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Biology A-Level > Enzymes (Chapter 3) > Flashcards

Enzymes (Chapter 3) Flashcards

1
Q

Define an enzyme

A

A globular protein that catalyses metabolic reactions

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2
Q

What is a catalyst?

A

A molecule which speeds up a chemical reaction but remains unchanged at the end of the reaction

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3
Q

What is an enzyme?

A

A biological catalyst made of protein with a specific tertiary structure and active site

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4
Q

Why are enzymes essential for life?

A

Because virtually every metabolic reaction in a living organism is catalysed by an enzyme

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5
Q

What is the name ending for most enzymes?

A

-ase

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6
Q

What are intracellular enzymes?

A

Enzymes that operate within cells

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7
Q

What are extracellular enzymes?

A

Enzymes that are secreted by cells and that catalyse reactions outside cells
- some organisms secrete enzymes outside their bodies

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8
Q

Describe the structure of enzymes as globular proteins

A

Enzyme molecules are coiled into a precise, 3D shape, with hydrophilic R groups (side-chains) on the outside of the molecule

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9
Q

What make enzymes soluble?

A

The hydrophilic R groups on the outside of the molecule

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10
Q

What is the active site?

A

A region of the enzyme, usually a cleft or depression, to which another molecule (substrate) or molecules can bind

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11
Q

Describe the lock and key hypothesis

A

The idea that the enzyme has a particular shape, into which the substrate fits exactly

  • the substrate is held in place by temporary bonds which form between the substrate and some of the R groups of the enzyme’s amino acids
  • this combined structure is called the enzyme-substrate complex
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12
Q

How is the enzyme specific for a substrate?

A

Each type of enzyme will usually act on only one type of substate molecule because the shape of the active site will only allow one shape of molecule to fit

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13
Q

What does catabolic mean?

A

Breaking down

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14
Q

What does anabolic mean?

A

Building up

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15
Q

Describe the induced fit hypothesis for enzyme action

A

It is mostly the same as lock and key but it adds the idea that the enzyme, and sometimes the substrate, can change shape slightly as the substrate molecule enters the enzyme, in order to ensure a perfect fit
- makes the catalysis even more efficient

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16
Q

What is formed briefly before the release of the product?

A

An enzyme-product complex

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17
Q

How do enzymes work?

A

1) Enzymes may catalyse the breakdown of a molecule as the interactions between the R groups of the enzyme and the atoms of the substrate can break bonds in the substrate molecule
2) Enzymes may catalyse the joining together of 2 molecules as the interactions can encourage formation of bonds in the substrate molecule

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18
Q

How fast is enzyme catalysis?

A

Very rapid e.g. breakdown of H2O2 into H2O and O2

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19
Q

What is the activation energy?

A

The energy temporarily given to a substrate to convert it to the product in a chemical reaction

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20
Q

How do enzymes affect the activation energy?

A

Enzymes decrease the activation energy of the reaction which they catalyse, making it easier to turn substrate into product

  • they do this by holding the substrate(s) in such a way that their molecules react more easily and their bonds become weaker
  • reactions catalysed by enzymes will take place rapidly at a much lower temp then they otherwise would
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21
Q

Why are enzymes needed when one way of increasing the rate of metabolic reactions is to increase the energy of the reactants is by heating them?

A

It is not enough to give most substrates the activation energy which they need to change into products

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22
Q

Describe what happens during the course of the reaction of the breakdown of H2O2 by catalase

A

1) The reaction begins very swiftly - bubbles of O2 are released quickly and a large vol of O2 is collected in the first minute
2) But as the reaction continues, the rate at which O2 is released gradually slows down
3) The reaction gets slower and slower until it eventually stops completely

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23
Q

Explain why the breakdown of H2O2 by catalase begins very swiftly

A
  • When the enzyme + substrate are first mixed, there are a large number of substrate molecules - at any moment, virtually every enzyme molecule has a substrate molecule at its active site
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24
Q

What does the rate at which the breakdown of H2O2 depend on?

A

How many enzyme molecules there are and the speed at which the enzyme can convert substrate to product, release it and bind with another substrate molecule

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25
Q

What happens as the breakdown of H2O2 continues?

A

As more and more substrate is converted into product, there are fewer and fewer substrate molecules to bind with enzymes
- therefore, the reaction gets slower and slower until it stops

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26
Q

Describe the graph for the rate of an enzyme controlled reaction

A

The graph is always steepest at the beginning (the initial rate of reaction) and then increases as a curve until it flattens out

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27
Q

How can you measure the initial rate of reaction by the release of O2?

A

Measure the amount of O2 given off in the first 30s and then convert to cm3/min or just work out gradient

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28
Q

What are the factors that affect enzyme action?

A

1) Enzyme concentration
2) Substrate concentration
3) Temperature
4) pH

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29
Q

Describe the effect of enzyme concentration on enzyme action

A
  • Higher enzyme conc, faster reaction
  • Overall, same amount of product will be formed eventually regardless of different enzyme conc because the substrate conc is the same
30
Q

How would you compare the effect of different enzyme concentrations on reaction rate and why?

A
  • Plot enzymes concentration against initial rate of reaction - it should increase linearly and be directly proportional
  • You use the initial rate of reaction because once the reaction is under way, the amount of substrate in each reaction begins to vary because substrate is converted into product at different rates - this ensures that the difference in reaction rate is only caused by the difference in enzyme concentration
31
Q

Explain the effect of enzyme concentration on enzyme action

A
  • The more enzyme present, the more active sites will be available for the substrate
  • As long as there is plenty of substrate available, the initial rate of reaction increases linearly with enzyme conc
32
Q

Describe the effect of substrate concentration on enzyme action

A

As substrate concentration increases, the initial rate of reaction also increases

33
Q

Explain the effect of substrate concentration on enzyme action

A

The more substrate molecules there are, the more often an enzyme’s active site can bind with one

34
Q

Why is there a limit on the initial rate of reaction in relation to substrate concentration?

A
  • Because if substrate conc keeps increasing but enzyme conc remains constant, there comes a point where every enzyme active site is working continuously
  • If more substrate is added, the enzyme simply cannot work faster - substrate molecules are effectively ‘queuing up’ for an active site to become vacant
  • The enzyme is working at its maximum possible rate, know as Vmax
35
Q

Describe the effect of temperature on the rate of reaction

A
  • At low temperatures the reaction takes place very slowly
  • As the temp rises, the rate of reaction begins to increase significantly
  • However above a certain temperature (the optimum temp), the molecule begins to denature and the rate of reaction falls until it completely denatures
36
Q

Why does the enzyme catalysed reaction take place slowly at low temperatures?

A
  • Molecules are moving relatively slowly
  • Substrate molecules will not collide often with active sites, and so binding between substrate and enzyme is a rare event (less enzyme-substrate complexes)
37
Q

Why does the rate of reaction increase as temperature increases?

A
  • As temp rises, the enzyme and substrate molecules move faster as they have more kinetic energy
  • Collisions happen more frequently, so that substrate molecules enter the active site more often (more enzyme-substrate complexes)
  • When they collide, they also do so with more force, making it easier for bonds to be formed or broken so that the reaction can occur
38
Q

Why does the rate of reaction fall after a certain temperature?

A
  • Above a certain temp, the structure of the enzyme molecule vibrates so energetically that some of the bonds (esp H-bonds) holding the enzyme molecules in its precise shape begin to break up
  • The enzyme molecules begins to lose its shape and activity and is said to be denatured (often irreversible)
  • At first, the substate molecule fits less well into the active site of the enzyme, so the rate of reaction begins to slow down
  • Eventually, the substrate no longer fits at all, or can no longer be held in the correct position for the reaction to occur
39
Q

What is the optimum temperature?

A

The temperature at which an enzyme catalyses a reaction at the maximum rate

40
Q

What is the optimum temperature of most human enzymes?

A

Roughly 40 degrees

41
Q

What are we ensuring by keeping our bodies at roughly 37 degrees?

A

That enzyme-catalysed reactions occur at close to their maximum rate

42
Q

Describe the effect of pH on the rate of reaction?

A

The rate of reaction rises until the optimum pH and then falls as the pH increases

43
Q

What is the optimum pH of most enzymes?

A

7

44
Q

What is pH?

A

A measure of the conc of H+ ions in a solution

45
Q

How does pH affect the enzyme and therefore the rate of reaction?

A
  • H+ ions (or OH- ions) can interact with the R groups of amino acids e.g. by affecting ionisation of the group
  • This affects the ionic bonding between the groups, which in turn affects the 3D arrangement of the enzyme molecule
  • The shape of the active site may change and reduce the chances of the substrate molecule fitting into it
  • The ions can also block up the active site
  • A pH which very different from the optimum pH can cause denaturation of an enzyme
46
Q

What can use when investigating pH?

A

Buffer solutions - these have a particular pH and maintain it even if the reaction taking place would otherwise cause the pH to change
- you add a measured volume of buffer to your reacting mixture

47
Q

How could a competitive inhibitor inhibit an enzyme’s function?

A
  • If it is very similar in shape to the substrate and there is a higher conc of inhibitor than substrate, it becomes less likely that the substrate will collide with an empty site
  • Therefore, the rate of enzyme activity decreases
  • This is known as competitive inhibition e.g. ethanol is a competitive inhibitor to ethylene glycol
48
Q

How is competitive inhibition reversible (not permanent)?

A

Because it can be reversed by increasing substrate concentration

49
Q

What does there has to be for a competitive inhibitor to inhibit enzyme function?

A

A higher concentration of inhibitor than substrate

50
Q

What is non-competitive inhibition?

A

When a molecule binds to another part of the enzyme, rather than the active site, but still affects the enzyme’s function

51
Q

How does non-competitive reversible inhibition inhibit an enzyme’s function?

A
  • When the inhibitor is bound to the enzyme it can seriously disrupt the normal arrangement of H-bonds and hydrophobic interactions holding the enzyme in its 3D shape
  • The resulting distortion ripples across the molecule to the active site, making the enzyme unsuitable for the substrate
  • Therefore, enzyme activity rapidly decreases
  • While the inhibitor is attached to the enzyme, the enzyme’s function is blocked no matter how much substrate is present
52
Q

Although inhibition of enzyme function can be lethal, how can it be essential?

A

It can control metabolic reactions so that they don’t ‘run wild’

53
Q

How can metabolic reactions be controlled?

A
  • By using the end-product of a chain of reactions as a non-competitive inhibitor (end-product inhibition)
  • As the enzyme converts substrate to product, it is slowed down because the end-product binds to another part of the enzyme and prevents more substrate binding
  • The end-product can then lose its attachment to the enzyme and be used elsewhere, allowing the enzyme to reform to it active state
  • As product levels fall, the enzyme is able to top them up again
54
Q

What is the turnover number/second (rate)?

A

The number of molecules of substrate that one molecule of enzyme converts into product in one second (related to Vmax) e.g. average enzyme = 1000

55
Q

What are the two different ways of comparing the efficiency of different enzymes?

A

1) To measure the theoretical maximum rate (velocity) -Vmax - of the reaction it catalyses
2) Km

56
Q

What happens at Vmax?

A

All the enzyme molecules are bound to substrate molecules - the enzyme is saturated with substrate

57
Q

What happens to the rate of reaction as substrate concentration increases?

A

The rate of reaction increases until the reaction reaches Vmax

58
Q

Why can’t you accurately read off Vmax from a substrate concentration graph?

A
  • The curve is asymptotic
  • Therefore, in practice, the curve never flattens out
  • In theory, it does at an infinite conc, but this is impossible to measure
59
Q

How can you find Vmax using a graph?

A
  • Plot 1/S (substrate concentration) against 1/V (velocity, i.e. rate)
  • This plot is called a double reciprocal plot
  • You can then plot the infinite substrate concentration because 1/infinity = 0 which can be plotted (the graph is a straight line)
  • Therefore Vmax can be found accurately (y-intercept = 1/Vmax)
60
Q

What is the Michaelis-Menten constant (Km)?

A

The substrate concentration at which an enzymes works at half its max rate (1/2 of Vmax)

61
Q

What happens at Km and what does this mean?

A
  • At Km, 1/2 of the active sites of the enzyme are occupied by the substrate
  • The higher the affinity of the enzyme for the substrate, the lower substrate concentration is needed for this to happen
  • Therefore, Km is a measure for the affinity of an enzyme for its substrate
  • The higher the affinity, the lower the Km and the quicker the reaction will proceed to Vmax (although it is not directly affected)
62
Q

What is a way to think about Vmax and Km?

A 
Vmax = max speed of car
Km = acceleration
63
Q

What does Vmax give information about?

A

The maximum rate of reaction that is possible (but not necessarily under cell conditions)

64
Q

What does Km measure?

A

The affinity of the enzyme for the substrate
- the higher the affinity, the more likely that the product will be formed when a substrate molecule enters the active site, rather than the substrate simply leaving the active site again before a reaction takes place

65
Q

How do you find Km from a double reciprocal plot?

A

-1/Km (x-intercept) - in negative region of x-axis

66
Q

What are two significances of Vmax and Km?

A

1) For a commercially important enzyme, the performance of different enzymes can be compared
2) An enzyme’s preference for different substrates can be compared quantitively

67
Q

What are some commercial applications of enzymes?

A

Medicine, food technology

68
Q

Why do people use immobilised enzymes?

A

They keep costs down as they can be reused/recycled

69
Q

How is the enzyme lactase immobilised?

A

Using alginate beads (enzyme is inside beads)

  • Milk is then allowed to run through the column of beads
  • Lactase hydrolyses the lactose in milk into glucose and galactose
  • Therefore, the milk is lactose-free
70
Q

What are the advantages of using immobilised lactase?

A
  • W/o it, you would not be able to get the enzyme back and the milk would be contaminated with enzyme
  • You can keep and re-use the enzyme and the milk is enzyme-free
  • Immobilised enzymes are also more tolerant of temp and pH changes than enzymes in solution (bc held more firmly in shape and less exposed)
71
Q

What is an inhibitor?

A

A substance that slows down the rate of an enzyme-catalysed reaction

Biology A-Level (44 decks)

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