Proteins & Enzymes

Question 1

What do all proteins contain?

Answer 1

contain the elements Nitrogen (N), Carbon (C), and Hydrogen (H) and oxygen (O). Some also contain sulphur (S)

Question 2

Structure of amino acids

Answer 2

R groups can be:

Positively charged +
Negatively charged -
Hydrophilic* attracted to water
Hydrophobic* repelled by water

Question 3

Describe how a peptide bond is formed between two amino acids to form a dipeptide

Answer 3

1. Condensation reaction
2. Between amine and carboxyl
3. Forms peptide bond

Question 4

Number of peptide bonds

Answer 4

The number of peptide bonds in the chain will be one less than the total number of amino acids originally joined together

Question 5

Protein Structure

Answer 5

• Polymer of amino acids;
• Joined by peptide bonds;
• Formed by condensation reactions

primary structure:
number and sequence of amino acids in a polypeptide chain

secondary structure:
way the polypeptide chain folds or coils into alpha helixes or/and beta pleated-sheets. The secondary structure is held together by WEAK hydrogen bonds between each amino acid

tertiary structure:
further folding of the polypeptide chain into a specific complex 3D shape.
3 different bonds:
1. WEAK HYDROGEN BONDS (between H and O)
2. IONIC BONDS, also weak bonds, which form between oppositely charged R groups.
3. DISULPHIDE BRIDGES, Strong bonds between R groups containing -SH groups

quaternary structure:
two or more polypeptide chains joined together

Haemoglobin has 4 polypeptide chains.
This is a GLOBULAR, functional protein.

Collagen has 3 polypeptide chains.
This is a FIBROUS, structural protein.

Question 6

Describe & explain how you could use the biuret test to distinguish a solution of enzyme, lactase, from a solution of lactose

Answer 6

Add Biuret reagent to both solutions

Lactase enzyme will give a purple colour
Because Lactase is a protein;

Lactose reducing sugar will remain blue

Question 7

How do enzymes increase the rate of reaction

Answer 7

lowering the activation energy needed for a chemical reaction.

They do this by distorting the bonds in the substrate during the formation of an ENZYME SUBSTRATE COMPLEX

Question 8

LOCK & KEY MODEL

Answer 8

1. The active site is rigid and does not change shape
2. The substrate binds to the enzymes active site
3. The substrate fits exactly into the active site – they are complementary
4. Products are formed and no longer fit into the active site, so is released
5. The enzyme is free to take part in another reaction

Question 9

Sucrase does not hydrolyse lactose.

Use your knowledge of the way in which enzymes work to explain why

Answer 9

1. Lactose has a different shape
2. Does not bind to active site of enzyme
3. The substrate and the enzyme active site are not complementary;
4. No ES complexes form

Question 10

Induced Fit Model

Answer 10

• The substrate enters the enzymes active site and binds to it Forming the ESC
• The binding of the substrate molecule induces the change in the shape of the active site.
• The ‘slight’ change in shape of the specific 3D tertiary structure of the active site, distorts the bonds within the substrate molecule which lowers the Ea of the reaction.
• When substrate leaves, the active site returns to its original shape

Question 11

Describe one way that the lock and key model is different from the induced fit model

Answer 11

1. Active site does not change shape
2. substrate is
3. complementary

Question 12

An enzyme catalyses only one reaction. Explain why.

Answer 12

1. Enzyme has active site is a specific shape;
2. Only one substrate binds

Question 13

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

Answer 13

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

Question 14

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

Answer 14

1. Change in primary structure changes
   sequence of amino acids;
2. Hydrogen bonds and Ionic bonds and Disulphide bonds form in different positions;
3. Alters the tertiary structure of the enzyme
4. No Enzyme-Substrate complexes can be formed

Question 15

Factors affecting Enzyme action: Temperature

Answer 15

If the temperature is increased, beyond the optimum, then the atoms have more kinetic energy. This causes the weak Hydrogen bonds and ionic bonds to break, this causes a change in the specific tertiary structure, which may change the shape of the active site.

no longer be complementary to the enzymes substrate, meaning less enzyme substrate complexes can be formed.

The enzyme is denatured and can no longer catalyse any chemical reactions.

Question 16

Factors affecting Enzyme action: pH

Answer 16

If the pH is changed from the optimum then the charge on the R groups of the amino acids are altered and bonds are broken.

The active site changes shape and not specifically complementary. Less enzyme substrate complexes can be formed and the rate of the reaction decreases either side of the optimum. The enzyme is denatured.

Question 17

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

Answer 17

1. Increases then plateaus
2. It plateaus as all active sites occupied
3. maximum number of Enzyme-Substrate complexes per second;

Question 18

Competitive inhibitor

Answer 18

• have similar structure to substrate
• bind to active site and temporarily prevent substrate from binding
• fewer ESC form
• reduce rate of reaction, fewer products formed
• takes longer for products to form

Question 19

Non-competitive inhibitor

Answer 19

• different shape to substrate, can not bind
• binds to allosteric site
• changes tertiary structure shape
• no ESC form
• rate of reaction slow - same effect as reducing number of enzymes

Question 20

Diabetes mellitus is a disease that can lead 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 cannot be taken orally.

Answer 20

1. Broken down by enzymes
2. Insulin no longer functional

Question 21

What is the proteome of a cell?

Answer 21

The full range of proteins a cell can produce