Enzymes - Biological Molecules

Question 1

What are enzymes?

Answer 1

Globular proteins
So have a specific shape that is determined by the number and sequence of amino acids in the polypeptide chain

Question 2

What do enzymes do?

Answer 2

Act as biological catalysts

Question 3

What is a catalyst?

Answer 3

Catalysts increase the rate of a chemical reaction without undergoing permanent changes themselves

Question 4

What is an enzyme’s functional region?

Answer 4

• large molecules with a small functional region
• known as active site
• made up of a small number of amino acids which form a small depression

Question 5

Why are enzymes important?

Answer 5

• without enzymes reactions would be as slow as to not proceed at all
• control reactions so that metabolic reactions can continue
• not used up in reaction and are free to be used again

Question 6

What is activation energy?

Answer 6

• activation energy is the minimum amount of energy needed to initiate the reaction

Question 7

How do enzymes lower activation energy?

Answer 7

• in reaction, the enzymes provides a template for reactants to come together in the correct orientation
• active site holds the substrates, stretching and bending critical chemical bonds that must be broken

Question 8

How does the enzyme-substrate complex form?

Answer 8

The substrate is held inside the active site by temporary bonds that form between the amino acids of the active site and the substrate molecule

Question 9

Why is the active site specific?

Answer 9

• has specific shape due to tertiary structure
• any change in the shape of the protein affects the shape of the active site and so the function of the enzyme

Question 10

What is the lock and key model?

Answer 10

• active site is similar to a lock and the substrate like a key
• complimentary shapes
• explains many properties such as specificity, excessive heat stopping enzymes working and the fact that molecules similar to the substrate can inhibit enzyme action

Question 11

What aspects of enzyme catalysis cannot be explained by the lock and key model?

Answer 11

The fact that enzyme activity can be altered by a molecule binding to it at a site other than the active site

Question 12

What is the induced fit model?

Answer 12

• prior to binding together, the substrate and the active site are not precisely complementary in shape
• only when the substrate binds does the active site change and molds closely around the substrate

Question 13

Why is induced fit a better model than lock and key?

Answer 13

• it reflects the flexibility of enzymes and allows for the active site of an enzyme changing its conformation to facilitate binding to the substrate

Question 14

What happens when one amino acid changes?

Answer 14

• any change in primary structure will ultimately change the 3D structure of an enzyme and so alter its function
• primary structure is different so bonds, I.e. hydrogen, disulphide, form in different places and change the tertiary structure

Question 15

How does substrate concentration affect enzyme activity?

Answer 15

• as conc increases, rate of reaction will increase proportionally until all enzymes are saturated, then it will level off
• increasing substrate increases number of collisions, assuming their are enough enzymes present
• graph : directly proportional and then levels off (see notes)

Question 16

How does enzyme concentration affect enzyme activity?

Answer 16

• increasing conc of enzyme present will also increase the rate of reaction, but this is limited by the amount of substrate present (enzyme is reused, substrate is not)
• if there is plenty of substrate available, initial rate of reaction is directly proportional to the concentration of the enzyme
• twice the number of enzyme molecules would provide twice the number of active sites for the substrate to collide with
• graph : directly proportional (see notes)

Question 17

How does temperature affect enzyme activity?

Answer 17

• increase in temperature increases the kinetic energy of enzyme and substrate molecules - they move faster
• increases the likelihood that enzyme and substrate will collide with each other
• rise of 10° doubles the rate of reaction as long as temperature is below the optimum
• abode the optimum an increase in temperature and consequent increases in kinetic energy cause hydrogen bonds to break and the tertiary structure is lost, active site is changes shape and enzyme is eventually denatured
• graph : see notes

Question 18

How does pH affect enzyme activity?

Answer 18

• changing the pH alters the concentration of H+ or OH- ions in the surrounding solution
• a small change alters the charges on amino acids that make up the active site, so the substrate is is unable to bind
• a large pH change breaks the bonds that hold the tertiary structure together, affects the formation of hydrogen and ionic bonds between -NH2 and -COOH groups and so affects the shape of the active site which leads to the enzyme being denatured
• graph : see notes

Question 19

How do enzyme inhibitors affect rate of enzyme activity?

Answer 19

• chemicals that slow down the rate of enzyme controlled reactions
• they attach to the enzyme
• if it attaches with a covalent bond, then it will be a permanent inhibition (can be reversed in some cases, if a weak bond)

Question 20

How does competitive inhibition work?

Answer 20

• compete for active site on the enzyme
• have a SIMILAR shape to substrate and block the active site so the substrate can’t bind to the enzyme
• if more substrate added, competitive inhibitor’s effect is lessened
• graph : see notes

Question 21

How does non-competitive inhibition work?

Answer 21

• binds somewhere other than the active site and alters the shape of the enzyme
• adding more substrate will not affect the rate of reaction

Question 22

Required practical

Answer 22

• the effect of temperature on the rate of reaction catalysed by trypsin
• write an X halfway down the sides of 3 test tubes
• add milk solution
• add trypsin and pH buffer to 3 other test tubes
• add to water bath for 10mins
• add trypsin and buffer to milk solution and mix
• add back into the water bath
• measure time taken for X to appear
• record time
• find time taken at 30, 40, 50, 60

Question 23

How to calculate rate of reaction on a graph?

Answer 23

Rate = change in y axis / change in x axis
Or
Dependant / independent
Or rate = 1/T (time taken)

Question 24

What methods alter the number of successful collisions?

Answer 24

• change in temperature
• enzyme concentration
• substrate concentration
• competitive inhibitors

Question 25

What methods change the shape of the active site?

Answer 25

• high temperatures - above optimum
• non-competitive inhibitors
• pH