1.4 PROTEINS

Question 1

Describe

amino acid general structure

Answer 1

• central C
• -COOH carboxyl group
• -NH₂ amine group
• -R variable side group
• -H hydrogen

Question 2

Describe

test of proteins

Answer 2

Biuret test
1. add equal volume of sodium hydroxide at room temp
2. add dropos of dilute copper (II) sulfate solution
3. +ive result: colour changes from blue to purple

Question 3

State and explain

number of amino acids and how they differ

Answer 3

20

differ only by side ‘R’ group

Question 4

Describe

dipeptide and polypeptide formation

Answer 4

• condensation reaction forms peptide bond (-CONH-) & eliminates molecule of water
• dipeptide: 2 amino acids
• polypeptide: 3 or more

Question 5

State

number of levels in protein structure

Answer 5

4

Question 6

Define

primary structure of proteins

Answer 6

• sequence, number & type of amino acids in the polypeptide
• determined by sequence of codons on mRNA

Question 7

Define

secondary structure of proteins

Answer 7

Hydrogen bonds form between O δ- attached to -C=O & H δ+ attached to -NH

Question 8

Describe

2 types of secondary protein structures

Answer 8

α-helix:
* all N-H bonds on same side of protein chain
* spiral shape
* H-bonds parallel to helical axis

β-pleated sheet:
* N-H & C=O groups alternate from one side to the other

Question 9

Define and name

tertiary structure of proteins

Answer 9

3D structure formed by firther folding of polypeptide
* disulfide bridges
* ionic bonds
* hydrogen bonds

Question 10

Describe

type of bonds in tertiary structures of proteins

Answer 10

• disulfide bridges: strong covalent S-S bonds
• ionic bonds: relatively strong bonds between charged R groups (changed by pH)
• hydrogen bonds: numerous & easily broken

Question 11

Define

quaternary structure of proteins

Answer 11

• functional proteins with more than one polypeptide
• precise 3D structure held together by the same types of bond as tertiary structure
• may involve addition of prosthetic groups

Question 12

Describe

globular protein structure and functions

Answer 12

• spherical & compact
• hydrophilic R groups face outwards & hydrophobic R groups face inwards = usually water-soluble
• involved in metabolic processes e.g. enzymes

Question 13

Describe

fibrous proteins structure and function

Answer 13

• can form long chains or fibres
• insoluble in water
• useful for structure and support e.g. collagen

Question 14

Outline

use of chromatography in identifying amino acids

Answer 14

1. use capillary tube to spot mixture onto pencil origin line & place chromatography paper in solven
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 llight to see spots
4. calculate Rf values & match to database

Question 15

Define

enzyme

Answer 15

• biological catalysts for intra & extrecellular reactions
• specific tertiary structure determines shape of active site, complementary to a specific subestrate
• formations of ES complexes lowers activation energy of metabolic reactions

Question 16

Explain

induced fit model of enzyme action

Answer 16

• shape of active site is not directly complementary to substrate & is flexible
• conformational change enables ES complexes to form
• strains substrate bonds, lowering the activation energy

Question 17

Describe

change in models of enzyme action

Answer 17

• lock & key model: rigid shape of active site complementary to only 1 substrate
• induced fit model: explains binding at allosteric sites changing active site shape

Question 18

Describe

process to identify activation energy from an energy level diagram

Answer 18

difference between free energy of substrate & peak of curve

Question 19

Name

5 factors that affect rate oof enzyme-controlled reactions

Answer 19

• enzyme conc.
• substrate conc.
• inhibitor conc.
• pH
• temp

Question 20

Explain

effect of substrate conc on enzyme controlled reactions

Answer 20

given that enzyme is in excess, rate increases proportionally to substrate conc.

rate levels when max no. ES complexes form

Question 21

Explain

effect of enzyme conc on enzyme controlled reactions

Answer 21

given that substrate is in excess, rate increases proportionally to enzyme conc.

rate levels when max no. ES complexes form

Question 22

Explain

effect of temp on enzyme controlled reactions

Answer 22

rate increases as kinetic energy increases & peaks at optimum temp

above optimum, ionic & H-bonds in 3⁰ structure break = changes in active site shape so no longer complementary (denaturation)

Question 23

Explain

effect of pH on enzyme controlled reactions

Answer 23

enzymes have a narrow optimum pH range

outside range, H⁺/OH⁻ ions interact with H-bonds & ionic bonds in 3⁰ structure = denaturation

Question 24

Contrast

competitive and non-competitive inhibitors

Answer 24

competitive:
* similar shape to substrate = bind to active site
* don’t stop reactions; ES complex forms when inhibitor releases
* increasing substrate conc, decreases effect

non-competitve:
* bind at allosteric binding site
* may permanently stop reactions; triggers active site to change shape
* increasing substrate conc, has no impact

Question 25

Outline

process to calculate rate of reaction from a graph

Answer 25

• calculate gradient of line or gradient of tangent to a point
• initial rate: draw tangent at t=0

Question 26

Explain

the advantages to calculate initial rate

Answer 26

represents maximum rate of reaction before concentration of reactants decreases & ‘end product inhibition’

Question 27

State

formula pH

Answer 27

pH=-log₁₀[H⁺]