Enzymes & Metabolism

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Metabolism: Enzymes & Reactions

Metabolism is the sum of all chemical reactions that take place within cells and organisms; the molecules involved are called metabolites.
Many metabolic reactions occur in multiple stages, each catalysed by a separate enzyme; a series of interlinked reactions is a metabolic pathway.
Metabolic reactions are broadly classified as anabolic (building large molecules from smaller ones) or catabolic (breaking down large molecules into smaller ones).
Anabolic reactions are endergonic, often involve condensation reactions, and store energy (e.g. photosynthesis, protein synthesis, glycogen formation).
Catabolic reactions are exergonic, often involve hydrolysis reactions, and release energy (e.g. respiration, deamination, digestion).
Both anabolism and catabolism are made up of enzyme-catalysed reactions and are coupled to ATP, the principal energy carrier in cells.

Enzymes as Biological Catalysts

Enzymes are globular proteins that act as biological catalysts, speeding up reactions without being used up or permanently changed.
They are reusable, so only a small number is needed to catalyse reactions.
Without enzymes, the rate of chemical reactions in organisms would be too low to support life.
Enzymes ensure substrate molecules are orientated correctly and close enough for a reaction to occur.
The cell controls which enzymes are produced, giving it control over the chemical reactions in the cytoplasm.

The lock and key model

Enzyme Action & Specificity

The active site is the specific region of an enzyme where the substrate binds; its shape and chemical properties are complementary to the substrate.
This complementary fit is called enzyme-substrate specificity; the shapes are complementary, not identical.
Substrates collide randomly with the active site; a reaction occurs only if the collision is at the correct orientation and speed.
A temporary enzyme-substrate complex forms; the substrate is in a transitional state, products are formed, and the enzyme is released unchanged.
The specificity of an enzyme is determined by the complex 3D shape of the protein, which is itself determined by the sequence of amino acids.
The active site is made of only a few amino acids, but their interactions within the 3D shape allow catalysis by binding the substrate, holding it in position, and lowering the energy needed for the reaction.

Induced-Fit Hypothesis

The original lock-and-key model proposed that the active site is precisely complementary to the substrate, like a key fitting a lock.
The modified induced-fit hypothesis states that the enzyme and its active site (and sometimes the substrate) change shape slightly as the substrate enters.
These shape changes are called conformational changes and ensure an ideal binding arrangement.
This maximises the ability of the enzyme to catalyse the reaction.
Our current understanding of enzyme-substrate interactions is based on the induced-fit hypothesis.

Denaturation

Enzymes can be denatured by high temperatures or extremes of pH.
Bonds (e.g. hydrogen bonds) holding the enzyme's precise 3D shape start to break; peptide bonds holding amino acids together are not broken.
This permanently changes the shape of the active site, preventing the substrate from binding.
Denaturation has occurred if the substrate can no longer bind, and the reaction no longer takes place.
Denaturation often causes the enzyme to become insoluble and form a precipitate.
Very few human enzymes function above 50°C; even temperatures exceeding 40°C can denature human enzymes because the body maintains about 37°C.

Denaturation of an enzyme

Factors Affecting Enzyme Activity

Enzymes have a specific optimum temperature at which they catalyse a reaction at the maximum rate.
Lower temperatures slow reactions because molecules have less kinetic energy, fewer successful collisions occur, and collisions have less energy.
Higher temperatures speed up reactions because molecules have more kinetic energy, more successful collisions occur, and collisions have more energy.
As temperatures continue to increase, the rate drops sharply as the enzyme begins to denature.
Each enzyme has an optimum pH; extremes of pH alter hydrogen bonding and cause irreversible denaturation (e.g. pepsin works best at pH 2, some bacterial enzymes at pH 9–10).
pH is measured on a logarithmic scale of hydrogen ion concentration; a 10× increase in hydrogen ion concentration lowers pH by 1 unit.
As substrate concentration rises, the rate of reaction increases until active sites become saturated; beyond this, increasing substrate concentration causes no further increase in rate.
At saturation, the rate can be increased by increasing enzyme concentration to make more active sites available.

Effect of temperature on enzyme activity

Investigating Enzyme Activity: Skills

The rate of an enzyme-catalysed reaction can be measured by the rate of formation of a product or the rate of disappearance of a substrate.
Catalase breaks down hydrogen peroxide into water and oxygen; the volume of oxygen produced in a set time measures the rate.
Amylase hydrolyses starch into maltose and glucose; starch can be tested with iodine in potassium iodide solution, which turns blue-black if starch is present.
Colorimetry can measure light absorbance or transmission in reactions involving a colour change; as starch is broken down, transmission increases or absorbance decreases.
A colorimeter is calibrated (e.g. with weak iodine solution as 100% transmission) and a calibration graph of starch concentration vs absorbance/transmission is plotted.
Serial dilutions are used to make a range of concentrations from a stock solution.
The rate of reaction can be calculated as 1 ÷ time taken (s⁻¹), or from the gradient of a graph of product/substrate quantity against time.

Investigating enzyme activity

Activation Energy

Activation energy is the amount of energy needed by the substrate to become unstable enough for a reaction to occur and for new products to be formed.
Enzymes speed up reactions because they reduce the stability of bonds in the substrate.
Enzymes lower the activation energy needed to catalyse a reaction; the energy released is unchanged but the activation energy required is lowered.
The rate of reaction is therefore quicker.
If energy is released to the surroundings, the reaction is exergonic; if energy is absorbed, it is endergonic.
Endergonic and exergonic reactions are defined by the net intake or output of energy, unlike endothermic and exothermic reactions which are defined by thermal energy only.

Immobilised Enzymes

Enzymes can be immobilised by attachment to an inert substance (e.g. glass), entrapment within a matrix (e.g. alginate gel), or entrapment within a partially permeable membrane.
Immobilised enzymes are used in industries such as food processing, environmental management, pharmaceuticals and manufacturing.
Advantages include: the product is uncontaminated with enzyme, so no further processing or filtering is needed.
The immobilised enzyme can be reused multiple times, which is efficient and cost-effective.
Reusing the enzyme avoids the need to separate it from the product in downstream processing.
Immobilised enzymes have greater tolerance of temperature and pH changes and are often more stable.
Substrates can be exposed to higher enzyme concentrations than with enzymes in solution, increasing the rate of throughput.

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练习题

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1.Which of the options below refers to the features of catabolism?
Easy
- AExergonic, condensation reaction, an example is polypeptide synthesis
- BEndergonic, hydrolysis reaction, an example is respiration
- CEndergonic, condensation reaction, an example is photosynthesis
- DExergonic, hydrolysis reaction, an example is deamination
2.Which of the following statements apply to enzymes? 1. They speed up the rate of chemical reactions in the body 2. They are fibrous proteins 3. The active site is where the substrate binds 4. The shape of the active site is complementary to the shape of the substrate molecule
Easy
- AI, III and IV
- BI, II and III
- CI and IV
- DII and III
3.An enzyme catalysed reaction is heated to a temperature of 65 °C. Which of the following best explains the effect this temperature increase would have on the enzymes?
Medium
- AThe enzyme and substrate molecules will gain kinetic energy and collide more frequently
- BThe bonds in the enzyme will vibrate more and break, which will cause a permanent change in the shape of the active site
- CThe bonds in the enzyme will vibrate more and break, causing a temporary change in the shape of the active site
- DThe active site of the enzyme will permanently change shape due to the increased speed of collisions between the substrate molecule and the active site
4.Which of the following statements is not true about enzymes in metabolic pathways?
Easy
- AAll metabolic reactions are catalysed by an enzyme.
- BAll proteins are enzymes.
- CAll enzymes are proteins.
- DAll enzymes reduce activation energy of metabolic reactions in living organisms.
5.The following statements are about enzymes: 1. The speed that they function at can be reduced by competitive inhibitors. 2. Their primary structure is translated from mRNA. 3. They can be embedded in the plasma membrane of a cell. 4. They all have quaternary structures. Which of these statements are correct for all enzymes?
Medium
- A1 and 3 only
- B1, 2 and 3 only
- C2, 3 and 4 only
- DAll of them are correct
6.Enzymes alter the activation energy required for a biochemical reaction to take place. Which of the following gives the best definition of activation energy?
Medium
- AThe energy input required to break bonds in order for a reaction to take place.
- BThe energy is released as bonds breaks in a reaction.
- CThe energy required to allow substrates and enzymes to be closely aligned at the active site.
- DThe energy given off when an enzyme-substrate complex forms.
7.The four statements below are examples of the two types of metabolism, anabolism and catabolism. Which one of the statements represents a type of metabolism which is different to the other three options?
Medium
- ADeamination of polypeptides to form urea.
- BFormation of glycosidic bonds between glucose and fructose.
- CDepletion of fat stores during a period of starvation.
- DAnaerobic respiration.
8.The following statements apply to enzyme-catalysed reactions. Which row of the table best describes the sequence of events in an anabolic reaction?
Medium
- ASubstrate binds to active site, enzyme-substrate complex forms, product is released, enzyme is reused
- BSubstrate binds to active site, enzyme-substrate complex forms, enzyme is permanently changed, product is released
- CSubstrate binds to active site, enzyme-substrate complex forms, product is released, enzyme is denatured
- DSubstrate binds to active site, enzyme-substrate complex forms, product is released, enzyme is broken down

更多题目

9.When investigating the rate of reaction of the enzyme lipase on the hydrolysis of triglycerides, the pH must be maintained to prevent the lipase denaturing. Why might fluctuations in pH occur?
Medium
- AThe products of hydrolysis will decrease the pH.
- BLipase has a very low optimum pH.
- CThe products of hydrolysis will increase the pH.
- DThe water molecules produced as a byproduct of hydrolysis decreases pH.
10.The breakdown of hydrogen peroxide to water and oxygen is catalysed by the enzyme catalase. In an investigation into the effect of temperature on the rate of reaction of catalase, mashed liver (which contains catalase) was added to hydrogen peroxide. What would the independent variable be in this experiment?
Medium
- AThe change in mass of the liver after a given time.
- BThe temperature of the solution.
- CThe mass of liver added at the start.
- DThe volume of oxygen produced.
11.Which of the following statements about enzymes is not correct?
Medium
- AThey can have multiple active sites.
- BThey are composed of one (or more) polypeptide chain(s).
- CEster bonds play an important role in maintaining shape.
- DThey catalyse both anabolic and catabolic reactions.
12.A student wished to use a starch solution of 0.01% for an enzyme experiment. A stock solution of 1.0% starch solution was provided. Which of the following dilutions would be the best method to achieve the desired concentration with greatest precision, using standard laboratory equipment?
Medium
- ADilute 1 cm³ of stock solution with 99 cm³ of water
- BDilute 1 cm³ of stock solution with 9 cm³ of water, then dilute 1 cm³ of this with 9 cm³ of water
- CDilute 1 cm³ of stock solution with 999 cm³ of water
- DDilute 10 cm³ of stock solution with 90 cm³ of water
13.Which of the following statements about metabolism are correct? 1. Metabolism involves chemicals called metabolites. 2. Metabolism involves reactions in a linear chain. 3. Metabolism involves reactions in a cycle. 4. Metabolism involves only the breakdown of molecules.
Medium
- AI and IV only
- BII and III
- CAll of the statements
- DI, II and III
14.Which of the following are features of enzymes? (select all that apply)
Medium
- AThey are globular proteins
- BThey are fibrous proteins
- CThey have an active site
- DThey are used up in the reaction
- EThey lower activation energy
15.Which of the following statements about anabolic reactions are correct? (select all that apply)
Medium
- AThey build large molecules from smaller ones
- BThey are exergonic
- CThey often involve condensation reactions
- DThey release energy
- EThey store energy in chemical form
16.Enzymes are fibrous proteins.
Easy
True or false?
17.The active site of an enzyme is complementary in shape to its substrate.
Easy
True or false?
18.Enzymes are permanently changed by the reactions they catalyse.
Medium
True or false?
19.Match the enzyme with its optimum pH.
Medium
- Pepsin
- Amylase
- Catalase
- pH 2
- pH 7
- pH 7.6
20.Put the steps of an enzyme-catalysed reaction in the correct order.
Medium
- Substrate binds to active site
- Enzyme-substrate complex forms
- Product is released
- Enzyme is reused
21.Match each enzyme with the pH at which it works best.
Medium
- Pepsin (stomach)
- Amylase
- Bacterial enzymes of the gut
- pH 2
- pH 7
- pH 9–10
22.Match each term with its correct description.
Medium
- Metabolism
- Metabolites
- Metabolic pathway
- All the chemical reactions that take place within cells and organisms
- The molecules involved in metabolic reactions
- A series of interlinked metabolic reactions
23.Match each property with the type of metabolism it describes.
Medium
- Endergonic
- Exergonic
- Condensation reactions
- Hydrolysis reactions
- Anabolism
- Catabolism
24.Put the steps of a catabolic enzyme-catalysed reaction in the correct order.
Medium
- Substrate collides with the enzyme's active site
- Enzyme-substrate complex forms
- Substrate is broken down into products
- Products are released and the enzyme is reused
25.Put the steps of the induced-fit hypothesis in the correct order.
Medium
- Substrate approaches the active site
- Enzyme and substrate change shape slightly (conformational change)
- An ideal binding arrangement is achieved
- Catalysis occurs and products are released
26.Which of the following are advantages of using immobilised enzymes? (select all that apply)
Medium
- AThe product is uncontaminated by enzyme
- BThe enzyme can be reused multiple times
- CThe enzyme has greater tolerance of temperature and pH changes
- DThe enzyme is permanently destroyed after one use
- EThe enzyme is always cheaper than free enzyme
27.Which of the following statements about enzyme denaturation are correct? (select all that apply)
Medium
- AHydrogen bonds holding the 3D shape begin to break
- BPeptide bonds between amino acids are broken
- CThe shape of the active site is permanently changed
- DThe substrate can no longer bind to the enzyme
- EThe enzyme is 'killed'
28.Which of the following are examples of anabolic reactions? (select all that apply)
Medium
- APhotosynthesis
- BProtein synthesis
- CFormation of glycogen from glucose
- DDeamination of proteins to release urea
- ERespiration
29.Which of the following statements about the effect of substrate concentration on enzyme activity are correct? (select all that apply)
Medium
- AIncreasing substrate concentration increases the frequency of collisions with active sites
- BAt saturation, increasing substrate concentration further increases the rate
- CAt saturation, adding more enzyme increases the rate
- DActive sites become saturated at high substrate concentration
- EThe rate of reaction is unlimited by substrate concentration
30.Enzymes are used up in the reactions they catalyse.
Easy
True or false?
31.The lock-and-key model states that the enzyme's active site is precisely complementary to the substrate.
Easy
True or false?
32.Immobilised enzymes can be reused and are more stable to temperature and pH changes.
Easy
True or false?
33.A colorimeter is used to follow the breakdown of starch by amylase. As the reaction proceeds, what happens to light transmission and absorbance?
Medium
- ATransmission increases and absorbance decreases
- BTransmission decreases and absorbance increases
- CBoth transmission and absorbance increase
- DBoth transmission and absorbance decrease
34.Which of the following is the best definition of activation energy?
Medium
- AThe energy needed by the substrate to become unstable enough for a reaction to occur
- BThe energy released when products are formed
- CThe energy required to bring the enzyme and substrate together
- DThe energy given off when an enzyme-substrate complex forms
35.An enzyme-catalysed reaction is carried out at 25 °C and then at 65 °C. Which statement best explains the difference in rate?
Hard
- AAt 65 °C the enzyme denatures because bonds vibrate more and break, changing the active site shape
- BAt 65 °C the enzyme and substrate have more kinetic energy so the rate is always higher
- CAt 65 °C the enzyme temporarily changes shape but can still catalyse the reaction
- DAt 65 °C the substrate denatures, preventing the reaction
36.The table gives the rates of several enzyme-catalysed reactions. Which enzyme catalyses the reaction with the fastest rate of product formation?
Medium
- ACitrate synthase (1.0 × 10⁶ s⁻¹)
- BAconitase (5.2 × 10³ s⁻¹)
- CFumarase (9.2 × 10⁴ s⁻¹)
- DMalate dehydrogenase (3.7 × 10⁷ s⁻¹)
37.In an investigation into the effect of temperature on the rate of reaction of catalase, mashed liver (which contains catalase) was added to hydrogen peroxide. What would the independent variable be in this experiment?
Medium
- AThe change in mass of the liver after a given time.
- BThe temperature of the solution.
- CThe mass of liver added at the start.
- DThe volume of oxygen produced.
38.Which statement about the protease enzymes is correct?
Hard
- AThe activity of the stomach protease of species Y is 2.5 times greater than that of species X.
- BThe activity of the intestinal protease of species X is significantly higher than the stomach protease of species X.
- CThe activity of the stomach protease of species X is 3.5 times less than that of species Y.
- DThe activity of the intestinal proteases of both species X and Y are significantly different.
39.Which of the following statements about immobilised enzymes are correct? (select all that apply)
Medium
- AThey can be reused multiple times.
- BThey are more tolerant of temperature and pH changes.
- CThey contaminate the product and must be filtered out.
- DThey allow substrates to be exposed to higher enzyme concentrations.
- EThey are always less stable than enzymes in solution.
40.A student investigates the effect of pH on catalase activity. They use buffer solutions to keep the pH constant. Why must a buffer solution be used?
Medium
- ATo prevent the pH from changing as the reaction proceeds and denaturing the enzyme.
- BTo provide the optimum pH for the enzyme.
- CTo neutralise the hydrogen peroxide.
- DTo increase the rate of reaction.
41.What is the name of the model that proposes the enzyme and substrate change shape slightly upon binding?
Easy
42.An enzyme-catalysed reaction produces 30 cm³ of gas in 10 seconds. Calculate the rate of reaction in cm³ s⁻¹.
Medium
43.Complete the sentence about enzyme action.
Medium
The substrate binds to the ____ of the enzyme, forming an enzyme-substrate ____.
44.An enzyme-catalysed reaction produces 60 cm³ of gas in 20 seconds. Calculate the rate of reaction in cm³ s⁻¹.
Medium
45.A reaction takes 40 seconds to complete. Calculate the rate of reaction in s⁻¹ (to 2 decimal places).
Medium
46.Complete the sentence about the induced-fit hypothesis.
Easy
In the induced-fit hypothesis, the enzyme and substrate change ____ slightly as the substrate enters the active site.
47.Complete the sentence about metabolism.
Medium
Metabolism can be divided into ____ reactions, which build large molecules, and ____ reactions, which break down large molecules.
48.Name the model of enzyme action in which the active site is precisely complementary to the substrate, like a key fitting a lock.
Medium
49.What is the name of the temporary complex formed when a substrate binds to an enzyme's active site?
Medium
50.An enzyme-catalysed reaction produces 45 cm³ of gas in 15 seconds. Calculate the rate of reaction in cm³ s⁻¹.
Medium
51.A reaction takes 25 seconds to complete. Calculate the rate of reaction in s⁻¹ (to 2 decimal places).
Medium
52.Complete the sentence about enzyme kinetics.
Medium
When all active sites are occupied, the enzyme is said to be ____.
53.Explain how enzymes lower the activation energy of a reaction.
Hard
54.Explain how the induced-fit hypothesis differs from the lock-and-key model of enzyme action.
Hard
55.Explain why the rate of an enzyme-catalysed reaction decreases at temperatures above the optimum.
Medium
56.Suggest one reason why the activity of the intestinal protease is higher than the stomach protease in these fish.
Hard
57.An enzyme-catalysed reaction produces 30 cm³ of gas in 10 seconds. Calculate the rate of reaction in cm³ s⁻¹.
Medium
- A4 cm³/s
- B5 cm³/s
- C3 cm³/s
- D2 cm³/s
58.An enzyme-catalysed reaction produces 60 cm³ of gas in 20 seconds. Calculate the rate of reaction in cm³ s⁻¹.
Medium
- A3 cm³ s⁻¹
- B5 cm³ s⁻¹
- C2 cm³ s⁻¹
- D4 cm³ s⁻¹
59.A reaction takes 40 seconds to complete. Calculate the rate of reaction in s⁻¹ (to 2 decimal places).
Medium
- A0.03 s⁻¹
- B1.03 s⁻¹
- C2.03 s⁻¹
- D-0.97 s⁻¹
60.An enzyme-catalysed reaction produces 45 cm³ of gas in 15 seconds. Calculate the rate of reaction in cm³ s⁻¹.
Medium
- A2 cm³ s⁻¹
- B4 cm³ s⁻¹
- C3 cm³ s⁻¹
- D5 cm³ s⁻¹
61.A reaction takes 25 seconds to complete. Calculate the rate of reaction in s⁻¹ (to 2 decimal places).
Medium
- A1.04 s⁻¹
- B-0.96 s⁻¹
- C0.04 s⁻¹
- D2.04 s⁻¹

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