2.1.4 - Enzymes (set B - factors affecting enzymes)

Question 1

give 4 factors that affect the rate of an enzyme controlled reaction?

Answer 1

• temperature
• pH
• enzyme concentration
• substrate concentration

Enzymes are also affected by the presence of inhibitors

Question 2

Explain the effect of temperature on enzymes?

Answer 2

Enzymes have a specific optimum temperature at which they catalyse a reaction at the maximum rate

• lower temps prevent reactions, or slow them down
• higher temps - speeds up reactions however can cause the enzyme to denature if its too warm

Question 3

Explain in depth why lower temperatures either prevent or slow down reactions with enzymes?

Answer 3

• molecules move relatively slow which decreases the frequency of successful collisions between substrate molecules and active site of the enzyme - less frequent enzyme-substrate complex formation
• substrate and enzyme collide with less energy - so less likely for bonds to be formed or broken (allowing reaction to occur)

Question 4

Explain in depth why higher temperatures speed up reactions with enzymes?

Answer 4

• molecules move more quickly as they have more kinetic energy - this increases the frequency of successful collisions between substrate molecules and active site of enzyme - therefore more frequent enzyme-substrate complex formation
• substrate and enzyme collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)

Question 5

Explain what happens to enzymes when the temperature increases too much?

Answer 5

Increase in temperature means the enzyme has more kinetic energy so it vibrates more, which can the lead to the hydrogen bonds that hold the enzyme in shape breaking changing the proteins tertiary structure - results in the active site permanentely changing shape which means it can no longer bind to the specific, complementary substrate

• enzyme is now denatured and no longer functions as a catalyst (rate of reaction is 0)

Question 6

Explain the graph relating to enzymes and temperature change?

Answer 6

Steady initial increase in rate of reaction - as molecuels gain kinetic energy,increasing frequency of collisions between enzyme and substrate molecules

Flat line when enzyme reaches optimum temperature,this is where rate is faster

Steep decrease - as enzyme starts to denature (hydrogen bonds break, changing the proteins tertiary structure permanentely damaging the active site) the rate of reaction falls

Question 7

Explain the link between pH and briefly explain the effect it has on enzymes - give an example of some enzymes and their optimum pH?

Answer 7

All enzymes have an optimum pH value, most human enzymes work best at pH 7 (neutral) - however some work better at pH 2 like pepsin (found in the acidic conditions of the stomach)

• changes in the pH can distrust the bonding in the enzyme, causing it too denature

Question 8

Explain in depth why changes in pH can cause the enzyme to denature - reference bonding and protein structure?

Answer 8

Above and below the optimum pH, the H+ and OH- ions found in acids and alkaline can mess with the ionic bonds and hydrogen bonds that hold the enzymes tertiary structure in place - can make the active site change shape causing the enzyme to become completely denatured

Question 9

Explain a method of investigating the effect of pH on the rate of an enzyme-catalysed reaction?

Answer 9

Use buffer solutions to measure the rate of reaction at different pH values, each buffer solution can have a specific pH and can maintain the specific pH

A measured volume of the buffer solution is added to the reaction mixture (the same volume of each buffer solution should be added for each pH value that is being investigated)

Question 10

Explain a benefit of using buffer solutions to investigate the effect of pH on rate of enzyme reactions - reference is ability to maintain pH?

Answer 10

• Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
• also have a specific pH

Question 11

Explain the main difference between temperature and pH effect on enzymes?

Answer 11

Temperature can both affect the speed of which molecules are moving (therefore the number of collisions between enzyme and substrate) and can denature enzymes

• pH however does not affect collision rate but only disrupts the ability of the substrate to bind with the enzyme, reducing the number of successful collisions until the active site changes so much that no more successful collisions occur

Question 12

Explain the effect of substrate concentration on enzyme controlled reactions?

Answer 12

The greater the substrate concentration the higher the rate of reaction - this is because more substrate molecules increases the likelihood of enzyme-substrate complex formation however if the enzyme concentration remains fixed, and the amount of substrate is increased past a certain point it can lead to all the active sites becoming saturated - no further increase in reaction rate

Question 13

Explain how active sites become saturated when substrate concentration is further increased?

Answer 13

If enzyme concentration remains fixed but substrate is increased past a certain point, however, all available active sites eventually become saturated - any further increase in substrate concentration will not increase the reaction rate

• when all the active sites are full, any substrate molecule that are added have nowhere to bind in order to form an enzyme-substrate complex

Question 14

Explain the graph regarding substrate concentration?

Answer 14

linear increase in reaction rate as substrate is added, which then plateaus when all active sites become occupied

Steeper increase in rate of reaction as substrate concentration increases however it becomes less steep before flatting out (due to active sites all becoming saturated, no further substrate will alter the rate of reaction)

Question 15

Explain the effect of enzyme concentration on enzymes?

Answer 15

The more enzyme molecules there are in a solution, the more likely a substrate molecule is to collide with one and form an enzyme-substrate complex - increasing the concentration of enzymes increases the rate of reaction

If the amount of substrate is limited - then there’s enough enzyme molecules to deal with the substrates, adding more will have no further effect

Question 16

Explain what an increase in enzyme concentration increases the rate of reaction?

Answer 16

The higher the enzyme concentration in a reaction mixture, 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.

Question 17

Explain the graph regarding the correlation between enzyme concentration and rate of reaction?

Answer 17

Linear increase which continues as long as sufficient substrate is available, increases as more active sites become available

• if substrate amount is limited -further increase in enzymes will not increase rate of reaction (substrate becomes limiting factor)

Question 18

Explain what happens when substrate concentration is continually increased but enzyme concentration is kept constant?

Answer 18

there eventually comes a point where every enzyme active site is working continuously - At this point, the substrate molecules are effectively ‘queuing up’ for an active site to become available, At this stage, the enzyme is working at its maximum possible rate, known as Vmax (V stands for velocity).

Question 19

What is an enzymes specific optimum temperature?

Answer 19

The temperature at which they catalyse a reaction at the maximum rate

Question 20

What are inhibitors, give the two types?

Answer 20

Molecules that prevent enzymes from carrying out their normal function of catalysis by stopping them or slowing them down - can be reversible or irreversible

• two types - competitive and non-competitive

Question 21

Outline how a competitive inhibitor works?

Answer 21

Molecule or part of a molecule that has a similar shape to substrate of an enzyme compete with substrate to fit into enzymes active site - blocks substrate from forming an enzyme-substrate complex, enzyme now inhibited

• most bind temporarily - so the effect is reversible (exceptions for example aspirin)

Question 22

Outline how a non-competitive inhibitor works?

Answer 22

Bind to the enzymes at an alternate site (allosteric site) which alters the shape of the active site by changing the enzymes tertiary structure - preventing substrate from binding to it (active site no longer complementary to substrate)

• enzyme now inhibited

Question 23

Explain the main difference between competitive and non-competitive inhibitors?

Answer 23

Competitive inhibitors compete with substrates to form an enzyme-substrate complex - where as non-competitive inhibitors attach to the allosteric site and change the enzymes tertiary structure so substrates can’t even bind to from a complex (don’t compete)

Question 24

Explain the effect of competitive inhibitors on rate of reaction?

Answer 24

Reduces the rate of reaction for a given concentration of substrate - does not change the V max of the enzyme it inhibits

• increasing substrate concentration means there’s more substrates than inhibitors so V max can still be reached

Question 25

Does increasing substrate concentration affect competitive inhibitors?

Answer 25

Yes - increasing substrate concentration decreases the competitive inhibitors effect

Question 26

Does increasing substrate concentration affect non-competitive inhibitors?

Answer 26

No - increasing substrate concentration has no effect on the inhibitor, as the enzymes active site remains changes so ES complex can not form

• increasing the concentration of inhibitors decreases the rate of reaction further

Question 27

Give an example of a competitive inhibitor?

Answer 27

Statins are competitive inhibitors of an enzyme used in the synthesis of cholesterol - prescribed to help people reduce blood cholesterol concentration

Question 28

Give an example of an irreversible non-competitive inhibitor?

Answer 28

Cannot be moved from the part of the enzyme that they are attached to (form covalent bonds) - often very toxic

• for example organophosphates used as insecticides and herbicides - inhibits an an enzyme necessary for nerve impulse transmission (can lead to paralysis and death if ingested)

Question 29

Explain the meaning of the term end-product inhibition?

Answer 29

Term used for enzyme inhibition that occurs when the product of a reaction acts as an inhibitor to the enzyme that produce it (serves as negative feedback control mechanism of the reaction)

Synthesis of ATP is regulated this way (ATP acts as the inhibitor)

Question 30

Explain the significance of the amount of end product?

Answer 30

• if end product is high - it binds non-competitively to an enzyme in the pathway, blocking further production of itself
• when the end product falls, inhibition ends and the pathway restarts

Question 31

Explain why and how metabolic reactions are controlled?

Answer 31

Must be tightly controlled and balanced so that no single enzyme continuously and uncontrollably generate more of a particular product

• controlled by using end-product of a particular sequence of metabolic reactions as a non-competitive, reversible inhibitor

Question 32

Explain why some non-reversible inhibitors are considered metabolic poisons?

Answer 32

Non-reversible inhibitors can prevent enzymes from function permanently by forming covalent bonds with them - this means the reaction which the enzyme catalyses stops, the process to produce more enzymes requires a slow process of transcription and translation

Question 33

Give some examples of non-reversible inhibitors which are provisions - give the enzymes/reactions they affect?

Answer 33

• cyanide acts as a non-reversible inhibitor of cytochrome oxidase (enzyme that catalases reactions in aerobic respiration) - can be fatal, due to length of time to produce new enzymes
• lead acts as non-reversible inhibitor of ferrochelatase (enzyme involved in production of haem for haemoglobin)

Question 34

Outline the role of non-reversible inhibitors in medicine - give 2 examples?

Answer 34

• penicillin - non reversible inhibitor of an enzyme that helps form cross-links in bacterial cell walls - allows for destruction of bacteria cells due to lack of cell wall
• aspirin - non reversible inhibitor for enzyme that helps produce prostaglandins (stimulates pain) - provides pain relief

Question 35

Give the uses of 3 inhibitors - provide an example for each?

Answer 35

• natural poisons - eg cyanide rervsibel inhibitor for enzyme, prevents ATP production
• biocides - eg glyphosate which blocks the synthesis of amino acids
• drugs - eg penicillin - blocks production of bacteria cell walls

Question 36

Explain what cofactors are?

Answer 36

Non-protein component of an enzyme that is required in order for the enzyme to function - two main types - activators and coenzymes (organic cofactors)

• example chloride ions (Cl-) is a cofactor for amylase

Question 37

Explain what coenzymes are?

Answer 37

Organic, non-protein molecules whose role is to transport chemical groups between enzymes, linking together controlled enzyme reactions - can bind temporarily or permanently to enzyme

• eg vitamin C and ATP

Question 38

Give 3 examples of coenzymes?

Answer 38

• vitamins (important source of coenzymes - especially vitamins of B group)
• NADP - coenzyme for photosynthesis
• NAD and FAD - coenzymes in respiration

Question 39

Explain what a prosthetic group - give an example?

Answer 39

Cofactor that is a permanent part of the enzyme - help form enzymes final 3D shape

• Zn2+ - prosthetic group in carbonic anhydrase as it forms part of the enzymes active site

Question 40

Explain the importance of co-factors for enzymes?

Answer 40

Some enzymes can only function properly if another non-protein substance is present - they are inactive, until the substance combines and changes the enzymes tertiary structure (allows for substrates to bind and form ES complex)

Question 41

Explain what activators are - give examples?

Answer 41

Inorganic groups that are permanently bound to the enzyme - type of prosthetic group

• examples - iron, zinc and copper

Question 42

Give an example of a cofactor?

Answer 42

Chloride ions act as a cofactor for amylase - inorganic ions help to stabilise the structure of the enzyme and take part in the reaction at the active site

• chloride ions must be present for amylase to digest starch

Question 43

Explain the role of the coenzymes NAD and FAD in respiration?

Answer 43

During reactions in respiration, coenzymes NAD and FAD are alternately reduced and oxidised, transferring energy in the form of hydrogen ions