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AS Biology > Chapter 4 - Enzymes > Flashcards

Chapter 4 - Enzymes Flashcards

1
Q

What are enzymes?

A

Biological catalysts that interact with substrate molecules to facilitate chemical reactions

Usually globular proteins

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2
Q

Why are enzymes important?

A

The allow reactions to happen quickly without extreme conditions (e.g. high temperatures and pressures) that are not possible in living cells because they would damage the cell components

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3
Q

What is a substrate?

A

A substance used, or acted on, by another process or substance, e.g. a reactant in an enzyme-catalysed reaction

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4
Q

What is anabolism?

A

Anabolic reactions of metabolism that construct molecules from smaller units

  • These reactions require energy from the hydrolysis of ATP, and are catalysed by enzymes
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5
Q

What is catabolism?

A

Catabolic reactions of metabolism that break molecules down into smaller units

  • These reactions release energy, and are catalysed by enzymes
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6
Q

What is metabolism?

A

The sum of all of the different reactions and reaction pathways happening in a cell or organism

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7
Q

What is Vmax?

A

Maximum initial velocity or rate of an enzyme-catalysed reaction

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8
Q

Describe the mechanism of enzyme action

A

Enzymes help the molecules collide successfully, and therefore reduce the activation energy required

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9
Q

What is the active site of an enzyme?

A

An area within the tertiary structure of the enzyme that has a shape that is complementary to the shape of a specific substrate molecule

  • This allows the enzyme to bind to a substrate with specificity
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10
Q

What is the ‘lock and key’ hypothesis of enzyme action?

A

In the same way that only the right key will fit into a lock, only a specific substrate will ‘fit’ the active site of an enzyme

Enzyme + Substrate —> ESC —> Enzymes + Products

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11
Q

How does the enzyme act on the substrate in the ‘lock and key’ hypothesis?

A
  • The substrate and enzyme molecules each have KE and are constantly moving randomly
  • If a substrate molecule successfully collides with an enzyme molecule, an ESC forms
  • The substrate is held in such a way by the enzyme that the right atom-groups are close enough to react
  • The R-groups within the active site of the enzyme will also interact with the substrate forming temporary bonds
  • These put strain on the bonds within the substrate, which also helps the reaction along
  • The substrate is converted into the product, an EPC formed, and then the products leave the enzyme
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12
Q

What is the ‘induced fit’ hypothesis of enzyme action?

A

Modified lock and key explanation for enzyme action: the active site of the enzyme is modified in shape by binding to the substrate

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13
Q

How does the enzyme act on the substrate in the ‘induced fit’ hypothesis?

A
  • The active site of the enzyme changes shape slightly as the substrate enters to fit it better
  • An ESC is formed, and non-covalent R-group interactions (e.g. hydrogen bonds, ionic attractions, van Der Waals forces and hydrophobic interactions) bind the substrate molecule to the enzyme’s active site
  • This can weaken bonds in the substrate, lowering the activation energy for the reaction
  • The substrate is converted into the product forming an EPC
  • As the product molecules have a slightly different shape from the substrate molecule, they detach from the active site
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14
Q

What are intracellular enzymes?

A

Enzymes that act within cells

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15
Q

Give an example of an intracellular enzyme

A

Catalase

  • Hydrogen peroxide is a toxic product of many metabolic pathways
  • Enzyme catalase break it down into O2 and H2O quickly, preventing its accumulation
  • Found in both animal and plant tissues
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16
Q

What are extracellular enzymes?

A

Enzymes that act outside the cell that made them

  • In some organisms e.g. fungi, they work outside the body
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17
Q

Describe the process of the digestion of starch

A
  1. Starch polymers are broken down into maltose (disaccharide) by amylase which is produced by the salivary glands and the pancreas. It’s released in saliva into the mouth, and in pancreatic juice into the small intestine
  2. Maltose is broken down into glucose (monosaccharide) by maltase in the small intestine
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18
Q

Describe the process of the digestion of proteins

A
  • Trypsin is a protease that catalyses the digestion of proteins into smaller peptides which can be broken further into amino acids by other proteases
  • It’s produced in the pancreas and released with the pancreatic juice into the small intestine, where it acts on proteins
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19
Q

Why does increasing the temperature initially increase the rate of reaction?

A

If a reactant mixture containing enzyme and substrate molecules is heated:

  • Both types of molecule will gain KE and move faster
  • This will increase the rate of successful collision
  • The rate of formation of ESCs increases and the rate of reaction increases, increasing the number of EPCs per second, up to a point
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20
Q

What is an enzyme’s optimum temperature?

A

The temperature at which the enzyme has the highest rate of activity

  • Around 40˚C in humans
  • 70˚C for thermophilic bacteria
  • Below 5˚C for psychrophilic organisms
21
Q

What does denaturation mean?

A

Change in the tertiary structure of a protein or ensyme, resulting in a loss of normal function

22
Q

What happens when the temperature is increased past the optimum?

A
  • Increasing temperature makes molecules vibrate
  • This may break some of the weak bonds (e.g. hydrogen and ionic bonds) that hold the tertiary structure of the enzyme’s active site
  • As the active site shape begins to change, the substrate won’t fit in so well, and the rate of reaction begins to decrease
  • As more heat is applied, the active site completely and irreversibly changes so that it is no longer complementary to the substrate
  • The reaction can’t proceed at all
  • The enzyme is denatured
  • (primary structure is not altered as heat doesn’t break peptide bonds)
23
Q

What is the temperature coefficient (Q10)?

A

A measure of how much the rate of reaction increases with a 10˚C temperature increase.

Usually taken as 2 for enzyme-controlled reactions i.e rate of reaction doubles with 10˚C increase

24
Q

Why does pH affect enzymes?

A
  • A change in pH refers to a change in hydrogen ion concentration
  • Active site is only the right shape at a certain hydrogen ion concentration
  • Hydrogen bonds and ionic bonds between amino acid R-groups hold proteins in their precise 3D shapes
  • Hydrogen ions interact with polar and charged R-groups, affecting the interaction of the R-groups with each other
  • The more hydrogen ions present (low pH) the less R-groups can interact with each other leading to bonds breaking and the shape of the enzyme changing. The reverse is true when less hydrogen ions present (high pH)
  • Shape of enzyme will change, so it can only function within a narrow pH range
25
Q

Why do enzymes only work within a narrow pH range?

A
  • Small changes of pH either side of the optimum slow ROR because the shape of the active site is disrupted
  • If normal optimum pH is restored, the hydrogen bonds can reformat the active site’s shape is restored
  • At extremes of pH, the active site may be permanently changed; the enzyme is denatured
26
Q

What happens when the concentration of a substrate is increased?

A
  • Higher collision rate, so the rate of reaction increases
  • Leads to the formation of ESCs at a faster rate
  • The rate of reaction increases up to its maximum (Vmax) - at this point all of the active sites are occupied and no more ESCs can be formed until products are released from active sites
  • The only way to increase the ROR would be to add more enzyme or increase the temperature
  • If the concentration of enzyme is increased, more active sites are available so the ROR can rise towards a higher Vmax.
27
Q

What is an inhibitor?

A

A factor that prevents or reduces the rate of an enzyme-catalysed reaction.

There are 2 types: competitive and non-competitive

28
Q

What is a competitive inhibitor?

A

An inhibitor that competes with substrate to bind to active site on an enzyme

29
Q

How does competitive inhibition work?

A
  1. A molecule or part of a molecule that has a similar shape to the substrate of an enzyme can fit into the active site of the enzyme
  2. This blocks the substrate from entering the active site, preventing the enzyme from catalysing the reaction
  3. The enzyme cannot carry out its function and is said to inhibited
30
Q

What is the effect of competitive inhibition?

A
  • Substrate and inhibitor molecules present in a solution will compete with each other to bind to the active sites of the enzymes catalysing the reaction
  • The rescues the number of substrate molecules binding to active sites in a given time and slows down the rate of reaction
  • Degree of inhibition will depend on the relative concentrations of substrate, inhibitor, and enzyme
  • Most competitive inhibitors only bind temporarily to the active site
31
Q

What is the effect of a competitive inhibitor on rate of reaction?

A
  • Reduces the rate of reaction for a given concentration of substrate
  • Doesn’t change the Vmax of the enzyme
  • If substrate concentration is increased, there will be so much more substrate than the inhibitor that the original Vmax can still be reached
32
Q

Give 2 examples of competitive inhibitors

A

Statins

  • Competitive inhibitors of an enzyme used in cholesterol synthesis
  • Prescribed to help people reduce blood cholesterol concentration
  • High blood cholesterol levels can result in heart disease

Aspirin

  • Irreversibly inhibits the active site of COX enzymes, preventing the synthesis of prostaglandins and thromboxane, the chemicals responsible for producing pain and fever
33
Q

What is a non-competitive inhibitor?

A

A inhibitor that binds to an enzyme at an allosteric site

34
Q

How does non-competitive inhibition work?

A
  • Inhibitor binds to the enzyme at a location other than the active site. This alternative sites called an allosteric site
  • Binding of the inhibitor causes the tertiary structure of the enzyme to change, meaning the active site changes shape
  • Active site no longer has complementary shape to substrate, so its is unable to bind to the enzyme
  • Enzyme cannot carry out its function and is said to be inhibited
35
Q

What is the effect of a nin-competitive inhibitor on rate of reaction?

A
  • Increasing concentration of enzyme or substrate will not overcome the effect of a non-competitive inhibitor
  • Increasing the concentration of inhibitor will decrease rate of reaction further as more active sites become unavailable
  • Lower Vmax
36
Q

Give examples of irreversible non-competitive inhibitors

A

Organophosphates

  • Used as insecticides and herbicides
  • Irreversibly inhibit the enzyme acetyl cholinesterase (necessary for nerve impulse transmission)
  • Can lead to muscle cramps, paralysis and death if ingested

Protein Pump Inhibitors (PPI)

  • Used to treat long-term indigestion
  • Irreversibly block an enzyme system responsible for secreting hydrogen ions into the stomach
  • Makes PPIs very effective in reducing the production of excess acid which, if left untreated, can lead to the formation of stomach ulcers
37
Q

What is end-product inhibition?

A

The product of a reaction inhibits the enzyme required for the reaction.

  • Negative-feedback control mechanism for the reaction
  • Excess products are not made and resources are not wasted
  • Example of non-competitive reversible inhibition
38
Q

Describe the metabolic pathway of respiration

A
  • Results in the production of ATP
  • Glucose is broken down
  • Addition of 2 phosphate groups to glucose molecule
  • The addition of the 2nd phosphate group results in the initial breakdown of the glucose molecule, and is catalysed by the enzyme phosphofructokinase (PFK)
  • This enzyme is competitively inhibited by ATP

Therefore ATP regulates its own production

39
Q

Describe how ATP regulates its own production

A

When ATP levels are high…

  • More ATP binds to the allosteric site of PFK, preventing the addition of the 2nd phosphate group to glucose
  • Glucose is not broken down, and ATP is not produced at the same rate

As ATP is used up…

  • Less binds to PFK
  • PFK is able to catalyse the addition of the 2nd phosphate group to glucose
  • Respiration resumes, leading the the production of more ATP
40
Q

What are cofactors needed for? (3)

A
  • Needed by enzymes in order to carry out their function as biological catalysts
  • May transfer atom groups from one reaction to another in a multi-step pathway
  • May form part of the active site of an enzyme
41
Q

What is a cofactor?

A

Non-protein components necessary for the effect functioning of an enzyme

42
Q

Where do inorganic cofactors come from?

A

The diet as minerals e.g. iron, calcium, chloride and zinc ions

43
Q

Give an example of an enzyme that needs an inorganic cofactor

A

Amylase

  • Contains a chloride ion that is necessary for the formation of a correctly shaped active site
44
Q

What is a coenzyme?

A

An organic cofactor

  • Derived from vitamins
  • e.g. vitamin B3 is used to synthesise NAD (nicotinamide adenine dinucleotide), a coenzyme responsible for the transfer of hydrogen atoms between molecules involved in respiration
  • e.g. vitamin B5 is used to make Coenzyme A, which is essential in the breakdown of fatty acids and carbohydrates in respiration
45
Q

How are prosthetic groups different to other cofactors?

A

Prosthetic groups are cofactors required by certain enzymes to carry out their catalytic function

  • Some cofactors are loosely or temporarily bound to the enzyme protein in order to activate them, but prosthetic groups are tightly bound and form a permanent feature of the protein
  • e.g. Zn2+ ions form an important part of the structure of carbonic anhydrase, an enzyme necessary for the breakdown of CO2
46
Q

What are inactive precursor enzymes?

A

Enzymes produce in a inactive form

  • Esp. enzymes that can cause damage within the cells producing them or tissues where they are released
  • OR enzymes whose action needs to be controlled and only activated under certain conditions
47
Q

How do precursor enzymes work?

A
  • They need to undergo a change in shape (tertiary structure), esp.to the active site to be activated
  • This happens with the addition of a cofactor
  • Before the cofactor is added, the precursor protein is called an apoenzyme
  • When the cofactor is added and the enzyme activated, it is called a holoenzyme
48
Q

How else can the change in tertiary structure be brought about?

A
  • The action of another enzyme, e.g. protease, which cleaves certain bonds in the molecule
  • A change in conditions e.g. pH or temperature can result in a change in tertiary structure and activate a precursor enzyme
  • These types of precursor enzymes are called zymogens or proenzymes
49
Q

Give an example of precursor activation

A
  • When inactive pepsinogen is released into the stomach to digest proteins, acid pH transforms it into the active enzyme pepsin
  • This adaptation protects body tissues against the digestive action of pepsin

AS Biology (5 decks)

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