proteins

Question 1

Describe the general structure of an amino acid

Answer 1

• Contains a COOH group
• Contains an R variable group
• Contains an NH2 amino group

Question 2

Describe a method to test for proteins in a sample

Answer 2

1. Add equal volumes of dilute sodium hydroxide (NaOH) to a sample at room temperature
2. Add drops of dilute copper (II) sulfate solution (CuSO4), swirl to mix
3. A negative result = solution remains blue, a positive result is when the colour changes from blue to purple

Question 3

How many amino acids are there and how do these differ from each other

Answer 3

20

differ by side ‘R’ group

Question 4

Explain how do dipeptides and polypeptides form?

Answer 4

• Condensation reaction forms a peptide bond between (-CONH-) and eliminates a molecule of water
• Dipeptides are formed from 2 amino acids
• Polypeptides are formed from 3 or more amino acids

Question 5

How many levels of protein structure are there

Answer 5

4

Question 6

Describe the primary structure of a protein

Answer 6

• Sequence, number and type of amino acids in the polypeptide
• Determined by a sequence of codons on mRNA

Question 7

Describe the secondary structure of a protein

Answer 7

• Hydrogen bonds form between negatively charged nitrogen, oxygen and positively charged hydrogen
• Causes polypeptide chain to fold into an alpha helix or a beta pleated sheet

Question 8

Describe the tertiary structure of a protein

Answer 8

• 3D structure formed by further folding of polypeptide due to: disulfide bridges, ionic bonds or hydrogen bonds

Question 9

Describe disulphide bridges in the tertiary structure of proteins

Answer 9

Strong covalent S-S bonds between molecules of amino acid cysteine

Question 10

Describe ionic bonds in the tertiary structure of proteins

Answer 10

Strong bonds between charged R groups

Question 11

Describe hydrogen bonds in the tertiary structure of proteins

Answer 11

Numerous and easily broken

Question 12

Describe the quaternary structure of a protein

Answer 12

• Forms when there is more than one polypeptide chain
• Held together by disulphide bridges, hydrogen bonds or ionic bonds to form an alpha helix and a beta-pleated sheet

Question 13

Describe the structure and function of globular protein

Answer 13

• Spherical and compact
• Hydrophilic R groups face outwards and hydrophobic R groups face inwards so are water soluble
• Involved in metabolic processes

Question 14

Describe the structure and function of fibrous protein

Answer 14

• Can form long chains or fibres
• Insoluble in water
• Useful for structure and support

Question 15

Define enzymes

Answer 15

Biological catalysts for intra and extracellular reactions

Question 16

What determines the shape of an enzyme active site

Answer 16

The tertirary structure

Question 17

How is the activation energy of metabolic reactions lowered

Answer 17

The formation fo enzyme-substrate complexes lowers the activation energy of metabolic reactions

Question 18

Explain the induced fit model of enzyme action

Answer 18

• The enzyme active site isn’t completely complementary to the substrate
• Active site shape changes as substrate binds and enzymes-substrate complex forms in an ideal binding agreement
• This stresses/distorts the bonds in the substrate which maximises the ability of the enzyme to catalyse the reaction

Question 19

Explain the lock and key model of enzyme action

Answer 19

• The active site is a rigid, fixed shape
• The active site is completely complementary to 1 substrate
• After a collision, enzyme-substrate complex forms leading to a reaction.

Question 20

Name 5 factors that affect the rate of enzyme-controlled reactions

Answer 20

1. Enzyme concentration
2. Substrate concentration
3. Concentration of inhibitors
4. pH
5. Temperature

Question 21

How does substrate concentration affect the rate of an enzyme-controlled reaction?

Answer 21

• As the number of substrate molecules increases, the likelihood of enzyme-substrate complex formation increases
• If the enzyme concentration remains fixed but the amount of substrate is increased, all active sites will become saturated and the substrate concentration will not increase the reaction rate
• When the active sites of enzymes are all saturated, any substrate molecules that are added will have nowhere to bind in order to form an enzyme-substrate complex

Question 22

How does enzyme concentration affect the rate of an enzyme-controlled reaction?

Answer 22

• The higher the enzyme concentration in a reaction mixture, the greater the number of active sites available and the greater the likelihood of enzyme-substrate complex formation

Question 23

How does a low temperature affect the rate of enzyme-controlled reactions

Answer 23

• Molecules move relatively slow and lower the frequency of successful collisions between substrates
• There are less frequent enzyme-substrate complex formations.
• Substrate and enzymes collide with less energy, making it less likely for bonds to be formed or broken which stops the reaction from occurring.

Question 24

How does a high temperature affect the rate of enzyme-controlled reaction

Answer 24

• Molecules move more quickly and there is a higher frequency of successful collisions between substrate molecules and the active site of enzymes. There are more frequent enzyme-substrate complex formations.
• Substrate and enzyme collide with more energy, making it more likely for bonds to be formed or broken which allows the reaction to occur.

Question 25

How does pH affect the rate of enzyme-controlled reaction

Answer 25

• Enzymes are denatured at extremes of pH.
• The hydrogen and ionic bonds that hold the tertiary structure of the protein together can break
• Altering the shape of the active site so enzyme-substrate complexes form less easily

Question 26

Compare competitive and non-competitive inhibitors

Answer 26

• Competitive inhibitors have a similar shape to the substrate so bind to the active site, non-competitive inhibitors bind at the allosteric binding site
• Competitive inhibitors do not stop reactions, enzyme-substrate complexes form when an inhibitor is released, and non-competitive stop reactions because they trigger to change in shape
• Competitive inhibitors = increasing substrate concentration decreases their effect, non-competitive inhibitors = increasing substrate concentration has no impact on their effect

Question 27

How can you reduce the effect of the competitive inhibitor?

Answer 27

Increasing substrate concentration reduces the effect of the inhibitor as the level of inhibition is dependent on relative concentrations.

Question 28

Why are enzymes specific?

Answer 28

• The shape of the active site (and its specificity) is determined by the ternary structure of the protein that makes up the enzymes