Enzymes

Question 1

What are enzymes?

Answer 1

• biological catalysts
• globular proteins
• interact with substrate molecules increasing the rate of reaction without the need for harsh environments.

Question 2

What are the roles of enzymes?

Answer 2

• synthesis of large-polymer based components e.g. cellulose forms the walls of plant cells
• anabolic (building up) and catabolic (breaking down) reactions
• digestion
• metabolism

Question 3

What is meant by Vmax?

Answer 3

Maximum velocity or rate at which enzymes catalyse a reaction. Happens when all active sites are saturated with substrate.

Question 4

What is the specificity of the enzyme?

Answer 4

• when many different enzymes are produced by living organisms, as each enzyme catalyses one biochemical reaction, of which there are thousands in any given cell.

Question 5

How do enzymes work?

Answer 5

• molecules in a solution move and collide randomly. For a reaction to happen they need to collide in the right orientation.
• energy needs to be provided for the reaction to start. This is activation energy.
• enzymes help the molecules collide successfully and therefore reduce the amount of activation energy required.

Question 6

What is the lock and key hypothesis?

Answer 6

• area of enzyme complementary to specific substrate molecule. This area is called the active site.
• when substrate is bound to active site an enzyme-substrate complex is formed
• they then react and the products are formed in an enzyme product complex
• products released leaving enzyme unchanged and able to react again

Question 7

How does the enzyme help a reaction take place?

Answer 7

• substrate is held by the enzyme so atom-groups are close enough to react.
• R-groups within the active site will also interact with the substrate forming temporary bonds that put a strain on the bonds within the substrate which helps the reaction proceed.

Question 8

What is the induced fit hypothesis?

Answer 8

• active site of the enzyme changes shape slightly as the substrate enters
• initial interaction between enzyme and substrate is relatively weak
• but these interactions rapidly induce changes in the enzyme’s tertiary structure that strengthen binding, putting strain on the substrate
• can weaken bonds in the substrate therefore lowering the activation energy for the reaction

Question 9

What are intracellular enzymes?

Answer 9

enzymes that act within cells

Question 10

Describe the production and release of extracellular enzymes

Answer 10

• protein made on RER
• protein packaged into transport vesicles
• vesicles move to golgi apparatus via cytoskeleton
• vesicles fuse with cis face of golgi as protein enters
• protein modified into specific 3D enzyme shape
• released from golgi trans face in secretory vesicle
• fuse with cell surface membrane
• enzymes released by exocytosis

Question 11

What are extracellular enzymes?

Answer 11

• work outside the cell that made them
• break down large nutrient molecules into smaller molecules in the process of digestion
• e.g. in humans amylase and trypsin

Question 12

What is the first step of digesting starch?

Answer 12

• starch polymers are partially broken down into maltose, which is a disaccharide
• enzyme involved is amylase
• amylase is produced by the salivary glands (saliva in mouth) and the pancreas (pancreatic juice in small intestine)

Question 13

What is the second step of digesting starch?

Answer 13

• maltose is broken down into glucose, which is a monosaccharide
• enzyme involved is maltase
• maltase is present in the small intestine
• glucose small enough to be absorbed by cells lining digestive system and so absorbed into bloodstream

Question 14

How do we digest protein?

Answer 14

• pepsin (protease produced by stomach wall) breaks proteins down into shorter peptide chains
• trypsin (protease produced by pancreas and released with pancreatic juice into the small intestine) produces even smaller chains
• other proteases break these down into amino acids which are then absorbed into the bloodstream

Question 15

Explain how catabolism and anabolism are related to metabolism

Answer 15

• catabolism is the breaking down of molecules
• anabolism is the building of molecules
• reactions include both and metabolism is the sum of these reactions

Question 16

Why have the models of enzyme action changed over time?

Answer 16

• improved technology
• continually investigated
• more evidence, so more accurate representation

Question 17

How does temperature increase the rate of reaction?

Answer 17

• more heat more kinetic energy
• more successful collisions between enzymes and substrates
• more ESC/EPC/product produced

Question 18

How does temperature denature the enzyme?

Answer 18

• more vibrations within molecules
• breaking hydrophilic and hydrophobic interactions, hydrogen and ionic bonds
• tertiary structure changes
• enzyme then denatures after optimum temperature
• substrate no longer complementary to shape of active site

Question 19

What is meant by Q10 (temperature coefficient)?

Answer 19

• The amount the rate of reaction increases when temperature is raised by 10c
• enzymes have a Q10 of 2 (this means the rate of reaction doubles for every 10C rise in temperature)

Question 20

What is the equation for Q10?

Answer 20

Q10 = rate of reaction at (T + 10C) / rate of reaction at TC

Question 21

How would you expect thermophile enzyme structure to differ from ours?

Answer 21

• more disulphide bridges that are less likely to break under high temperatures
• more cysteine amino acids in primary structure to form more strong disulphide bridges
• means active site less likely to denature

Question 22

Why are there different enzymes in different parts of the digestive system?

Answer 22

• enzymes have different optimum pas
• conditions are changing as enzymes are needed
• molecules have changed shapes through digestive system also

Question 23

How can ph affect enzyme structure?

Answer 23

• deviation away from optimum ph (H ion concentration)
  alters enzyme structure by interfering with the hydrogen bonds and ionic bonds between R-groups
• proteins can renature when ph returns to optimum as long as deviation wasn’t too significant

Question 24

Enzymes with low optimum temperatures tend to have flexible structures. Why is this necessary?

Answer 24

• at low temperatures kinetic energy is low
• substrates/enzymes move slowly so fewer collisions
• collisions have less energy increasing flexibility of active site
• so increases chance of successful collision

Question 25

How does increasing substrate concentration affect the rate of reaction?

Answer 25

• higher collision rate with the active site of enzymes and the formation of more enzyme substrate complexes.
• rate of reaction therefore increases.

Question 26

What are inhibitors?

Answer 26

• molecules that prevent enzymes from carrying out their normal function of catalysis (or slow them down).
• two types of inhibition: competitive and non-competitive

Question 27

How does competitive inhibition work?

Answer 27

• Competitive inhibitor interferes with active site of enzyme so substrate can’t bind
• The substrate and the competitive inhibitor compete for the active site, reducing the number of ESC’s made in a given time, slowing the rate of reaction.
• Binding is normally temporary, so effect is reversible (aspirin is an exception)

Question 28

What will determine the degree of inhibiton?

Answer 28

• relative concentrations of enzyme, substrate and the inhibitor

Question 29

How does a competitive inhibitor effect the rate of reaction?

Answer 29

• reduces it for a given concentration of substrate
• does not change the Vmax of the enzyme it inhibits
• if substrate concentration is increased enough there will be so much more substrate than inhibitor that the original Vmax can still be reached.

Question 30

How do statins work as competitive inhibitors?

Answer 30

• inhibit enzyme used in the synthesis of cholesterol

- statins prescribed to reduce blood cholesterol concentration as high concentration can result in heart disease.

Question 31

How do aspirins work as competitive inhibitors?

Answer 31

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

Question 32

What is non-competitive inhibition?

Answer 32

• Inhibitor binds to a region other than the active site (an allosteric site)
• Causes tertiary shape of the enzyme to change shape, so active site changes shape
• Substrate no longer complementary to the active site
• Enzyme therefore inhibited

Question 33

How does a non-competitive inhibitor effect the rate of reaction?

Answer 33

• increasing concentration of substrate or enzyme will not overcome effect of a non-competitive inhibitor.
• increasing concentration of the inhibitor will decrease the rate of reaction further as more active sites become unavailable.

Question 34

What are examples of irreversible, non-competitive inhibitors?

Answer 34

• Usually toxic
• Organophosphates (insecticides and herbicides) inhibit acetyl cholinesterase, an enzyme necessary for nerve impulse transmission. Can lead to paralysis or death.
• Proton pump inhibitors (PPI’s), used to treat long term indigestion, block a system used to secrete H+ into the stomach. Makes PPIs effective in reducing production of excess acid, which if left untreated can lead to formation of stomach ulcers.

Question 35

What is end-product inhibition?

Answer 35

• term used for enzyme inhibition that occurs when the product of a reaction acts as an inhibitor to the enzyme that produces it.
• serves as a negative feedback control mechanism for the reaction: excess products not made and resources not wasted.

Question 36

What is an example of end-product inhibition?

Answer 36

• The ATP produced in respiration non-competitively inhibits an enzyme (PFK) involved in the activation of glucose at the beginning of the respiration reaction.

Question 37

How does the body obtain cofactors?

Answer 37

• inorganic cofactors via the diet as minerals, including iron, calcium, chloride, and zinc ions.

Question 38

How does the body obtain coenzymes?

Answer 38

• from vitamins, a class of organic molecules found in the diet.

Question 39

How do coenzymes and cofactors work?

Answer 39

• transfer atoms or groups from one reaction to another in a multi-step pathway
• or form part of the active site of an enzyme
• only difference is coenzymes are organic molecules

Question 40

What is an example of a cofactor?

Answer 40

• amylase, which catalyses the breakdown of starch contains a chloride ion that is necessary for the formation of a correctly shaped active site.

Question 41

What are examples of coenzymes?

Answer 41

• vitamin B3 is used to synthesise NAD
• NADP, which plays a similar role in photosynthesis is also derived from vitamin B3.
• vitamin B5, which is used to make coenzyme A, which is essential in the breakdown of fatty acids and carbohydrates in respiration.

Question 42

What are prosthetic groups?

Answer 42

• Sub set of cofactors, they are required by certain enzymes to carry out their catalytic function
• Tightly bound to the enzyme protein and form a permanent feature of it.

Question 43

What is an example of a prosthetic group?

Answer 43

• zinc ions form an important part of the structure of carbonic anhydrase, an enzyme necessary for the metabolism of carbon dioxide.

Question 44

What are inactive precursor enzymes?

Answer 44

• enzymes produced in inactive form particularly ones 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

Question 45

Why do precursor enzymes need to change shape (tertiary structure) and how does it happen?

Answer 45

• to be activated
• addition of a cofactor
• by action of another enzyme, such as protease, which cleaves certain bonds in the molecule.
• in some cases it’s a change of condition (zymogens or proenzymes)

Question 46

What is a precursor enzyme called before and after the cofactor is added?

Answer 46

• before: apoenzyme

- after: holoenzyme

Question 47

What is an example where conditions changing cause the precursor enzyme’s tertiary structure to change?

Answer 47

• when inactive pepsinogen is released into the stomach to digest proteins, the acid ph brings about the transformation into the active enzyme pepsin. This adaption protects the body tissues against the digestive action of pepsin.