Enzymes

Lessons

Senior 4 · Biology

Enzymes

4 lessons

Lesson 4 of 4

The importance of enzymes in living organisms

25 min

Why sorghum is sprouted before brewing

To make the traditional sorghum drink ikigage, the grain is first soaked and left to sprout for a few days before it is dried and ground. Yeast cannot ferment starch, only sugars. A germinating seed, however, switches on genes for amylase to break down its starch store for the growing embryo. Sprouting (malting) fills the grain with amylase, which then turns the starch into sugars that yeast can use. People were using enzymes long before anyone knew what they were. This lesson pulls together why enzymes matter: inside cells, in digestion, and in industry and medicine.

Definition

Metabolism: all the chemical reactions in an organism. Anabolic reactions build large molecules from small ones (for example protein synthesis); catabolic reactions break large molecules down (for example respiration, digestion).

Metabolic pathway: a sequence of reactions, each catalysed by its own enzyme, in which the product of one reaction is the substrate of the next.

End-product (feedback) inhibition: control of a pathway in which its final product inhibits an enzyme near the start of the pathway.

Immobilised enzyme: an enzyme fixed to or trapped in an inert material, such as beads of alginate gel, so it can be reused and kept out of the product.

Enzymes run and control metabolism

Without enzymes, the reactions of life would be far too slow at body temperature. Because each step needs its own enzyme, a cell controls a pathway simply by controlling its enzymes: making more or less of them, or switching them on and off.

Figure 1. End-product inhibition. When the cell has enough D, D inhibits enzyme 1, usually by binding to a site away from the active site; the whole pathway slows, and no materials are wasted making B and C. When D is used up, enzyme 1 is released and the pathway speeds up again: negative feedback.

For example, when a cell has plenty of ATP, ATP inhibits an enzyme early in glycolysis, so glucose is not broken down faster than energy is needed.

Process	Enzyme(s)	Role
Respiration	Dehydrogenases, ATP synthase	Release energy from glucose and make ATP
Photosynthesis	Rubisco	Fixes carbon dioxide into organic molecules
Copying DNA	DNA polymerase, DNA ligase	Make new DNA before cell division
Defence	Lysozyme (in tears and saliva)	Breaks down bacterial cell walls
Blood clotting	Thrombin	Converts soluble fibrinogen to insoluble fibrin
Detoxification	Catalase	Breaks down toxic hydrogen peroxide

Worked example 1: reasoning about feedback

In Figure 1, a cell suddenly starts using up D very fast. Predict and explain what happens to (a) the amount of D, (b) the activity of enzyme 1, (c) the rate of the whole pathway.

(a) D falls, because it is used faster than it is made.
(b) With less D, fewer D molecules are bound to enzyme 1, so more enzyme 1 molecules are active.
(c) The pathway speeds up, making more D. As D builds up again, it inhibits enzyme 1 and the rate settles: the level of D is held steady by negative feedback.

Enzymes in digestion

Large food molecules cannot cross the gut wall. Extracellular enzymes hydrolyse them into small, soluble molecules that can be absorbed.

Where	Enzyme	Substrate → products	Conditions
Mouth	Salivary amylase	Starch → maltose	About neutral; needs chloride ions
Stomach	Pepsin	Proteins → polypeptides	Acid (pH about 2) from hydrochloric acid
Small intestine (from the pancreas)	Pancreatic amylase, trypsin, lipase	Starch → maltose; polypeptides → shorter peptides; lipids → fatty acids and glycerol	Alkaline (about pH 8); bile emulsifies fats, giving lipase a larger surface
Wall of the small intestine	Maltase, sucrase, lactase, peptidases	Disaccharides → monosaccharides; peptides → amino acids	Alkaline

When one enzyme is missing, the effects show. Lactase breaks down milk sugar, and about 65 % of people worldwide make much less lactase after infancy (MedlinePlus Genetics, 2023). Undigested lactose reaches the large intestine, where bacteria ferment it, causing bloating and diarrhoea. Many such people can still eat fermented milk such as yoghurt or ikivuguto, because bacteria have already broken down much of the lactose.

Enzymes in industry and medicine

Use	Enzyme	What it does
Biological washing powder	Proteases, lipases, amylases (often from bacteria that tolerate heat and alkali)	Hydrolyse protein stains (blood, egg), fats and starch at low wash temperatures, saving energy
Fruit juice	Pectinase	Breaks down pectin in the cell walls, so more juice comes out and it is clearer
Brewing and baking	Amylase (from malted grain or microbes)	Turns starch into sugars for yeast to ferment
Lactose-free milk	Lactase, often immobilised	Hydrolyses lactose into glucose and galactose
Cheese making	Chymosin (rennin)	Clots milk protein
Meat tenderising	Papain (papaya), bromelain (pineapple)	Partly digests the proteins of tough meat
Testing blood glucose	Glucose oxidase on a test strip	Reacts only with glucose; the reaction produces a signal read by a meter, used by people with diabetes
DNA testing (PCR)	Taq polymerase	Copies DNA at high temperature

Industry often immobilises enzymes. The enzyme can be reused many times, the product is not contaminated with enzyme, and the fixed enzyme is often more stable to heat and pH. The costs are setting it up, and a slightly lower rate, because the substrate has to reach the trapped enzyme.

Worked example 2: does pectinase pay?

Two 100 g samples of crushed mango pulp were kept at 40 °C for 20 minutes, one with pectinase and one with water, then filtered for 10 minutes (illustrative results). With pectinase, $68\,\mathrm{cm^3}$ of juice was collected; without, $41\,\mathrm{cm^3}$ . (a) Calculate the percentage increase in yield. (b) Why was the control kept at the same temperature for the same time?

(a) $\frac{68 - 41}{41} \times 100 = \frac{27}{41} \times 100 = 65.9\,\%$ , about two-thirds more juice.
(b) So that the only variable differing between the two is the enzyme; any juice released by warming or standing alone shows up in the control too.

Worked example 3: the best wash temperature

Egg-stained cloth was washed in a biological powder at different temperatures, and the time for the stain to disappear was recorded (illustrative):

Temperature / °C	20	30	40	50	60
Time / min	50	26	15	22	stain remained after 60

Rate $\propto \frac{1}{\text{time}}$ : at 40 °C, $\frac{1}{15} = 0.067\,\mathrm{min^{-1}}$ ; at 20 °C, $\frac{1}{50} = 0.020\,\mathrm{min^{-1}}$ . The optimum is near 40 °C.
Above it the protease begins to denature (slower at 50 °C); at 60 °C it has been denatured, so the stain is not removed.
Evaluation: washing at 40 °C rather than at a hot 60 °C removes the stain faster and uses less energy to heat water. Wool and silk are made of protein, so they should not be washed in biological powder.

Amoeba Sisters: "Enzyme Examples, Cofactors/Coenzymes, Inhibitors, and Feedback Inhibition". Watch for real examples of enzymes and how feedback inhibition controls a pathway.

Warning

Common mistakes

Saying digestive enzymes "absorb" food. They hydrolyse it; absorption is a separate step through the gut wall.
Writing that end-product inhibition stops the pathway for good. It is reversible: as the product is used, the enzyme works again.
Thinking biological washing powder works better in very hot water. Hot water denatures its enzymes.
Calling lactose intolerance an allergy. It is a shortage of the enzyme lactase, not an immune reaction.

Tip

Exam technique. For any industrial use, give three things: the enzyme, its substrate and product, and the advantage (faster, works at lower temperature, specific so few by-products, reusable if immobilised). For "evaluate" questions, add one limitation (cost, denaturation, needs the right pH).

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Practice: the importance of enzymes

Distinguish between anabolic and catabolic reactions, with one enzyme-catalysed example of each.
Explain why sorghum grain is allowed to sprout before it is used for brewing.
Explain how end-product inhibition prevents a cell from wasting resources.
Name the enzyme, substrate and products in (a) the stomach's digestion of protein, (b) the making of lactose-free milk.
In a juice experiment, 50 g of apple pulp gave $18\,\mathrm{cm^3}$ of juice without pectinase and $27\,\mathrm{cm^3}$ with it. Calculate the percentage increase.
Give two advantages and one disadvantage of using immobilised enzymes in industry.
Someone feels bloated after drinking fresh milk but not after drinking ikivuguto. Explain why.

Show answersHide answers

Anabolic reactions build large molecules from small ones (DNA polymerase building DNA); catabolic reactions break them down (amylase hydrolysing starch; respiration).
During germination the grain makes amylase, which breaks down its stored starch into maltose and other sugars; yeast can ferment sugars but not starch, so the sprouted grain provides the sugar for fermentation.
When enough end product has been made, it inhibits the first enzyme of the pathway, so intermediates are not made that would not be used; when the product is used up, the inhibition is removed and the pathway restarts.
(a) Pepsin: proteins → polypeptides. (b) Lactase: lactose → glucose + galactose.
$\frac{27 - 18}{18} \times 100 = 50\,\%$ .
Advantages (any two): reused many times; the product is not contaminated with enzyme; often more stable to heat and pH; continuous production is possible. Disadvantage: costly to set up, or a lower rate because the substrate must reach the trapped enzyme.
The person makes little lactase, so lactose in fresh milk reaches the large intestine, where bacteria ferment it, producing gas. In ikivuguto, bacteria have already fermented much of the lactose, so there is less left to cause symptoms.

Quick check

In a pathway A → B → C → D, product D inhibits the first enzyme. What happens when the cell has more D than it needs?

Key points

Every step of metabolism is catalysed by its own enzyme, so cells control pathways by controlling enzymes; end-product inhibition is negative feedback.
Digestive enzymes hydrolyse large food molecules into small, absorbable ones; a missing enzyme (such as lactase) causes symptoms.
Industry uses enzymes because they are specific, fast and work in mild conditions: washing powders, juice, brewing, lactose-free milk, cheese, test strips and PCR. Immobilised enzymes can be reused.

1Why sorghum is sprouted before brewing

2Enzymes run and control metabolism

3Enzymes in digestion

4Enzymes in industry and medicine

Why sorghum is sprouted before brewing

Enzymes run and control metabolism

Enzymes in digestion

Enzymes in industry and medicine