying the product destroys the enzyme permanently.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A competitive inhibitor: a. cytochrome oxidase in respiration 2. Cyanide is poisonous because it inhibits: b. binds to the active site 3. The number of substrate molecules one enzyme converts per second is the: c. turnover number 4. Inhibition of the first enzyme in a pathway by the final product is: d. end-product inhibition Show answer
1 → b (binds to the active site) · 2 → a (cytochrome oxidase in respiration) · 3 → c (turnover number) · 4 → d (end-product inhibition)
- It resembles the substrate.
- Aerobic respiration stops.
- It can be thousands per second.
- It regulates the pathway.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
cofactors · inhibitors · competitive · a competitive inhibitor- Methanol poisoning is treated by giving ethanol, which competes for the same enzyme. Ethanol acts as .
- Adding more substrate restores the rate of an inhibited reaction to normal. The inhibitor is .
- Non-protein substances needed for some enzymes to work are .
Show answer
1: a competitive inhibitor · 2: competitive · 3: cofactors
- It reduces conversion of methanol to toxic products.
- Substrate outcompetes it.
- Ions, coenzymes, prosthetic groups.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Many coenzymes are made from: a. lowering the activation energy 2. Enzymes that break bonds by adding water are: b. hydrolases 3. Enzymes speed up reactions by: c. vitamins 4. Heavy metals such as lead and mercury act as: d. non-competitive inhibitors Show answer
1 → c (vitamins) · 2 → b (hydrolases) · 3 → a (lowering the activation energy) · 4 → d (non-competitive inhibitors)
- NAD comes from vitamin B3.
- Digestive enzymes are hydrolases.
- Less energy is needed to start the reaction.
- They disrupt enzyme structure.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
salivary amylase · anabolic · bonds that hold the tertiary structure · the substrate molecules- Chloride ions activate .
- High temperatures denature enzymes by breaking .
- Reactions that build large molecules are .
Show answer
1: salivary amylase · 2: bonds that hold the tertiary structure · 3: anabolic
- They are inorganic cofactors.
- The active site loses its shape.
- Example: protein synthesis.