Proteins and enzyme controlled reactions

Question 1

describe the structure of an amino acid

Answer 1

alpha carbon bonded to hydrogen, the amine group (NH2), the carboxyl group (COOH), and an R group

Question 2

name the elements within an amino acid

Answer 2

carbon, hydrogen, oxygen, nitrogen, sulphur (only sometimes if found in R group)

Question 3

name the type of bond formed when amino acids bond together in a condensation reaction

Answer 3

peptide bond

Question 4

what is produced when 2 amino acids join in a condensation reaction?

Answer 4

dipeptide + water

Question 5

describe the primary structure of protein

Answer 5

a sequence of amino acids joined by peptide bonds in a condensation reaction

Question 6

describe the secondary structure of protein

Answer 6

• protein folds into a beta-pleated sheet or coils into an alpha helix
• hydrogen bonds form between carboxyl and amine group of adjacent amino acids

Question 7

describe the tertiary structure of protein

Answer 7

• protein folds into a 3D shape
• hydrogen bonds, ionic bonds and disulphide bridges form between R-groups which gives rise to a 3D shape

Question 8

describe the quaternary structure of protein

Answer 8

more than one polypeptide chain held together in a precise structure by hydrogen bonds and disulphide bridges (not all proteins have a quaternary structure)

Question 9

describe the biochemical test for proteins

Answer 9

-add an equal volume of the sample and Biuret’s solution
-a purple/lilac colour indicates presence of protein
-a negative result is no colour change (stays blue)

Question 10

What is the activation energy and what do enzymes do to it?

Answer 10

The minimum amount of energy needed to start a reaction

Enzymes will lower the activation energy

Question 11

How is the lock and key model different to the induced fit model?

Answer 11

The lock and key model suggests that enzymes and substrates fit together like a lock and key, and that the substrate is complementary to the enzyme so can immediately be broken down.

The induced fit model suggests that the substrate and enzyme are initially different shapes and so the enzyme must change shape in order to bind to the substrate and break it down. As the enzyme changes shape it breaks the bonds within the substrate which breaks it down

Question 12

What is denaturation?

Answer 12

When there is an extreme pH or temperature change which can break down the hydrogen bonds in the tertiary structure of the enzyme. This causes the active site to change shape meaning the substrate and the enzyme are no longer complementary so cannot form an enzyme substrate complex.

Question 13

How does temperature effect enzyme activity?

Answer 13

As temperature increases, rate of reaction also increases because the enzymes and substrates gain more kinetic energy, meaning there are more successful collisions so more enzyme substrate complexes will form.

At 37 degrees, enzyme activity is at its optimum

After 37 degrees the rate of reaction begins to decrease, because the high temperature begins to break down the hydrogen bonds in the tertiary structure of the enzyme which causes the active site to change shape meaning that the substrate can no longer bind to it. This means that enzyme substrate complexes cannot form.

Question 14

Describe an investigation to observe the effect of hydrogen peroxide concentration on the activity of catalase

Answer 14

• Measure 20cm^3 pureed potato in a conical flask and push in a bung
• Fill a measuring cylinder with water and insert over a trough of water and insert the rubber tube into the measuring cylinder
• Add 2cm^3 5 vol hydrogen peroxide into the conical flask and time for 30 seconds
• After 30 seconds record the volume of oxygen in the measuring cylinder
• Repeat with different concentrations of hydrogen peroxide
• Calculate rate of oxygen production using: amount of oxygen formed divided by time taken

Question 15

How does pH effect enzyme activity?

Answer 15

The optimum pH for enzymes is 7 and either side of the optimum the rate of reaction will decrease.

Extreme pHs will disrupt the ionic/hydrogen/disulphide bonds in the tertiary structure at the active site meaning that enzyme substrate complexes can no longer form. A decrease in pH increases the concentration of H+ ions which breaks hydrogen and ionic bonds, which causes the tertiary structure to change shape

Question 16

What is the saturation point of an enzyme?

Answer 16

The point at which all of the active sites are occupied - limiting reactant will eventually be enzyme concentration

Question 17

How does enzyme concentration effect enzyme activity?

Answer 17

If the substrate is in excess, the rate of reaction will steadily increase because more active sites are available. If substrate amount is limited, an increase in enzyme concentration will eventually have no effect and so rate will level off

Question 18

What is a competitive inhibitor and what is its effect?

Answer 18

• A molecule which is also complementary to an enzymes active site and a similar shape to the substrate molecule
• prevents substrate from colliding with the active site of the enzyme - less successful collisions so rate of reaction decreases as less enzyme substrate complexes are formed
• effect can be reduced by increasing substrate concentration
• at a certain substrate concentration, rate levels off because active sites are all occupied

Question 19

What is the effect of an irreversible competitive inhibitor?

Answer 19

• bind permanently to active site, so increasing substrate concentration will have no effect

Question 20

What is a non-competitive inhibitor and what is its effect?

Answer 20

• Binds to an alternative site on an enzyme molecule which causes the tertiary structure of the enzyme to change
• shape of active site changes, so no longer complementary to substrate so no enzyme substrate complexes will form
• rate of reaction is reduced and the effect cannot be overcome by increasing substrate concentration