Proteins And Enzymes

Question 1

Structure of an amino acid

Answer 1

H2N-CHR-COOH
NH2 = amine group
R = variable side chain. Can be:
- positive
- negative
- hydrophobic
- hydrophilic
COOH = carboxylic acid group

Question 2

Describe how a dipeptide is formed

Answer 2

Condensation reaction
Produces peptide bond
OH from carboxylic group combines with H from amine group in second amino acid
Water produced

Question 3

Describe the structure of proteins

Answer 3

Polymer of amino acids
Joined by peptide bonds
Formed by condensation reaction
Primary structure is number and order of amino acids
- Secondary structure is folding of polypeptide chain into alpha helix and beta pleated sheets due to Hydrogen bonding
- tertiary structure is 3D shape due to hydrogen bonds and ionic bonds and disulfide bridges
- quaternary structure is 2 or more polypeptide chains joined together

Question 4

Describe how an enzyme substrate complex increases the rate of reaction

Answer 4

• reduces activation energy
• due to bending bonds

Question 5

When a pathogen causes an infection, plasma cells secrete antibodies that destroy this pathogen .
Explain why these antibodies are only effective against a specific pathogen

Answer 5

• Antigens have a specific tertiary structure
• Antibody is complementary to antigen
• antibody-antigen complex forms

Question 6

Define activation energy

Answer 6

The minimum amount of energy required for a successful chemical reaction

Question 7

How do enzymes increase the rate of the reaction

Answer 7

• lowers activation energy
• stressing/distorting bonds in substrate
• during formation of enzyme-substrate complex

Question 8

Describe how a change in the base sewuence of a DNA coding for an enzyme may result in a non functional protein

Answer 8

• change in primary structure changes sequence of amino acids
• hydrogen/ionic bonds and disulphide bonds form in different positions
• alters the tertiary structure of the enzyme’s active site
• no enzyme-substrate complexes can be formed

Question 9

What is a proteome of a cell?

Answer 9

Full range of proteins a cell is able to produce

Question 10

Describe and explain how you can use the bieuret test to distinguish a solution of lactase enzyme from a solution if lactose

Answer 10

• add bieuret solution to both solutions.
• Lactase enzyme = purple
• because lactase is a protein

Question 11

Sucrase doesnt hydrolyse lactose. Use your knowledge of the way in which enzymes work to explain why

Answer 11

• lactose has a different shape
• doesnt bind to the sucrase active site
• substrate and active site are not complementary
• No enzyme substrate complexes form

Question 12

Describe the induced fit of enzyme action

Answer 12

• active site not complementary
• active site changes shape as substrate binds
• distorts/stresses bonds within substrate
  -lowers activation energy
• enzyme-substrate complex forms

Question 13

Describe one way the lock and key model differs from the induced fit model

Answer 13

LAK: Active site doesnt change shape whereas IF: active site DOES change shape
- active site rigid vs flexible
- substrate is complementary to active site

Question 14

An enzyme catalyses only one reaction. Explain why.

Answer 14

Enzymes active site has a specific tertisry structure
Only one substrate binds

Question 15

Suggest why a protein can ve the substrate for two different enzymes

Answer 15

• different parts of protein have different amino acif sequences so have different shapes
• each enzyme active site is a specific shape and complementary to a different part of the protein

Question 16

Diabetes mellitus is a disease that can lesd to an increase in blood glucose concentration. Some diabetics need insulin injections. Insulin is a protein so it cannot be taken orally. Suggest why insulin cab not ve taken orally.

Answer 16

• broken down by enzymes
• insulin no longer functional

Question 17

What is the effect of substrate concentration on the rate of an enzyme controlled reaction

Answer 17

• increases then plateaus
• plateaus because no active sites are available/all AS are filled
• max number of enzyme-substrate complexes form per second

Question 18

Explain how a competitive inhibitor works

Answer 18

• inhibitor similar shape to substrate
• inhibitor enters active site
• less substrate binds/ fewer ESC per second formed

Question 19

Describe how a non-competitive inhibitor works

Answer 19

Attaches to enzyme at allosteric site (site other than AS)
- changes shape of active site
- so active site no longer complementary to substrate
- less/no substrate binds and fewer ESC form

Question 20

Explain the results without inhibitor (curve A) shown in figure 6

Answer 20

• increases because more ESC form
• levels off because no free active sites

Question 21

Figure 6 shows that max initial rate of reaction when a competitive inhibitor was present ( curve B) is different from when a non-competitive inhibitor was present (curve C)

Explain this difference.

Answer 21

• competitive inhibitor binds to active sites vs non competitive inhibitor binds to allosteric site
• competitive inhibitor binding doesnt change shape of AS vs non competitice inhinitors do
• with competitive inhibitor, high sub concentration = enzyme still available but non competigive at high sub conc = enzymes no longer available
• at high sub conc, ES collisions likelihood increases with comp inhibitor but not possible with noncomp inhibitor

Question 22

describe how amino acids join to form a polypeptide so there is always a NH2 at one end and COOH at other.

Answer 22

• . One NH2 group joins to a COOH group to form a peptide
  bond;
• (So in chain) there is a free NH2 group
  at one end and a free COOH group
  at the other