Enzymes

Unit 5 Enzymes

Key unit competence: Explain the role of enzymes as biological catalysts and the factors that affect their activity.

Key ideas

• Enzymes are biological catalysts: proteins that speed up chemical reactions in living organisms without being used up or changed at the end of the reaction.
• The substance an enzyme acts on is its substrate; what is formed are the products. Examples: amylase breaks starch into maltose; protease (pepsin, trypsin) breaks proteins into amino acids; lipase breaks fats into fatty acids and glycerol; catalase breaks hydrogen peroxide into water and oxygen.
• Properties of enzymes: they are proteins; they are specific (one enzyme acts on one substrate); they work in small amounts and can be reused; they are affected by temperature and pH; many reactions they catalyse are reversible.
• Lock and key model: the enzyme has an active site with a shape that fits only its substrate (the key). The substrate binds to form an enzyme-substrate complex; the reaction takes place; the products leave and the enzyme is free again.
• Temperature: activity increases as temperature rises up to the optimum temperature (about 37 °C for human enzymes) because molecules move faster and collide more. Above the optimum, the active site changes shape and the enzyme is denatured; activity falls quickly. At low temperatures enzymes are inactive but not destroyed.
• pH: each enzyme has an optimum pH: pepsin in the stomach works best at about pH 2 (acid); salivary amylase at about pH 7; trypsin at about pH 8 (alkaline). Very high or low pH denatures enzymes.
• Other factors: substrate concentration (rate rises until all active sites are busy), enzyme concentration (more enzyme, faster rate if substrate is plentiful) and inhibitors (substances such as heavy metals that block enzymes).
• Intracellular enzymes work inside cells (respiration enzymes); extracellular enzymes are secreted and work outside cells (digestive enzymes).
• Uses of enzymes: digestion, respiration, photosynthesis; industry: biological washing powders (proteases, lipases), brewing and baking (yeast enzymes), sweetening porridge or making syrups with amylase, tenderising meat (papain from pawpaw), cheese making (rennin).

Common misconceptions

• "Enzymes are killed by high temperature." No: enzymes are not living; they are denatured (their shape changes permanently).
• "Enzymes stop working in the cold because they are destroyed." No: at low temperature they are only inactive; they work again when warmed.
• "One enzyme can digest any food." No: enzymes are specific; amylase cannot digest protein.
• "Enzymes are used up in the reaction." No: they are released unchanged and reused many times.

Worked examples

Practice questions

1. Define an enzyme and state three properties of enzymes.

   Show answer

   An enzyme is a protein that acts as a biological catalyst. Properties: protein in nature; specific; not used up; work in small amounts; affected by temperature and pH; denatured by high temperature.

   Common mistake: Saying enzymes are living.

2. Explain the lock and key model of enzyme action.

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   The enzyme's active site has a specific shape; only the substrate with the matching shape fits (like a key in a lock), forming an enzyme-substrate complex; the reaction occurs; products leave; the enzyme is unchanged.

   Common mistake: Saying the enzyme changes into the product.

3. Explain the effect of temperature on enzyme activity.

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   Activity rises with temperature up to the optimum because molecules move faster and collide more; above the optimum the enzyme is denatured (active site loses its shape) and activity falls sharply.

   Common mistake: Saying enzymes die at high temperature.

4. Name the substrate and products of: (a) amylase; (b) lipase; (c) catalase.

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   (a) Starch → maltose. (b) Fats → fatty acids and glycerol. (c) Hydrogen peroxide → water and oxygen.

   Common mistake: Writing glucose as the product of amylase.

5. Distinguish between intracellular and extracellular enzymes, with an example of each.

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   Intracellular enzymes work inside the cell that made them (respiration enzymes, catalase); extracellular enzymes are secreted and work outside the cell (amylase, pepsin, lipase in the gut).

   Common mistake: Saying all enzymes work in the gut.

6. Give three uses of enzymes in industry or at home.

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   Proteases and lipases in washing powders; yeast enzymes in brewing and baking; papain to tenderise meat; rennin in cheese making; amylase to make syrups/sweet porridge.

   Common mistake: Listing uses of hormones.

7. Describe an experiment to show the effect of pH on amylase.

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   Set tubes of starch solution with buffers of different pH, add equal amylase to each at 37 °C, take drops every minute onto iodine on a tile; record the time until iodine stays brown. The shortest time shows the optimum pH.

   Common mistake: Changing the temperature at the same time as pH.

8. Explain why food kept in a refrigerator stays fresh longer.

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   Low temperature slows the enzymes of microorganisms and of the food itself, so decay and reproduction of microbes are slow.

   Common mistake: Saying cold kills all microbes.

Matching and drag-and-drop

1. 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. Protease enzymes break down proteins into:	a. denatured
   2. Which enzyme is used to tenderise meat?	b. active site
   3. The part of an enzyme where the substrate binds is the:	c. Papain from pawpaw
   4. When an enzyme is heated to a high temperature, it is:	d. amino acids

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   1 → d (amino acids) · 2 → c (Papain from pawpaw) · 3 → b (active site) · 4 → a (denatured)

   • Amino acids are absorbed in the small intestine.
   • Papain is a protease.
   • Its shape fits the substrate.
   • Its active site changes shape permanently.

2. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   37 °C  ·  inhibitors  ·  hormones  ·  intracellular enzymes
   1. Substances that slow or stop enzyme action are .
   2. The optimum temperature of most human enzymes is about .
   3. Enzymes that work inside the cell where they are made are .
   Show answer

   1: inhibitors · 2: 37 °C · 3: intracellular enzymes

   • Heavy metals such as lead are inhibitors.
   • This is normal body temperature.
   • Respiration enzymes are intracellular.

3. 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. Enzymes are said to be specific because:	a. each acts on one type of substrate
   2. Biological washing powders contain:	b. molecules move slowly and collide less
   3. Low temperatures slow down enzymes because:	c. proteases and lipases
   4. The complex formed when substrate binds an enzyme is the:	d. enzyme-substrate complex

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   1 → a (each acts on one type of substrate) · 2 → c (proteases and lipases) · 3 → b (molecules move slowly and collide less) · 4 → d (enzyme-substrate complex)

   • The active site fits one substrate.
   • They digest protein and fat stains.
   • Warming the enzyme restores activity.
   • Products then leave the active site.

4. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   unchanged and can be used again  ·  biological catalysts made of protein  ·  hormones carried in the blood  ·  starch into maltose
   1. Amylase breaks down .
   2. At the end of a reaction, an enzyme is .
   3. Enzymes are .
   Show answer

   1: starch into maltose · 2: unchanged and can be used again · 3: biological catalysts made of protein

   • Salivary amylase starts starch digestion in the mouth.
   • Only small amounts are needed.
   • They speed up reactions in living things.

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