Proteins, Enzymes and Digestion

Question 1

How would you test to see if a food contained lipids?

Answer 1

By doing the emulsion test.

Add water and shake. If an emulsion forms, a protein is present.

Question 2

Why do lipids form an emulsion in water?

Answer 2

Because fats are soluble in ethanol but insoluble in water. So when you shake in water, the globules disputes forming a white emulsion.

Question 3

What’s the letter that denotes the variable group of an amino acid?

Answer 3

R

Question 4

What is the bond between amino acids in polypeptides called?

Answer 4

Peptide bonds.

Question 5

How does temperature affect enzymes?

Answer 5

1. The rise in temperature makes enzyme’s molecule vibrate more
2. If the temperature goes above a certain level, the vibration breaks the bonds that hold the enzymes in shape.
3. The active site changes shape and the enzyme and substrate no longer fit together.
4. At this point, the enzyme is denatured, so no longer functions as a catalyst.

Question 6

What happens if the enzyme goes above or below the optimum pH?

Answer 6

The H+ and OH- ions found in acids and alkalis can damage the ionic and hydrogen bonds that hold the enzyme’s tertiary structure in place. This makes the active site change, so the enzyme is denatured.

Question 7

How does enzyme concentration affect the rate of reaction?

Answer 7

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

Question 8

What happens if the amount of substrate is limited?

Answer 8

There is no further effect. Because there’s enough substrate to enzyme molecules, so adding more enzymes wouldn’t do anything.

Question 9

What happens if there is a greater amount of substrate concentration?

Answer 9

Higher substrate concentration = faster reaction. Because more substrate concentration means a collision between substrate and enzyme is more likely and so more active sites will be used.

Question 10

For substrate concentration, what happens after the saturation point is met?

Answer 10

There are so many substrate molecules that the enzymes have as much as they can cope with - all the active sites are full - so adding more makes no difference.

Question 11

How could you prevent the rate of reaction decreasing over time?

Answer 11

Add more substrate concentration.

Question 12

What do competitive inhibitors look like?

Answer 12

The molecule have a similar shape to that of substrate molecules.

Question 13

What do competitive inhibitors do?

Answer 13

They compete with the substrate molecules to bind the active site, but no reaction takes place. Instead, they block the active site so that no substrate molecule can fit in it.

Question 14

How much the enzyme is inhibited by a competitive inhibitor depends on…

Answer 14

The relative concentration of the enzyme and substrate.

High concentration of inhibitor = nearly all active sites take up. Not much substrate will reach the enzyme.

High concentration of substrate = increased ROR (up to a point) because there is a higher chance of substrate getting to an active site before the inhibitor does.

Question 15

Methotrexate is a drug used in the treatment of cancer. It is a competitive inhibitor and affects the enzyme folate reductive.

Explain how the drug lowers the rate of reaction controlled by folate reductase.

Answer 15

• methotrexate drug competes with the substrate for an active site
• because they both have a complementary shape to the substrate
• this forms fewer enzyme substrate complexes

Question 16

Methotrexate only affects the rate of reaction controlled by folate reductase.

Explain why this drug doesn’t affect other enzymes.

Answer 16

Because other enzymes will have a different shape of substrate to methotrexate.

Question 17

Name the monomers that make up the active site of an enzyme.

Answer 17

Amino acids.

Question 18

Aspirin is an enzyme inhibitor. Explain how aspirin prevents substrate molecules being converted to product molecules.

Answer 18

Aspirin binds to the active site - which then means that the substrate can’t bind to the site as the aspirin is there, so an enzyme substrate complex is not formed.

Question 19

Scientists have investigated the effects of competitive and non-competitive inhibitors of the enzyme maltase.

Describe competitive and non-competitive inhibition of an enzyme.

Answer 19

• enzymes prevent the formation of an enzyme substrate complex
• competitor inhibitors have a similar shape to the substrate and binds to an active site of the enzyme.
• however, this can be overcome by adding more substrate
• non-competitive inhibitors bind to the allosteric site. This prevents the formation of an active site
• cannot be overcome by adding more substrate.

Question 20

How could you test to see if a food contains proteins?

Answer 20

Perform the biuret test.

Add biuret solution (sodium hydroxide and copper sulphate). If a protein is present, it would turn purple.