1.4

Question 1

What are amino acids?

Answer 1

The monomers from which proteins are made

Question 2

Draw the general structure of an amino acid

Question 3

Describe the test for proteins in a sample

Answer 3

Biuret test confirms presence of peptide bond
1. Add equal volume of sodium hydroxide to sample at room temperature.
2. Add drops of dilute copper (II) sulfate solution. Swirl to mix. (steps 1 & 2 make Biuret reagent).
3. Positive result: colour changes from blue to purple Negative result: solution remains blue.

Question 4

How many amino acids are there and how do they differ from one another?

Answer 4

20
They differ only by their side R group

Question 5

What bond does a condensation reaction between two amino acids form

Answer 5

A peptide bond

Question 6

How do dipeptides form?

Answer 6

Dipeptides are formed by the condensation of two amino acids

Question 7

How do polypeptides form?

Answer 7

Polly peptides are formed by the condensation of many amino acids

Question 8

A functional protein may contain…

Answer 8

One or more polypeptides

Question 9

How many levels of protein structure are there?

Answer 9

Four

Question 10

Define primary structure of a protein

Answer 10

-Sequence, number and type of amino acids in the polypeptide

Question 11

Define secondary structure of a protein

Answer 11

-The sequence of amino acids causes part of a protein molecule to bend into a-helix shapes or fold into B-pleated sheets
-The hydrogen in the -NH has a slightly positive charge while the oxygen in the -C=O has a slight negative charge
- As a result weak hydrogen bonds can form leading to alpha helices or beta pleated sheets.

Question 12

Describe the two types of secondary protein structure?

Answer 12

a-helix:
– all N-H bonds on the same side of protein chain
– Spiral shape
– H-bonds parallel to helical axis

B-pleated sheet:
– N-H and C=O groups, alternate from one side to the other

Question 13

Define tertiary structure of a protein. Name the bonds present

Answer 13

3-D Structure formed by further folding of polypeptide.
– Disulfide bridges.
– Ionic bonds.
– hydrogen bonds.

Question 14

Describe each type of bond in the tertiary structure of proteins

Answer 14

Disulfide bridges:
– strong covalent S-S bonds between molecules of the amino acid cysteine

Ionic bonds:
– relatively strong bonds between charged R groups (pH changes causes these bonds to break)

Hydrogen bonds:
– numerous and easily broken

Question 15

Define quaternary structure of a protein

Answer 15

– Functional proteins may consist of more than one polypeptide.
-Precise 3-D structure held together by the same types of bond a tertiary structure.
-May involve addition of prosthetic groups, E.G metal ions of phosphate groups

Question 16

Describe the structure and function of globular proteins

Answer 16

– Spherical and compact
– Hydrophilic, R groups face outwards and hydrophobic R groups face inwards = usually water soluble
-involved in metabolic processes, E.G enzymes and haemoglobin

Question 17

Describe the structure and function of fibrous proteins

Answer 17

– Can form long chains or fibres
-Insoluble in water
-useful for structure and support E.G.collagen in skin

Question 18

Outline how chromatography could be used to identify the amino acids in a mixture

Answer 18

1. Use capillary tube to spot mixture onto pencil origin line & place chromatography paper in solvent.
2. Allow solvent to run until it almost touches other end of paper. Amino acids move different distances based on relative attraction to paper & solubility in solvent.
3. Use revealing agent or UV light to see spots.
4. Calculate Rf values & match to database.

Question 19

What are enzymes?

Answer 19

• Biological catalysts for intra & extracellular reactions.
• Specific tertiary structure determines shape of active site, complementary to a specific substrate.
• Formation of enzyme-substrate (ES) complexes lowers activation energy of metabolic reactions.

Question 20

Explain the induced fit model of enzyme action

Answer 20

• Shape of active site is not directly complementary to substrate & is flexible.
• Conformational change enables ES complexes to
form.
• This puts strain on substrate bonds, lowering activation energy.

Question 21

How have models of enzyme action changed

Answer 21

– Initially lock and key model: rigid shape of active site, complimentary to only one substrate
– Currently induced fit model: also explains why binding at allosteric sites can change shape of active site

Question 22

How could a student identify the activation energy of a metabolic reaction from an energy level diagram?

Answer 22

Difference between free energy of substrate and peak of curve

Question 23

Name five factors that affect the rate of enzyme controlled reactions

Answer 23

– Enzyme concentration.
– substrate concentration.
– concentration of inhibitors.
– P H.
– temperature

Question 24

How does substrate concentration affect rate of reaction?

Answer 24

Given that enzyme concentration is fixed, rate increases proportionally to substrate concentration.

Rate levels of when maximum number of ES complexes form at any given time

Question 25

How does enzyme concentration of that rate of reaction?

Answer 25

– Given that substrate is in excess rate increases proportionally to enzyme concentration.
– rate levels off when maximum number of ES complexes form at any given time.

Question 26

How does temperature affect rate of reaction?

Answer 26

– Rate increases as kinetic energy increases and peaks at optimum temperature.
– above optimum, ionic and H-bonds in 3° structure break = active site, no longer complimentary to substrate (denaturation)

Question 27

How does pH affect the rate of reaction?

Answer 27

– Enzymes have a narrow optimum pH range.
– outside range, H+/OH- ions interact with H-bonds and ionic bonds in 3° structure = denaturation

Question 28

Contrast, competitive and non-competitive inhibitors

Answer 28

Competitive inhibitors:
– similar shape to substrate = bind to active site
– Do not stop reaction: E S. Complex forms when inhibitor is released.
– Increasing substrate concentration decrease is their effect

Noncompetitive inhibitors:
– bind at allosteric binding site
-May permanently stop reaction: triggers active site to change shape
-Increasing substrate concentration has no impact on their effect

Question 29

Outline how to calculate rate of reaction from a graph

Answer 29

– Calculate gradient of line or gradient of tangent to a point.

-Initial rate: draw tangent at t=0

Question 30

Outline how to calculate rate of reaction from raw data

Answer 30

Change in concentration of product or reactant / time

Question 31

Why is it advantageous to calculate initial rate?

Answer 31

Represents maximum rate of reaction before concentration of reactants decreases and ‘end product inhibition’

Question 32

State the formula for pH

Answer 32

pH = -log10[H+]

Question 33

What do the properties of an enzyme relate to?

Answer 33

The properties of an enzyme relate to the tertiary structure of its active site, and its ability to combine with a complimentary substrate(s) to form an enzyme-substrate complex

Question 34

What 2 chemicals make biurets reagent

Answer 34

Sodium hydroxide and dilute copper(iii) sulfate solution