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AQA A Level Biology > 3.1.4 Proteins > Flashcards

3.1.4 Proteins Flashcards

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1
Q

How would you test for proteins in a sample?

A

Biuret test: confirms the presence of peptide bonds
- Add an equal volume of sodium hydroxide to a sample at room temp
- Add drops of dilute copper sulphate solution and swirl to mix
- Positive result: change from blue to purple
- Negative result: remains blue

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2
Q

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

A

20 and by their side ‘R’ group

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3
Q

How does a Peptide bond form?

A

Condensation reaction between 2 amino acids

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4
Q

How are Dipeptides formed?

A

Condensation reaction between 2 amino acids

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5
Q

What are Amino acids?

A

Monomers from which proteins are made

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6
Q

How are Polypeptides formed?

A

Condensation reaction between many amino acids

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7
Q

How many levels of a Protein structure are there?

A

4

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8
Q

Define Primary structure of a protein

A

Sequence , number and type of amino acids in the polypeptide is determined by the sequence of codons on the mRNA

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9
Q

Define Secondary structure of a protein

A
  • It’s the shape that the chain of amino acids chains and is either alpha helix or beta pleated sheets
  • Hydrogen in the - NH has a slight positive charge
  • 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
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10
Q

Describe the 2 types of Secondary protein structure

A

Alpha helix
- All N-H bonds are on the same side of protein chain
- Spiral shape
- Hydrogen bonds are parallel to the helical axis
Beta pleated sheets
- N-H and C=O groups alternate from one side to the other

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11
Q

Define Tertiary structure of a protein and name bonds present

A

3D structure formed by the further folding of the polypeptide
- Disulfide bridges
- Ionic bonds
- Hydrogen bonds

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12
Q

Describe Disulfide bridges

A

Strong covalent s-s bonds between molecules of the amino acid cystiene

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13
Q

Describe Ionic bonds

A

Relatively strong bonds between charged R groups (PH changes cause bonds to break)

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14
Q

Describe Hydrogen bonds

A

Numerous and easily broken

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15
Q

Define Quaternary structure of a protein

A
  • Functional protein may consist of more than 1 polypeptide
  • Precise 3D structure is held together by the same types of bonds as the tertiary structure
  • May involve addition of prosthetic group e.g. metal ions or phosphate groups
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16
Q

Describe the structure and function of Globular proteins

A
  • Spherical and compact
  • Hydrophilic R groups face outwards and hydrophobic R groups face inward so usually water soluble
  • Involved in metabolic processes e.g. enzymes and haemoglobin
17
Q

Describe the structure and function of Fibrous proteins

A
  • Can form long chains or fibres
  • Insoluble in water
  • Useful for structure/support (collagen in skin)
18
Q

What are Enzymes?

A

Biological catalysts for intra and extracellular reactions
- Specific tertiary structure determines shape of active site which is complementary to a specific substrate
- Formation of enzyme-substrate (es) complexes lowers activation energy of metabolic reactions

19
Q

Explain the Induced fit model of enzyme action

A
  • Shape of active site is not directly complementary to substrate and is flexible
  • Conformational change enables the es complex to form
  • This puts strains on substrate bonds , lowering activation energy
20
Q

How have models of Enzyme action changed?

A
  • Initially lock and key model: rigid shape of active site is complementary to only 1 substrate
  • Currently induced fit model: also explains why binding at allosteric sites can change shape of active site
21
Q

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

A

Difference between free energy of substrate and peak of curve

22
Q

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

A
  • PH
  • Temp
  • Concentration of inhibitors
  • Substrate concentration
  • Enzyme concentration
23
Q

How does Enzyme concentration affect rate of reaction?

A
  • Given that substrate is in excess rate increases proportionally to enzyme concentration
  • Rate levels off when max number of es complexes form at any given time
24
Q

How does Substrate concentration affect rate of reaction?

A
  • Given that enzyme concentration is fixed , rate increases proportionally to substrate concentration
  • Rate levels off when max number of es complexes form at any given time
25
Q

How does Temperature affect rate of reaction?

A
  • Rate increases as kinetic energy increases and peaks at optimum temperature
  • Above optimum ionic and H-bonds in the 3D structure break and so the active site is no longer complementary to substrate (denaturation)
26
Q

How does PH affect rate of reaction?

A
  • Enzymes have a narrow optimum PH range
  • Outside the range H+/OH- ions interact with the H-bonds and ionic bonds in the 3D structure leading to denaturation
27
Q

Contrast competitive and non-competitive Inhibitors

A

Competitive:
- Similar shape to substrate so binds to active site
- Doesn’t stop reaction; es complex forms when inhibitor is released
- Increasing substrate concentration decreases their effect
Non-competitive:
- Binds at the allosteric binding site
- May permanently stop reaction and so triggers active site to change shape
- Increasing substrate concentration has no impact on their effect

28
Q

Why is it advantageous to calculate initial rate?

A

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

29
Q

State the formula for PH

A

PH = -log10 (H+)

30
Q

Outline how to calculate rate of reaction from raw data

A

Change in concentration of product or reactant / Time

31
Q

Outline how to calculate rate of reaction from a graph

A
  • Calculate gradient of line or gradient of tangent to a point
  • Initial rate = draw tangent at 0

AQA A Level Biology (23 decks)

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