Proteins

Question 1

Primary structure

Answer 1

Order of amino acids held together by covalent peptide bonds

Question 2

Secondary structure

Answer 2

folding of chain due to hydrogen bonds between c=o and N-H

• a-helix
• b-pleated sheet

Question 3

Tertiary structure

Answer 3

Further folding
-hydrogen bonds
-electrostatic bonds
-hydrophobic interactions
-disulphide bridges
-van der waals forces
cause attraction between different amino acids side groups

Question 4

hydrogen bonds

Answer 4

between non-ionised carboxyl group (COOH)
-amide groups (NH2)
-OH groups
can form between side chains or water

Question 5

electrostatic interactions

Answer 5

negative charge mutually attracts a positive charge

depends on pH

Question 6

Hydrophobic bond

Answer 6

non-polar substances aggregate in areas with less water

Question 7

disulphide bridges

Answer 7

covalent bond between 2 cysteine residues due to oxidation of SH groups

Question 8

Van der waals

Answer 8

weak
between non-polar molecules with complementary shapes
can induce opposite dipole in neighbouring molecules
attraction

Question 9

Bond strength

Answer 9

must be higher than kinetic energy of molecules in protein or structure will be unstable

Question 10

Bound water

Answer 10

water molecules have strong electrical polarity, causes water to bond to each other or other charged molecules.
less mobile than rest of water
Bound water refers to amount of water bound to macromolecules. Most proteins absorb some water
tightly bound - removed by freeze drying
loosely bound - centrifugation
unbound - filtration

Question 11

water binding capacity

Answer 11

tightly bound water

g water bound per g of protein when a dry protein powder is equilibrated with water vapour at 90-95% relative humidity

Question 12

water holding capacity

Answer 12

loosely bound water

g of water bound per g of protein after food sample is centrifuged

Question 13

Protein solubility

Answer 13

depends on protein-protein, protein-solvent, solvent-solvent interactions
ionic interactions promote protein-water interactions
hydrophobic interactions promote protein-protein interactions

Question 14

Isoelectric pH

Answer 14

protein usually net positive or negative
at isoelectric pH protein has no net charge
at this pH protein is least soluble due to reduced interactions with water

Question 15

Denaturation

Answer 15

heat, extreme pH, organic solvents
heat destabilise hydrogen bonds, electrostatic interactions and van der waals
hydrophobic interactions stabilised at high temps
denaturation decreases solubility

Question 16

Gel

Answer 16

Protein network that immobilise large amount go water
Intermediate phase between solid and liquid
-protein-protein infraction’s hold network together
-high viscosity
-high elasticity

Question 17

stages of gel formation

Answer 17

1. denaturation of native protein

2. gradual association to form gel matrix

Question 18

types of gel

Answer 18

1. aggregated - opaque due to large size of protein aggregates, high non-polar amino acids, irreversible
2. Clear - high water holding capacity, smaller fibrous particles, reversible if heated

Question 19

syneresis

Answer 19

• shrinkage of gel and expulsion of trapped liquid
• with time and temp
• as contraction takes place pores in matrix decrease in size and trapped liquid is squeezed out

Question 20

Emulsions

Answer 20

denaturing protein can increase emulsifying properties
due to amphipathic nature
hydrophilic parts of protein associate with polar phase (water) and hydrophobic parts associate with non-polar phases (oil/air)