3) Enzymes

Question 1

what type of protein are enzymes

Answer 1

globular (round - the hydrophobic parts of the protein fold inwards while the hydrophilic parts become arranged around the external surface. Globular proteins are water soluble.)
tertiary structure

Question 2

what do enzymes do ?

Answer 2

catalyse reactions inside cells (intracellular enzymes) or are secreted to catalyse reactions outside cells (extracellular enzymes)

Question 3

what is the lock and key hypothesis ?

Answer 3

idea that enzyme has particular shape which the substrate fits exactly
- enzyme is specific to the substrate

Question 4

enzymes involved in _______ are intracellular

Answer 4

(act within the cell it is produced) respiration, photosynthesis - metabolic enzymes

Question 5

enzymes involved in _______ are extracellular

Answer 5

(act outside of the cell it is produced) digestive enzymes

Question 6

what is the induced fit model ?

Answer 6

same as lock and key but adds the idea that the substrate can sometimes change shape slightly as the substrate molecule enters the enzyme, in order to ensure perfect fit

Question 7

how does the induced fit model work ?

Answer 7

• as substrate approaches enzyme, it induces a conformational change (3D shape).
• it changes shape BC the R groups of amino acids in active site interact w the substrate.
• it changes shape to fit the substrate.
• this stresses the substrate, reducing the activation energy

Question 8

what is activation energy ?

Answer 8

minimum amount of energy needed to raise molecules to a transition state

Question 9

explain how enzymes lower the activation energy needed to allow reactions to proceed.

Answer 9

• provide an alternative pathway.
• brings reactants close together.
• put a strain on reactants.
• so bonds break/form more easily.

Question 10

what are anabolic and catabolic enzymes ?

Answer 10

• Anabolism is the building of complex molecules from numerous simple ones (protein synthesis).
• Catabolism is the breakdown of complex molecules into numerous simple ones (break down of glucose).

Question 11

what are the 2 ways you can measure activity of enzyme ?

Answer 11

• rate of formation of product
• rate of disappearance of substrate

Question 12

what does a colorimeter do and how to interpret it ?

Answer 12

• measures light absorbance/transmission.
• darker the colour, larger the amount of light absorbance (eg: water = 0).
• one with lowest absorbance is optimum temp.

Question 13

how do you test for starch and what are the results?

Answer 13

• iodine solution.
• positive : blue/black.
• negative : orange/brown

Question 14

how to investigate temp effect on enzyme action ?

Answer 14

• choose range of 5 temps.
• increased temp = increased kinetic energy.
• use thermostatically controlled waterbath for temp.

Question 15

how does increasing temp/ optimum temp affect enzyme action ?

Answer 15

• increasing temp :
  kinetic energy increases,
  more successful collisions,
  more escs.
• optimum temp :
  bonds that stabilise tertiary structure are broken,
  enzyme loses its shape,
  active site altered, substrate can no longer bind to enzyme,
  enzyme denature.

Question 16

how to investigate effect of pH on enzyme action ?

Answer 16

• atleast 5 pHs
• use of buffer
• test w iodine/look for colour change/ time taken for colour change

Question 17

how does pH affect enzyme action ?

Answer 17

• affects ionic & hydrogen bonds responsible for specific tertiary shape.
• each enzyme has diff optimum pH range.
• extreme pH denature enzyme

Question 18

how does enzyme concentration affect enzyme action ?

Answer 18

• greater concentration = more frequent collisions between enzyme and substrate.
• faster rate of reaction.
• rate of reaction directly proportional.
• at v high enzyme concentrations, substrate may become limiting factor, so rate does not continue to increase if enzyme concentration is increased.

Question 19

what are competitive inhibitors ?

Answer 19

-molecules that sufficiently resemble the substrate in shape & occupy (block) active site.

-increasing concentration of substrate eventually overcomes inhibition as substrate molecules displace inhibitor.

Question 20

what are non-competitive inhibitors?

Answer 20

-chemically unlike the substrate molecule (reacts with bulk of enzyme, reducing access to active site)

-increasing substrate concentration cannot prevent binding of inhibitor- some inhibition remains at high substrate concentration.

Question 21

compare Vmax and Km of competitive and non-competitive inhibitors

Answer 21

• competitive : same Vmax, larger Km.
• non-competitive : smaller Vmax, same Km

Question 22

what does high & low Km mean in terms of affinity ?

Answer 22

• high Km = low enzyme affinity
• low Km = high enzyme affinity

Question 23

how to calculate Km ?

Answer 23

• pull a line at Vmax (highest point, eg: 10).
• calculate 1/2 Vmax (eg: 10/2)
• pull horizontal line to where the graph line is, then vertical line.
• read off Km value

Question 24

having a high affinity / low Km means ?

Answer 24

it needs a lower concentration of substrate to reach Vmax.

Question 25

Km is a measure of… ?

Answer 25

how tightly the substrate is bound to the enzyme

Question 26

what is enzyme immobilisation ?

Answer 26

attachment of enzymes to insoluble materials

Question 27

advantages of immobilisation

Answer 27

• no purification steps needed
• can be reused
• stable at temp and pH at which it is held and used (less likely to be denatured)

Question 28

disadvantages of immobilisation

Answer 28

• immobilisation mechanism that does not alter the shape or catalytic ability must be selected
• added expense
• if enzyme becomes detached, will appear in product as contaminant

Question 29

3 facts about immobilised enzymes

Answer 29

• attached to insoluble materials
• affects enzyme action
• has protection from hydrogen ions

Question 30

outline the differences between induced fit model and lock & key model

Answer 30

induced fit
1. shape of substrates not fully complementary to shape
of active site
2. active site flexible / moulds around substrates
3. provides better fit / fully complementary

lock & key
1. shape of substrates complementary to shape of active
site
2. active site does not change shape
3. substrate fits into active site