C1.1 Enzymes & Metabolism HL

Question 1

Enzyme-catalysed reactions can be _____ or _____

Answer 1

intracellular or extracellular

Question 2

give examples of intracellular reactions

Answer 2

glycolysis & Kreb’s cycle → in respiration

Question 3

give examples of extracelluar reactions

Answer 3

chemical digestion outside of cells in the lumen of the small intestine

Question 4

are metabolic reactions 100% efficient with enzymes, why?

Answer 4

no

heat will be generated and lost

e.g. while maintaining a constant body temperature

Question 5

define metabolic pathways. also state and explain

Answer 5

Metabolic pathways are chains or cycles of enzyme-catalysed reactions

• The product of one reactant is a reactant in the next

Initial substrate → intermediates → end-product

Question 6

give an example of a linear metabolic reaction

Answer 6

glycolysis in respiration

Question 7

give examples of enzyme-catalysed, cyclical metabolic pathways

Answer 7

the Kreb cycle in (cell) respiration

the Calvin cycle in photosynthesis

Question 7

what is competitive enzyme inhibition

Answer 7

inhibitor fits the active site and prevents the substrate from entering

Question 8

give an example of competitive enzyme inhibition and explain how it works

Answer 8

• Statin (stops someone from making too much cholesterol)
• used to treat high blood cholesterol (contributes to heart disease)
• binds to active site of enzyme HMG-CoA reductase
  
  • [this enzyme] catalyses one of the reactions in the metabolic pathway used to synthesise cholesterol in liver cells
  • rate-limiting step in the pathway
• if statins lower the rate → less cholesterol produced by the body

Question 9

what is allosteric site?

Answer 9

other site (somewhere other than the active site on the enzyme)

Question 9

what is non-competitive enzyme inhibition?

Answer 9

inhibitor fits into an allosteric site causing a conformational change in the active site → the substrate cannot attach to react

• no longer fits the substrate

Question 10

give an example of non-competitive enzyme inhibition

Answer 10

Isoleucine

• as concentration of isoleucine builds up → binds to allosteric site of the first enzyme in the chain (threonine deaminase)

Question 10

explain end product inhibition (using isoleucine as an example)

Answer 10

• after sufficient isoleucine is produced it blocks the enzyme from producing it → if too much end product is made
• if have enough end product → stop production = negative feedback
• end product bind to allosteric site
• how to restart? → use up end product → get rid of inhibition

Question 11

is inhibition reversible?

Answer 11

mostly reversible

Question 12

explain end product inhibition

Answer 12

inhibited enzyme has an allosteric site to which the end product binds

• binding changes shape of active site → prevent catalysis for as long as the end product remains bound
• the enzyme that is inhibited has an allosteric site to which the end product binds
• this binding changes the shape of the active site, preventing catalysis for as long as the end product remains bound
• if too much end-product is made, it will increasingly inhibit the first enzyme in the pathway
• effectively switches off the whole pathway and prevents synthesis of more end product
• if there is too little of the end product → minimal inhibition of the first enzyme
• the metabolic pathway will be open to produce more of the end product

Question 13

explain competitive inhibition in a general sense

Answer 13

• blocks the active site, preventing the substrate from entering
• the higher the concentration of inhibitor, the slower the rate of reaction
• even with competitive inhibition, the same maximum rate of reaction will be achieved if more substrate is added - bc we have not changed the number of enzymes available

Question 14

what does the graph of competitive enzyme inhibition look like?

Answer 14

same maximum rate of reaction

also same start point?

Question 15

explain non-competitive enzyme inhibition in a general sense

Answer 15

non-competitive inhibitors bind to an allosteric site on the enzyme

the active site is altered and the substrate cannot attach and react

Question 16

what does the graph of non-competitive enzyme inhibition look like?

Answer 16

not the same maximum rate of reaction

Question 17

as the concentration of non-competitive enzyme inhibition increase, what happens to the rate of reaction and why?

Answer 17

rate of reaction decrease

bc there are fewer function(al)? active sites available for reaction

the max rate of reaction is reduced - with fewer functional active sites, the enzyme has reduced ability to process the substrates, even if substrate concentration is increased

Question 18

explain mechanical-based inhibition

Answer 18

irreversible binding of an inhibitor (mechanical based inhibition) can cause chemical changes to the active site via covalent bond

Question 19

give an example of mechanical-based inhibition

Answer 19

penicillin

if cell wall of bacteria weakened (lysis occurs easily)

Question 20

explain the example of mechanical-based inhibition

Answer 20

• cells walls of bacteria prevent them from bursting when low external solute concentrations cause water to enter by osmosis and hydrostatic pressures inside the cell become very high
• enzyme: transpeptidase
• very important in the process of cell wall formation → cross-links strands of carbohydrate into one huge peptidoglycan molecule that forms the entire cell wall
• when bacteria grow: one enzyme breaks these links, allowing the wall to expand
• transpeptidase then remakes the links
• saprotrophic bacteria and fungi compete for food because they both secrete enzymes for extracellular digestions of carbon compounds in dead matter and then absorb the products of digestion
• The fungus Penicillium notatum produces penicillin
• binds to the active site of transpeptidase in the cell walls of bacteria and prevents the substrate of the enzyme from binding
• Penicillin forms a permanent covalent bond with a particular amino acid in the active site, binding irreversibly with the enzyme
• the enzyme that breaks cross-links in the bacterial cell wall continues working but the transpeptidase enzyme cannot work to reform these links
• the cell wall is weakened and the bacteria are killed by bursting (lysis)
• penicillium can then monopolize the food source
• the fungus only does this when food supplies are limited, because resources are needed for the synthesis and secretion of penicillium

Question 21

is mechanical-based inhibition competitive or non-competitive?

Answer 21

competitive

bc it binds to the active site