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OCR Biology Paper 1 > 2.1 - Enzymes > Flashcards

2.1 - Enzymes Flashcards

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1
Q

Define catalyst

A
  • Substance which speeds up rate of chemical reactions without being used up
  • Remains chemically unchanged
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2
Q

Define substrate

A

Reactant in enzyme-controlled reactions

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3
Q

Define metabolism

A

All the reactions that occur within an organism in order to maintain life

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4
Q

Describe the structure of an enzyme

A
  • Globular protein
  • Active site - specific place where a substrate binds
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5
Q

Outline the action of enzymes

A
  • Catalyse biological reactions
  • Substrate-specific
  • Lower activation energy of a chemical reaction
  • Substrate binds to active site
  • Enzyme–substrate complex formed
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6
Q

Define activation energy

A

Minimum amount of energy required for a reaction to occur

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7
Q

Outline the importance of enzymes to metabolic processes

A
  • Increase rate of reaction
  • Lower activation energy
  • A specific enzyme is required for each substrate
  • Metabolic pathway blocked if an enzyme is inhibited or absent
  • End-product inhibition can control metabolic pathways
  • Differences in metabolism as cells produce different enzymes during differentiation
  • Can affect metabolism at both cellular and whole organism level
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8
Q

Which protein structure is responsible for the specificity of the active site?

A
  • Tertiary (3D) structure
  • Held in place by hydrogen bonds, ionic bonds, disulfide bridges and hydrophilic &
    hydrophobic interactions
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9
Q

What is an intracellular enzyme?

A
  • Enzyme that works inside cells
  • e.g. catalase
  • Found inside liver cells
  • Breaks down hydrogen peroxide into water and oxygen
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10
Q

What is an extracellular enzyme?

A
  • Enzyme that works outside cells
  • e.g. amylase
  • Found in saliva and small intestine
  • Breaks down amylose into maltose
  • e.g. trypsin
  • Protease found in small intestine
  • Hydrolyses proteins into amino acids
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11
Q

Explain the old lock and key model of enzyme activity

A
  • Enzymes have specific active sites to which substrate(s) are complementary
  • Enzyme-substrate complex forms when substrate binds to active site
  • This reduces activation energy
  • Enzyme-product complex forms
  • Once reaction is complete, products leave and enzyme can work again
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12
Q

Explain the current induced fit model of enzyme activity

A
  • Enzymes have specific active sites to which substrate(s) bind
  • Enzyme active site and substrate not perfectly complementary
  • Enzyme-substrate complex forms
  • Enzyme changes shape once substrate is bound
  • Change in shape causes straining of bonds, weakening them
  • This reduces activation energy
  • Enzyme-product complex forms
  • Once reaction is complete, products leave and enzyme can work again
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13
Q

How is the rate of a reaction calculated?

A
  • Rate of reaction = volume of product produced / time
  • Rate of reaction = volume of substrate used up / time
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14
Q

What conditions affect the rate of an enzyme-catalysed reaction?

A
  • Temperature
  • pH
  • Substrate concentration
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15
Q

What is the temperature coefficient, Q10?

A

How much the rate of a reaction increases with a 10°C rise in temperature

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16
Q

What does a Q10 value of 2 signify?

A
  • Rate of reaction doubles with each 10°C temperature increase
  • Value usually shown by enzyme-controlled reactions
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17
Q

Describe and explain how temperature affects the rate of an enzyme catalysed reaction

A

Description:
- As temperature increases, enzyme activity increases
- At high temperatures, enzyme activity decreases

Explanation:
- Enzymes and substrates have more kinetic energy and collide more frequently as
temperature increases, forming more enzyme-substrate complexes in active site
- At high temperatures, enzyme is denatured and 3D (tertiary) shape of active site is
altered. No longer able to form enzyme-substrate complexes

18
Q

Describe and explain how pH affects the rate of an enzyme catalysed reaction

A

Description:
- Enzyme activity decreases as pH deviates from its optimum
- Optimum pH for most enzymes in a human is 7 (exceptions include protease found
in stomach with an optimum pH of 3)

Explanation:
- As pH deviates from its optimum, the increase or decrease in H+ ions causes
intermolecular bonds to break (e.g. hydrogen bonds, ionic bonds)
- Tertiary structure is altered as enzyme becomes denatured
- Enzyme-substrate complexes can no longer form

19
Q

Describe and explain how substrate concentration affects the rate of an enzyme catalysed reaction

A

Description:
- As substrate concentration increases, enzyme activity increases to a point
- Further increases in substrate concentration see no further increase in enzyme
activity

Explanation:
- As substrate concentration increases, there are more frequent collisions between
substrate and enzyme active site so rate increases
- Eventually all the enzyme active sites are occupied, so no further rate increase
takes place
- Enzyme concentration becomes limiting factor

20
Q

What is Vmax?

A

The maximum rate of a reaction

21
Q

What happens to Vmax if the enzyme concentration is increased?

A
  • Increases
  • Concentration of substrate eventually becomes limiting factor
22
Q

Describe an investigation into the effect of enzyme concentration on the rate of a reaction

A
  • Create stock solution of enzyme
  • Perform a serial dilution to produce a range of enzyme concentrations
  • Stock is diluted 10 fold each time using distilled water
  • Add equal volumes of a given concentration of the substrate to each dilution
  • Time how long it takes the enzyme to catalyse the reaction
  • e.g. by using a colorimeter
23
Q

Define inhibitor

A

Substances that reduce enzyme activity

24
Q

What are the two types of inhibitor?

A
  • Competitive
  • Non-competitive
25
Q

Describe how a competitive inhibitor works

A
  • Molecule, other than the substrate, binds to the enzyme’s active site
  • Inhibitor is structurally and chemically similar to the substrate so able to bind to active site
  • Competitive inhibitor blocks active site and prevents substrate binding
  • As inhibitor is in competition with substrate, its effects can be reduced by increasing
    substrate concentration
26
Q

Give an example of competitive inhibition

A
  • Statins are competitive inhibitors of an enzyme that synthesises cholesterol
  • Statins are prescribed to treat high cholesterol levels in blood
  • Aspirin inhibits the active site of COX enzymes
  • Prevents the synthesis of chemicals responsible for pain
27
Q

Define allosteric site

A

Region on the enzyme other than the active site

28
Q

Describe how a non-competitive inhibitor works

A
  • Inhibitor binds to an allosteric site on enzyme
  • Binding of inhibitor to allosteric site causes conformational change to enzyme’s active site
  • Active site and substrate no longer share specificity
  • Substrate cannot bind
  • As inhibitor is not in direct competition with substrate, increasing substrate levels cannot
    mitigate the inhibitor’s effect
  • Maximum rate of enzyme activity reduced
29
Q

Give and example of non-competitive inhibition

A
  • Organophosphates are used as insecticides and herbicides
  • Inhibit acetyl cholinesterase - necessary for nerve impulse transmission
  • Can lead to paralysis and death
  • Protein pump inhibitors are used to treat long-term indigestion
  • Block enzyme responsible for secreting H+ into the stomach
  • Reduces production of excess stomach acid
30
Q

Explain the control of metabolic pathways by end-product inhibition

A
  • Metabolic pathway is a series of enzyme-catalysed reactions
  • End-product acts as inhibitor of enzyme at beginning of pathway
  • Inhibitor can be competitive or non-competitive
  • More inhibition if end-product concentration rises
  • Prevents a build-up of intermediate products until end-product level reduces
  • Example of negative feedback
31
Q

What is the purpose of end-product inhibition?

A
  • Ensure levels of an essential product are tightly regulated
  • If product levels build up, the product inhibits the reaction pathway and decreases the rate
    of further product formation
  • If product levels drop, the reaction pathway will proceed unhindered and the rate of product
    formation will increase
32
Q

Give an example of end-product inhibition

A
  • ATP is produced in respiration
  • PFK enzyme required for first step of glycolysis (first stage of respiration)
  • ATP inhibits PFK
  • When ATP levels are high, more PFK is inhibited
  • Rate of respiration (and ATP production) slows
  • When ATP level are low, less PFK is inhibited
  • Rate of respiration (and ATP production) increases
33
Q

Why are inhibitors often used in medicine?

A

Can target pathogenic enzymes
- e.g. antimalarial medicines target enzymes involved in parasitic development

34
Q

What is a cofactor?

A
  • Non-protein components that help enzyme function
  • Binds loosely to enzyme
35
Q

What is the difference between a cofactor and a coenzyme?

A
  • Cofactors are inorganic molecules
  • Coenzymes are organic molecules
36
Q

How are most coenzymes derived?

A
  • From vitamins
  • e.g. vitamin B3 used to synthesise NAD
37
Q

How are inorganic cofactors obtained?

A
  • Through diet
  • e.g. iron, calcium, chloride and zinc ions
38
Q

Give an example of a cofactor

A
  • Chloride ions (Cl-)
  • Required for amylase to form complementary active site for starch
39
Q

What is precursor activation?

A
  • Activation of an enzyme by a cofactor
40
Q

Define apoenzyme

A

Inactive precursor protein

41
Q

Define holoenzyme

A

Activated enzyme

42
Q

What is a prosthetic group?

A
  • Molecules that bind and form permanent feature of the enzyme
  • e.g. zinc ions (Zn2+) form part of carbonic anhydrase enzyme
  • Required for metabolism of carbon dioxide

OCR Biology Paper 1 (20 decks)

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