Proteins: Enzymes

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Enzymes as Biological Catalysts

Enzymes are globular proteins that act as biological catalysts.
They are 'biological' because they function in living systems, and 'catalysts' because they speed up the rate of chemical reactions without being used up or changed.
The active site is a specific region on the enzyme where the substrate binds.
Virtually every metabolic reaction in living organisms is catalysed by an enzyme, making them essential for life.
Enzymes can be intracellular (produced and function inside the cell) or extracellular (secreted by cells and catalyse reactions outside cells, e.g. digestive enzymes in the gut).
Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions.

Enzyme Specificity and the Enzyme-Substrate Complex

The active site of an enzyme has a specific shape that is complementary to a specific substrate.
Substrates collide with the active site, and this must happen at the correct orientation and speed for a reaction to occur.
When an enzyme and its substrate join together, an enzyme-substrate complex forms temporarily.
The complex is only temporary; the enzyme catalyses the reaction and the product(s) are released, allowing the enzyme to be recycled for future reactions.
The specificity of an enzyme results from the complementary nature between the shape of the active site and its substrate(s).
The shape of the active site is determined by the complex tertiary structure of the protein, which is determined by the order of amino acids in the chain.
If the order of amino acids is altered, the resulting three-dimensional shape changes, affecting the active site.

How Enzymes Work: The Induced-Fit Model

The induced-fit model (or induced-fit hypothesis) is the currently accepted model of enzyme action.
In this model, the enzyme and its active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme.
These changes in shape are known as conformational changes.
This ensures an ideal binding arrangement between the enzyme and substrate, maximising the ability of the enzyme to catalyse the reaction.
The lock and key model (covered at GCSE) suggested that the rigid shape of the active site is a precise fit for the specific shape of the substrate.
New techniques have shown that proteins are not rigid structures; experiments showed that multiple regions of an enzyme molecule moved in response to the environment, especially when the substrate bound.
Evidence for the induced-fit model comes from X-ray diffraction techniques, which produced 3D pictures of the enzyme hexokinase before and after it bound to its substrate glucose, confirming that the active site changed shape after the substrate bound.

Activation Energy and the Mechanism of Catalysis

All chemical reactions are associated with energy changes, and for a reaction to proceed, there must be enough activation energy.
Activation energy is the amount of energy needed by the substrate to become unstable enough for a reaction to occur and for products to be formed.
Enzymes speed up chemical reactions because they influence the stability of bonds in the reactants.
The destabilisation of bonds in the substrate makes it more reactive.
Enzymes work by lowering the activation energy of a reaction by providing an alternative energy pathway.
The activation energy of a chemical reaction is lowered by the presence of a catalyst (i.e. an enzyme).

Required Practical: Measuring Enzyme Activity

The progress of enzyme-catalysed reactions can be investigated by measuring the rate of formation of a product (using catalase) or the rate of disappearance of a substrate (using amylase).
Catalase is an enzyme found in the cells of most organisms that breaks down hydrogen peroxide (a toxic by-product of metabolism) into water and oxygen.
In the catalase investigation, hydrogen peroxide and catalase are combined, and the volume of oxygen generated is measured over a set time to calculate the rate of reaction.
Amylase is a digestive enzyme that hydrolyses starch (the substrate) into maltose and glucose; it functions best at pH 7 and 37°C.
In the amylase investigation, amylase and starch are combined, and the reaction mixture is tested for starch at regular time intervals using iodine in potassium iodide solution.
Starch forms a blue-black colour with iodine solution; if no starch is present, the iodine solution remains yellow-brown.
The time taken for starch to be broken down can be measured, and the investigation can be repeated under a variety of conditions (e.g. altering pH, temperature, enzyme concentration or starch concentration) to compare reaction rates.
A colorimeter can measure light absorbance or light transmission and can be used in any enzyme-catalysed reaction that involves a colour change; as the colour breaks down, transmission increases or light absorption decreases.

Drawing Graphs and Using Tangents for Enzyme Rate Experiments

Line graphs should be used to present the results of enzyme rate experiments, with the independent variable on the x-axis and the dependent variable on the y-axis.
A line of best fit (straight or curved) should be added to identify trends; it must be smooth and have a balance of data points above and below the line.
For rate-concentration graphs, the line of best fit should be drawn through the origin (the reaction cannot occur if the concentration of enzyme or substrate is 0), but only if the data and trend allow it.
For linear graphs, the gradient is the same throughout, making it easy to calculate the rate of change (rate of change = change ÷ time).
The initial rate of reaction is the rate of reaction at the start of the line (i.e. where time = 0).
Many enzyme rate experiments produce non-linear graphs with an ever-changing gradient; in these cases, a tangent can be used to find the reaction rate at any one point on the graph.
To calculate the gradient of the tangent, use the formula: gradient = change in y-axis ÷ change in x-axis (rise over run).
When drawing a tangent, use a ruler and pencil to draw a perfectly straight line that just touches the curve at the chosen point, ensuring the curve remains visible.

Limiting Factors: Temperature

Enzymes have a specific optimum temperature – the temperature at which they catalyse a reaction at the maximum rate.
Lower temperatures either prevent reactions from proceeding or slow them down: molecules move relatively slowly, there is a lower frequency of successful collisions between substrate molecules and the active site, and therefore less frequent enzyme-substrate complex formation.
At lower temperatures, substrate(s) and enzyme collide with less energy, making it less likely for bonds to be formed or broken.
Higher temperatures speed up reactions: molecules move more quickly, there is a higher frequency of successful collisions, and therefore more frequent enzyme-substrate complex formation.
At higher temperatures, substrate(s) and enzyme collide with more energy, making it more likely for bonds to be formed or broken.
However, as temperatures continue to increase, the rate drops sharply as the enzyme begins to denature: bonds (e.g. hydrogen bonds) holding the enzyme in its precise shape start to break, causing the tertiary structure to change.
This permanently damages the active site, preventing the substrate from binding; denaturation has occurred if the substrate can no longer bind.
Very few human enzymes can function at temperatures above 50°C; humans maintain a body temperature of about 37°C, so temperatures exceeding 40°C will cause denaturation of enzymes.

Limiting Factors: pH

All enzymes have an optimum pH and are denatured at extremes of pH.
Hydrogen and ionic bonds hold the tertiary structure of the protein (i.e. the enzyme) together.
Below and above the optimum pH, solutions with an excess of H⁺ ions (acidic solutions) and OH⁻ ions (alkaline solutions) can cause these bonds to break.
This alters the shape of the active site, which means enzyme-substrate complexes form less easily; eventually, enzyme-substrate complexes can no longer form at all – complete denaturation has occurred.
Where an enzyme functions can be an indicator of its optimal environment; e.g. pepsin is found in the stomach, an acidic environment at pH 2, so its optimum pH is pH 2.
Buffer solutions each have a specific pH and maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change; they are used when investigating the effect of pH on enzyme rate.
The pH can be calculated using the equation: pH = -log₁₀ [H⁺].

Limiting Factors: Enzyme and Substrate Concentration

The higher the enzyme concentration, the greater the number of active sites available and the greater the likelihood of enzyme-substrate complex formation.
As long as there is sufficient substrate available, the initial rate of reaction increases linearly with enzyme concentration.
If the amount of substrate is limited, further increases in enzyme concentration will not increase the reaction rate as the amount of substrate becomes a limiting factor.
The greater the substrate concentration, the higher the rate of reaction: as the number of substrate molecules increases, the likelihood of enzyme-substrate complex formation increases.
If enzyme concentration remains fixed but substrate is increased past a certain point, all available active sites eventually become saturated, and any further increase in substrate concentration will not increase the reaction rate.
When the active sites are all full, any substrate molecules added have nowhere to bind to form an enzyme-substrate complex.
The enzyme is then working at its maximum rate, called Vmax (maximum velocity), and substrate molecules must wait for active sites to become available.

Limiting Factors: Inhibitors

An enzyme's activity can be reduced or stopped, temporarily, by a reversible inhibitor.
Competitive inhibitors have a similar shape to that of the substrate molecules and therefore compete with the substrate for the active site.
Non-competitive inhibitors bind to the enzyme at an alternative site, which alters the shape of the active site and therefore prevents the substrate from binding.
Increasing the concentration of an inhibitor reduces the rate of reaction, and eventually, if the inhibitor concentration continues to be increased, the reaction will stop completely.
For competitive inhibitors, countering the increase in inhibitor concentration by increasing the substrate concentration can increase the rate of reaction once more, as more substrate molecules mean they are more likely to collide with enzymes and form enzyme-substrate complexes.
For non-competitive inhibitors, increasing the substrate concentration cannot increase the rate of reaction once more, as the shape of the active site remains changed and enzyme-substrate complexes are still unable to form.
Reversible inhibitors can act as regulators in metabolic pathways through end-product inhibition: the end-product of a reaction binds to an alternative site on the original enzyme, changing the shape of the active site and preventing the formation of further enzyme-substrate complexes.
The end-product can then detach from the enzyme, allowing the active site to reform and the enzyme to return to an active state; as product levels fall, the enzyme begins catalysing the reaction once again, in a continuous feedback loop.

Control of Variables and Uncertainty

Enzyme rate experiments are carried out to determine the effect of changing a particular factor on the rate of a reaction catalysed by an enzyme.
Factors that can be changed include temperature, pH, enzyme concentration, and substrate concentration.
Only one of these variables must be changed during a particular experiment; this is known as the independent variable.
All other variables must be controlled (they must stay the same); these are known as the control variables.
For example, if investigating the effect of temperature on the rate of reaction, the pH, enzyme concentration and substrate concentration must be kept constant each time the experiment is carried out.
If these control variables are not kept constant, they could affect the results of the experiment, making the results invalid or unreliable.
Uncertainty is the amount of error your measurements might contain; results from experiments always contain some error because the sensitivity of the apparatus being used is limited.
A '±' sign tells you the range in which the true value lies; this range is called the margin of error.
The percentage error can be calculated using the formula: percentage error = (uncertainty ÷ measured value) x 100.

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1.Which of the following best describes an enzyme?
Easy
- AA globular protein that acts as a biological catalyst
- BA fibrous protein that provides structural support
- CA carbohydrate that speeds up chemical reactions
- DA lipid that lowers activation energy
2.What is the active site of an enzyme?
Easy
- AThe region where the substrate binds
- BThe entire surface of the enzyme
- CThe site where inhibitors permanently bind
- DThe part of the enzyme that is denatured first
3.According to the induced-fit model, what happens when a substrate binds to an enzyme?
Medium
- AThe active site and sometimes the substrate change shape slightly
- BThe enzyme and substrate remain rigid and perfectly complementary
- CThe substrate changes shape to fit the rigid active site
- DThe enzyme breaks down into amino acids
4.Which of the following statements about enzymes is correct?
Medium
- AThey lower the activation energy of a reaction
- BThey increase the activation energy of a reaction
- CThey are permanently changed by the reaction they catalyse
- DThey are consumed in the reaction
5.Which of the following are true about enzymes? (Select all that apply)
Medium
- AThey are globular proteins
- BThey have a specific active site
- CThey are used up in the reaction
- DThey lower activation energy
- EThey are always intracellular
6.Enzymes are fibrous proteins.
Easy
True or false?
7.The lock and key model is the currently accepted model of enzyme action.
Easy
True or false?
8.Match the enzyme-related term with its correct description.
Medium
- Active site
- Enzyme-substrate complex
- Denaturation
- Activation energy
- The energy needed for a reaction to occur
- The region where substrate binds
- Loss of enzyme shape and function
- Temporary complex formed when enzyme and substrate join

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9.Place the following steps of an enzyme-catalysed reaction in the correct order.
Medium
- Substrate binds to active site
- Enzyme-substrate complex forms
- Products are released
- Enzyme is free to catalyse another reaction
10.Which of the following statements about enzymes are correct? (Select all that apply)
Medium
- AEnzymes are globular proteins.
- BEnzymes speed up reactions by lowering the activation energy.
- CEnzymes are permanently altered by the reactions they catalyse.
- DEnzymes can be intracellular or extracellular.
- EEnzymes raise the activation energy of a reaction.
11.Match each enzyme term with its correct description.
Medium
- Active site
- Enzyme-substrate complex
- Denaturation
- Activation energy
- The region of an enzyme where a specific substrate binds.
- The temporary structure formed when an enzyme and substrate join together.
- Loss of the active site's shape due to extremes of heat or pH.
- The minimum energy required for a reaction to occur.
12.Place the following steps of an enzyme-controlled reaction in the correct order.
Medium
- Substrate collides with the active site.
- Enzyme-substrate complex forms.
- Enzyme catalyses the reaction.
- Products are released.
- Enzyme is free to be reused.
13.Which of the following best explains why an enzyme is specific to one substrate?
Medium
- AThe active site has a shape complementary to the substrate.
- BThe enzyme has a rigid shape that never changes.
- CThe substrate is always the same size as the enzyme.
- DThe enzyme lowers the activation energy of all reactions equally.
14.Enzymes are used up in the reactions they catalyse.
Easy
True or false?
15.The graph of an enzyme-catalysed reaction shows a curve that plateaus. Which method is used to find the initial rate of reaction?
Hard
- ADraw a tangent at time = 0 and calculate its gradient.
- BRead the value on the y-axis at the plateau.
- CCalculate the total product formed divided by the total time.
- DDraw a tangent at the plateau and calculate its gradient.
16.Which of the following are control variables when investigating the effect of temperature on enzyme activity? (Select all that apply)
Medium
- ApH
- BEnzyme concentration
- CSubstrate concentration
- DTemperature
- EVolume of oxygen produced
17.Which of the following statements about competitive and non-competitive inhibitors is correct?
Medium
- AA competitive inhibitor has a similar shape to the substrate and binds to the active site.
- BA non-competitive inhibitor binds to the active site and changes its shape.
- CIncreasing substrate concentration overcomes non-competitive inhibition.
- DCompetitive inhibitors permanently denature the enzyme.
18.Hydrogen peroxide is broken down by catalase into water and oxygen. The volume of oxygen produced after 1 minute was 18 cm³, 20 cm³ and 21 cm³ in three repeats. What is the mean volume of oxygen produced after 1 minute?
Easy
- A19.7 cm³
- B20.0 cm³
- C19.5 cm³
- D59.0 cm³
19.A protease hydrolyses protein. At 25°C, 1310 mg of protein is hydrolysed in 4 minutes. What is the rate of hydrolysis at 25°C?
Medium
- A0.33 mg s⁻¹
- B327.5 mg s⁻¹
- C5.46 mg s⁻¹
- D1310 mg s⁻¹
20.A student investigates the effect of pH on pepsin. At pH 4, 8 g of product is produced in 20 seconds. What is the rate of reaction at pH 4?
Medium
- A0.4 g s⁻¹
- B2.5 g s⁻¹
- C0.4 g min⁻¹
- D160 g s⁻¹
21.A colorimeter is used to follow the starch–amylase reaction. The percentage transmission is 80% with an uncertainty of ±0.5%. What is the percentage error of this measurement?
Medium
- A0.00625%
- B0.625%
- C6.25%
- D0.8%
22.A student uses a gas syringe to collect oxygen from the catalase–hydrogen peroxide reaction. The syringe reads to the nearest 1 cm³. What is the uncertainty of each volume reading?
Medium
- A±0.1 cm³
- B±0.5 cm³
- C±1 cm³
- D±2 cm³
23.In an enzyme rate experiment, a gas syringe collected 50 cm³ of oxygen with an uncertainty of ±0.5 cm³. Which calculation gives the percentage error of this measurement?
Medium
- A(50 ÷ 0.5) × 100
- B(0.5 ÷ 50) × 100
- C(0.5 × 50) ÷ 100
- D(50 − 0.5) × 100
24.A graph of rate of reaction against substrate concentration shows a curve that rises steeply and then plateaus. What is the name given to the maximum rate reached when all active sites are occupied?
Medium
- AOptimum rate
- BInitial rate
- CVmax
- DActivation energy
25.A graph of rate of reaction against enzyme concentration shows a straight line through the origin, but a graph of rate against substrate concentration levels off. Why does the substrate graph level off?
Medium
- AAll enzyme active sites are saturated with substrate
- BThe substrate molecules have been denatured
- CThe enzyme concentration has increased
- DThe activation energy has increased
26.In an experiment, molecule X reduces the rate of an enzyme-controlled reaction. Increasing the substrate concentration does not restore the original rate. Which type of inhibitor is molecule X?
Medium
- ACompetitive inhibitor
- BNon-competitive inhibitor
- CA coenzyme
- DA buffer
27.An enzyme in a metabolic pathway is inhibited by the end-product of the pathway. As the end-product concentration falls, the enzyme becomes active again. What is this process called?
Medium
- ACompetitive inhibition
- BEnd-product inhibition
- CDenaturation
- DActivation
28.A student investigates the effect of temperature on the rate of an enzyme-controlled reaction. Which of the following are control variables that must be kept constant? (Select all that apply)
Medium
- ApH
- BEnzyme concentration
- CSubstrate concentration
- DTemperature
- EVolume of oxygen produced
29.Which of the following statements about competitive inhibitors are correct? (Select all that apply)
Medium
- AThey have a similar shape to the substrate
- BThey bind to the active site
- CThey permanently denature the enzyme
- DIncreasing substrate concentration can overcome their effect
- EThey bind to an alternative site on the enzyme
30.Which of the following are valid ways to measure the rate of an enzyme-controlled reaction? (Select all that apply)
Medium
- AMeasuring the volume of oxygen produced over time
- BMeasuring the time taken for starch to disappear using iodine
- CMeasuring the change in light absorbance using a colorimeter
- DMeasuring the final mass of the enzyme after the reaction
- EMeasuring the temperature of the reaction mixture
31.A colorimeter can be used to follow the starch–amylase reaction because as starch is broken down, the percentage transmission increases.
Easy
True or false?
32.When investigating the effect of temperature on enzyme activity, the pH does not need to be controlled because it does not affect enzyme activity.
Easy
True or false?
33.In an enzyme rate experiment, increasing the enzyme concentration increases the rate of reaction only if there is sufficient substrate available.
Medium
True or false?
34.Match each experimental observation with the correct explanation.
Medium
- Rate of reaction increases as temperature rises from 10°C to 30°C
- Rate of reaction falls sharply above 50°C
- Rate of reaction plateaus as substrate concentration increases
- Rate of reaction increases linearly with enzyme concentration
- More frequent successful collisions between enzyme and substrate
- Hydrogen bonds break and the active site changes shape
- All active sites become occupied
- More active sites are available for substrate binding
35.Match each term with its correct description.
Medium
- Competitive inhibitor
- Non-competitive inhibitor
- End-product inhibition
- Denaturation
- Binds to an alternative site and changes the shape of the active site
- The end-product of a pathway inhibits an earlier enzyme
- Permanent loss of active site shape so substrate cannot bind
- Has a similar shape to the substrate and binds to the active site
36.Place the following steps in the correct order to measure the rate of the catalase–hydrogen peroxide reaction.
Medium
- Add hydrogen peroxide to the reaction vessel
- Add catalase to the hydrogen peroxide and start the timer
- Collect the oxygen produced in a gas syringe
- Record the volume of oxygen produced after a set time
- Calculate the rate of reaction
37.At 10°C, the iodine no longer changes colour after 300 seconds. What does this indicate?
Medium
- AAll the starch has been broken down
- BAll the amylase has been used up
- CThe iodine has been denatured
- DThe reaction has not started
38.At 50°C, the time taken for starch to disappear is 330 seconds, and at 60°C it is greater than 330 seconds. What is the best explanation for these results?
Medium
- AThe enzyme has been denatured at high temperatures
- BThe substrate concentration has increased
- CThe pH has decreased
- DThe enzyme concentration has increased
39.At pH 2, product production reaches a plateau at around 14.75 g. Why does the reaction stop increasing?
Medium
- AThe substrate has been used up
- BThe enzyme has been used up
- CThe temperature has increased
- DThe pH has changed
40.At pH 4, the reaction is slower than at pH 2. What is the best explanation?
Medium
- ApH 4 is further from pepsin's optimum pH, so fewer enzyme-substrate complexes form
- BpH 4 denatures pepsin completely
- CpH 4 increases the activation energy of the reaction
- DpH 4 increases the concentration of substrate
41.In an enzyme rate experiment, 50 cm³ of oxygen was produced with an uncertainty of ±0.5 cm³. Calculate the percentage error. Give your answer to 1 decimal place.
Hard
42.Complete the sentence about enzyme specificity.
Medium
The active site of an enzyme has a specific shape that is ____ to the shape of its substrate.
43.Name the model of enzyme action that replaced the lock and key model.
Medium
44.Complete the sentence about enzyme action.
Easy
The induced-fit model states that the enzyme's active site ____ slightly as the substrate binds, achieving an ideal binding arrangement.
45.A colorimeter measures percentage transmission. A reading has an uncertainty of ±0.5% and a measured value of 80%. Calculate the percentage error. Give your answer to 1 decimal place.
Medium
46.A tangent drawn to a curve at 0 seconds shows that 72 cm³ of product was produced in the first 20 seconds. Calculate the initial rate of reaction in cm³ s⁻¹.
Medium
47.The hydrogen ion concentration of a solution is 1.6 × 10⁻⁴ mol dm⁻³. Calculate the pH of the solution to 1 decimal place.
Medium
48.Ethanoic acid produced by wood ants has a hydrogen ion concentration of 8.39 × 10⁻⁶ mol dm⁻³. Calculate the pH of the ethanoic acid to 2 decimal places.
Medium
49.A student measures the volume of oxygen produced by catalase at five hydrogen peroxide concentrations. The mean volumes are 5.7 cm³, 9.3 cm³, 12.3 cm³, 17.7 cm³ and 19.7 cm³. Calculate the mean of these five values to 1 decimal place.
Hard
50.A protease hydrolyses 990 mg of protein in 4 minutes at 15°C. Calculate the rate of hydrolysis in mg s⁻¹ to 2 decimal places.
Hard
51.In an enzyme rate experiment, 25 cm³ of oxygen is produced with an uncertainty of ±0.5 cm³. Calculate the percentage error to 1 decimal place.
Medium
52.Complete the sentences about measuring enzyme activity.
Medium
Catalase breaks down hydrogen peroxide into water and ____. Amylase breaks down starch into maltose and ____.
53.Explain why enzymes are referred to as biological catalysts.
Hard
54.Describe how the induced-fit model differs from the lock and key model of enzyme action.
Hard
55.Explain why a very high temperature causes a decrease in the rate of an enzyme-catalysed reaction.
Hard
56.Explain how the induced-fit model accounts for the specificity of enzyme action.
Hard
57.Describe how the addition of a phosphate group to the non-functional form of enzyme X leads to the formation of the functional form.
Hard
58.In an enzyme rate experiment, 50 cm³ of oxygen was produced with an uncertainty of ±0.5 cm³. Calculate the percentage error. Give your answer to 1 decimal place.
Hard
- A0 %
- B2 %
- C3 %
- D1 %
59.A colorimeter measures percentage transmission. A reading has an uncertainty of ±0.5% and a measured value of 80%. Calculate the percentage error. Give your answer to 1 decimal place.
Medium
- A0.6 %
- B-0.4 %
- C1.6 %
- D2.6 %
60.A tangent drawn to a curve at 0 seconds shows that 72 cm³ of product was produced in the first 20 seconds. Calculate the initial rate of reaction in cm³ s⁻¹.
Medium
- A5.6 cm³ s⁻¹
- B3.6 cm³ s⁻¹
- C2.6 cm³ s⁻¹
- D4.6 cm³ s⁻¹
61.The hydrogen ion concentration of a solution is 1.6 × 10⁻⁴ mol dm⁻³. Calculate the pH of the solution to 1 decimal place.
Medium
- A3.8
- B2.8
- C5.8
- D4.8
62.Ethanoic acid produced by wood ants has a hydrogen ion concentration of 8.39 × 10⁻⁶ mol dm⁻³. Calculate the pH of the ethanoic acid to 2 decimal places.
Medium
- A4.08
- B7.08
- C5.08
- D6.08
63.A student measures the volume of oxygen produced by catalase at five hydrogen peroxide concentrations. The mean volumes are 5.7 cm³, 9.3 cm³, 12.3 cm³, 17.7 cm³ and 19.7 cm³. Calculate the mean of these five values to 1 decimal place.
Hard
- A12.9 cm³
- B14.9 cm³
- C11.9 cm³
- D13.9 cm³
64.A protease hydrolyses 990 mg of protein in 4 minutes at 15°C. Calculate the rate of hydrolysis in mg s⁻¹ to 2 decimal places.
Hard
- A6.13 mg s⁻¹
- B3.13 mg s⁻¹
- C5.13 mg s⁻¹
- D4.13 mg s⁻¹
65.In an enzyme rate experiment, 25 cm³ of oxygen is produced with an uncertainty of ±0.5 cm³. Calculate the percentage error to 1 decimal place.
Medium
- A4 %
- B1 %
- C3 %
- D2 %

Ghi chú ôn tập

## Enzymes: Globular Proteins as Biological Catalysts - Enzymes are **globular proteins** that act as **biological catalysts**: they speed up reactions without being used up or changed, and function in living systems. - **Active site**: the region where the substrate binds. The active site has a specific shape complementary to a specific substrate → **enzyme specificity**. - **Enzyme-substrate complex** forms temporarily; products are released and the enzyme is recycled. - **Activation energy** is the energy needed by the substrate to become unstable enough to react. Enzymes **lower activation energy** by providing an alternative energy pathway (they destabilise bonds in the substrate). - Enzymes can be **intracellular** (produced and function inside cells) or **extracellular** (secreted, act outside cells, e.g. digestive enzymes). ## Models of Enzyme Action - **Lock and key model** (GCSE): rigid active site is a precise fit for the substrate. - **Induced-fit hypothesis** (modern): the active site (and sometimes substrate) changes shape slightly as the substrate enters — **conformational changes** — giving an ideal binding arrangement and maximising catalysis. - Evidence: X-ray diffraction of hexokinase showed the active site changed shape after binding glucose. ## Required Practicals - **Catalase**: breaks down hydrogen peroxide → water + oxygen. Measure **volume of oxygen produced** over a set time (rate of product formation). - **Amylase**: hydrolyses starch → maltose and glucose. Test samples with **iodine in potassium iodide**: starch gives blue-black; no starch → yellow-brown. Measures rate of substrate disappearance. - **Colorimetry**: measures light absorbance/transmission; used for colour-change reactions (e.g. starch–amylase). Calibrate with weak iodine (100% transmission); use red filter; plot calibration graph of starch concentration vs absorbance/transmission. ## Graphs and Tangents - Line graphs: independent variable on x-axis, dependent variable on y-axis; draw a smooth line of best fit. - For non-linear graphs, draw a **tangent** at t = 0 to find the **initial rate**: - **Gradient = change in y-axis ÷ change in x-axis** - Example: 72 cm³ ÷ 20 s = 3.6 cm³ s⁻¹ ## Limiting Factors - **Temperature**: optimum temperature gives maximum rate. Lower temperatures → slower molecule movement, fewer successful collisions. Higher temperatures → more collisions and energy, but above optimum bonds (e.g. hydrogen bonds) break, tertiary structure changes, active site damaged → **denaturation**. Most human enzymes denature above ~40–50°C. - **pH**: each enzyme has an **optimum pH**. Extremes of pH break hydrogen and ionic bonds holding tertiary structure, altering active site shape → denaturation. E.g. pepsin (stomach) optimum pH 2. Buffer solutions maintain pH. - **pH = -log₁₀ [H⁺]** - **Enzyme concentration**: higher concentration → more active sites → greater likelihood of enzyme-substrate complexes. Initial rate increases linearly if substrate is sufficient; if substrate limited, rate plateaus. - **Substrate concentration**: higher substrate → more enzyme-substrate complexes. Eventually all active sites are saturated → **Vmax**; further increase has no effect. - **Inhibitors**: - **Competitive**: similar shape to substrate, competes for active site. Increasing substrate can overcome it; Vmax still reached. - **Non-competitive**: binds at an alternative site, alters active site shape, substrate cannot bind. Increasing substrate cannot overcome it; reduces Vmax. - **End-product inhibition**: end-product acts as a non-competitive reversible inhibitor in a feedback loop. ## Control of Variables & Uncertainty - Change only the **independent variable**; keep all others as **control variables** (temperature, pH, enzyme concentration, substrate concentration). - **Percentage error = (uncertainty ÷ measured value) × 100** - Example: (0.5 ÷ 50) × 100 = 1%

Enzyme: A globular protein that acts as a biological catalyst, speeding up the rate of chemical reactions without being used up or changed.
Active site: The specific region of an enzyme where the substrate binds; its shape is complementary to a specific substrate.
Enzyme-substrate complex: The temporary structure formed when an enzyme and its substrate join together before the reaction is catalysed and products are released.
Activation energy: The amount of energy needed by the substrate to become unstable enough for a reaction to occur and for products to be formed.
Denaturation: Loss of an enzyme's function due to extremes of heat or pH changing the shape of the active site, preventing substrate binding.
Induced-fit hypothesis: Model of enzyme action where the enzyme and active site (and sometimes the substrate) change shape slightly as the substrate enters, achieving an ideal binding arrangement.
Lock and key model: Older model stating the rigid active site of the enzyme is a precise fit for the specific shape of the substrate.
Optimum temperature: The temperature at which an enzyme catalyses a reaction at the maximum rate.
Optimum pH: The pH at which an enzyme works best; extremes of pH cause denaturation.
Vmax: The maximum velocity (rate) of an enzyme-catalysed reaction when all active sites are occupied.
Competitive inhibitor: A reversible inhibitor with a similar shape to the substrate that competes for the active site; its effect can be reduced by increasing substrate concentration.
Non-competitive inhibitor: A reversible inhibitor that binds at an alternative site, altering the shape of the active site and preventing substrate binding; increasing substrate concentration cannot overcome it.
End-product inhibition: Feedback control where the end-product of a metabolic pathway acts as a non-competitive, reversible inhibitor of an earlier enzyme.
Intracellular enzyme: An enzyme produced and functioning inside the cell.
Extracellular enzyme: An enzyme secreted by cells that catalyses reactions outside the cell, e.g. digestive enzymes in the gut.
Catalase: An enzyme found in most organisms that breaks down toxic hydrogen peroxide into water and oxygen.
Amylase: A digestive enzyme that hydrolyses starch into maltose and glucose; functions best at pH 7 and 37°C.
Colorimetry: A technique measuring light absorbance or transmission, used to follow enzyme-catalysed reactions that involve a colour change.
Initial rate of reaction: The rate of reaction at the start (time = 0), found by drawing a tangent to a curve and calculating its gradient.
Percentage error: A measure of uncertainty calculated as (uncertainty ÷ measured value) × 100.

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