Almost every reaction in a cell would be far too slow at 37 °C without help. Enzymes supply that help: globular proteins with an active site that holds one substrate, lowers the energy barrier and lets go of the products unchanged.
Enzymes are globular proteins that catalyse reactions either inside cells (intracellular, such as catalase) or after being secreted (extracellular, such as amylase in saliva). The substrate binds to the active site, a small region whose shape and charges are complementary to it, forming an enzyme-substrate complex. That makes the enzyme specific. In the lock-and-key hypothesis the site is rigid; in the induced-fit hypothesis it changes shape slightly as the substrate binds, straining its bonds. Either way the enzyme lowers the activation energy, so more collisions succeed at the same temperature, and it leaves the reaction unchanged. Reaction progress is followed by measuring product formed (oxygen from catalase) or substrate lost (starch with amylase), and the initial rate is used to compare conditions.
Fig. 1: a progress curve for catalase; the gradient of the tangent at the start gives the initial rate
Section 01
A globular protein with a pocket.
Rotate the enzyme in the simulator, then bring in the substrate. Watch the enzyme-substrate complex form, the bond break and the two products leave. Switch between lock and key and induced fit, and try the wrong substrate.
Globular protein: a polypeptide folded into a compact, roughly spherical tertiary structure, with hydrophilic R groups outside, so the enzyme is soluble.
Intracellular enzymes: work inside the cell that made them, for example catalase breaking down hydrogen peroxide, or the enzymes of respiration and DNA replication.
Extracellular enzymes: secreted by exocytosis and work outside the cell, for example salivary amylase and the protease trypsin in the small intestine.
Active site: a small cleft formed by a few amino acids; their R groups give it a shape and charge pattern complementary to one substrate.
Enzyme-substrate complex: the substrate is held in the active site by temporary bonds; the reaction happens here and the products then leave.
Lock-and-key hypothesis
Induced-fit hypothesis
active site is a rigid shape exactly complementary to the substrate
active site is flexible and changes shape slightly as the substrate binds
explains specificity well
also explains specificity, and why binding strains bonds in the substrate
the older, simpler model
better supported by modern evidence
Examiner trap
Say complementary, not the same shape. The substrate and active site fit together like a key and lock, so their shapes are complementary. Also, the enzyme is not used up: it is released unchanged and can bind another substrate molecule.
Section 02
Lowering the activation energy.
Fire 200 collisions with and without the enzyme. In the lab, the barrier drops from 160 to 80 kJ/mol with the enzyme, and the share of collisions with enough energy rises sharply even though the temperature has not changed.
Activation energy: the minimum energy that colliding molecules need for the reaction to occur.
How enzymes lower it: by holding the substrate in the right orientation, bringing reacting groups close together, and (induced fit) putting strain on bonds so they break more easily.
Result: at body temperature a much larger proportion of collisions succeed, so the rate is far higher without heating the cell.
What does not change: the energy of the reactants and products, so the overall energy change and the final equilibrium position are unaffected.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the simulator, what happens when the wrong substrate approaches the enzyme?
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How does the induced-fit model differ from the lock-and-key model?
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In the activation energy simulator the barrier falls from 160 to 80 kJ/mol with the enzyme. What is the effect at the same temperature?
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Which is an example of an extracellular enzyme?
Section 03
Following a reaction's progress.
Start the reaction in the simulator and watch the substrate curve fall as the product curve rises. Change the number of enzyme or substrate molecules and compare the initial rate reading.
Method
What is measured
How
catalase and hydrogen peroxide
rate of product formation
collect the oxygen in a gas syringe or upturned measuring cylinder and read the volume at regular intervals
amylase and starch
rate of substrate disappearance
remove samples at intervals and add to iodine solution; the blue-black colour fades as starch is hydrolysed to maltose
colorimeter
a colour change, measured objectively
select a filter of the complementary colour, zero with a blank, then record absorbance (or transmission) of the reaction mixture over time
Initial rate: draw a tangent to the curve at t = 0 and find its gradient (for example, cm3 of O2 per second).
Why initial rate: it is measured while the substrate concentration is still known and before products accumulate, so it is a fair comparison between conditions.
Colorimeter calibration: measure the absorbance of known starch concentrations with iodine to make a calibration curve, then read unknown concentrations from it.
Examiner trap
The curve levels off because the substrate has run out, not because the enzyme has been used up or denatured. If you add more substrate the reaction carries on at once.
Stage 2 · Exam
Exam-style questions
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Finish the four checks above to unlock the exam questions
Exam style+20 XP
A student measures oxygen released from hydrogen peroxide by catalase every 30 s. Why is the initial rate used to compare different catalase concentrations?
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Starch is hydrolysed by amylase and samples are tested with iodine in a colorimeter. Which change in the reading shows the reaction progressing?
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Which statement best explains why an enzyme is specific to one substrate?
Exam style+20 XP
In a catalase experiment, the volume of oxygen rises from 0 to 18 cm3 in the first 20 s along an almost straight line. What is the initial rate?
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How enzymes work, mastered.
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