Proteins

Question 1

Draw the general structure of an amino acid

Answer 1

(Amin group)(R group)(Carboxylgroup)

Question 2

Describe how to test for proteins in a sample

Answer 2

Biurets test:
1. Add equal volume of sodium hydroxide to a sample at room temperature
2. Add drops of dilute copper (II) sufate solution. Swirl to mix
3. Positive result = colour changes from blue to purple

Question 3

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

Answer 3

There are 20 amino acids and they differ by their r group

Question 4

How do dipeptides and polypeptides form

Answer 4

• Condensation reaction forms peptide bond & eliminates a molecule of water
• Dipeptide = 2 amino acids
• Polypeptide = 3 or more amino acids

Question 5

How many levels of protein structure are there?

Answer 5

4

Question 6

Define ‘primary structure’ of a protein

Answer 6

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

Question 7

Define ‘secondary structure’ of a protein

Answer 7

Hydrogen bonds form between the slightly positive carboxyl oxygen and slightly negative amine hydrogen

Question 8

Describe the two types of secondary protein structure

Answer 8

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

Question 9

Define ‘tertiary structure’ of a protein

Answer 9

3D structure formed by further folding of polypeptide

Question 10

Name the bonds present in tertiary structures

Answer 10

• Disulfide bridges
• Ionic bonds
• Hydrogen bonds

Question 11

Describe each type of bond in the tertiary structure of proteins

Answer 11

• 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 cause these bonds to break)
• Hydrogen bonds = numerous & easily broken

Question 12

Define ‘quaternary structure’ of a protein

Answer 12

• Functional proteins may consist of more than one polypeptide
• Precise 3D structure held together by the same types of bond as tertiary structure
• May involve addition of prosthetic groups e.g. metal ions or phosphate groups

Question 13

Describe the structure and function of globular proteins

Answer 13

• Spherical & compact
• Hydrophilic R groups face outwards & hydrophobic R groups face inwards so it is usually water soluble
• Involved in metabolic processes e.g. enzymes & haemoglobin

Question 14

Describe the structure and function of fibrous proteins

Answer 14

• Can form long chains or fibres
• Insoluble in water
• Useful for structure and support e.g. collagen in skin

Question 15

Describe the structure and function of fibrous proteins

Answer 15

• Can form long chains or fibres
• Insoluble in water
• Useful for structure and support e.g. collagen in skin