BMS1064: Wk7 - Proteins

Question 1

What is the primary structure of proteins?

How are peptides different to proteins?

Answer 1

Primary structure: the sequence of AAs in a polypeptide chain

Long chains of AAs (with amine and carboxyl groups).
R group differs between AAs, giving polar, non-polar and +/- charge characteristics.

Peptides = 50 AAs or less
Proteins = More than 50 AAs

Question 2

Describe the secondary structure of proteins.

Answer 2

Polypeptide folds as a result of hydrogen bonding between local amino acids.

Question 3

Describe the tertiary structure of proteins.

What are the different types?

Answer 3

Further folding of secondary structure as result of bonding between peptides/polypeptide regions.

LINEAR:
- structural and mechanical elements
- Connective tissue (collagen in tendons, cartilage and bone)

GLOBULAR:
- involved in transport and dynamic functions.
- soluble in aqueous environment (hydrophobic AAs inside and hydrophilic outside).

Question 4

Describe the quaternary structure of proteins.

Answer 4

Two or more polypeptides bound together. (e.g. haemoglobin)

Question 5

What is the isoelectric point of a protein?

Answer 5

The pH at which their net charge is zero.

Question 6

What are the 3 classifications of proteins?

Answer 6

Simple proteins = AAs only
Conjugated proteins = AAs + prosthetic group
Derived proteins = modified by chemical or enzymatic treatment

Question 7

What are the structural properties of proteins in food?

Answer 7

1. Formation of gels (gelation)
2. Stabalise emulsions and foams
3. Form films

Question 8

What are the effects of denaturing proteins (for food)?

Answer 8

Loss of solubility and activity
Changes in functional properties
- enhanced gel formation
- enhancedn stability for emulsions
- elastic films

Question 9

What is gelation / gel formation?

Answer 9

The association or cross linking of the protein chain to form a 3D network that traps or immobilises the water within it to form a rigid structure.

Question 10

Looking at globular and fibrous protiens, is gelation reversible or irreversible?

Answer 10

GLOBULAR proteins unfold on heating and the denatured chains aggregate to form thermally IRREVERSIBLE gels

FIBROUS proteins are able to form thermally REVERSIBLE gels (e.g gelatine from collagen).

Question 11

What are emulsions?

Answer 11

A mixture of two liquids that normally don’t mix (e.g. oil and water).

Question 12

_______ from milk and eggs are commonly used to stabalise oil-in-water emulsions and foams.

How do they do this?

Answer 12

Protiens

INTERFACIAL PROPERTIES
- Able to adsorb at the oil-water and air-water interfaces, preventing aggregation
- Small enough to diffuse to the interface at fast rate.
- Large enough to provide enough points of contact and increase the overall energy of adsorption.

Question 13

What affects interfacial activity?

Answer 13

• Molecular size
• Conformation (structure)
• Amino acid composition
• pH and solvent composition

Question 14

What are the benefits of using proteins to form edible films on food?

What are examples of some proteins that can form films?

Answer 14

• Barrier to moisture, O2 and CO2 - enhance shelf-life and appearance
• Biodegradable packaging
• Improve mechanical handling properties (size, texture etc)

e.g. Gelatine, whey, soy, corn and wheat gluten

Question 15

Name 5 enzymes used in food processing.

(Catalytic properties of proteins in food)

Answer 15

Invertase
Isomerase
Lipoxygenase
Protease
Lactase

Question 16

What does the enzyme Isomerase do in food science?

Why is this beneficial?

Answer 16

Converts glucose to fructose.

Fructose from corn is sweeter than glucose.
Corn syrups = mixture of fructose and glucose.

Question 17

What does the enzyme invertase do?

Why is this beneficial in food science?

Answer 17

Converts [sucrose] –> [glucose and fructose].

Reduces sugars for confectionary as there are more stable and less likely to crystalise.

Question 18

What does the enzyme lipoxygenase do?

What are the desriable and undesirable results of it being used in food science?

Answer 18

Converts Unsaturated FAs -> FA hydroperoxides.

Improves dough and flavour of bread. (desirable)
Causes fat rancidty (undesirable)

Question 19

How are proteases used in food science?

Answer 19

Meat tenderization.
Cheese making.
Flavouring

Question 20

How is lactase used in food science?

Answer 20

To convert lactose -> galactose and glucose

Makes food more digestable for those who are lactose intolerant. (e.g. lactase added to milk)

Question 21

What are the protein chemical reactions in food? (4)

Answer 21

1. Carboxyl group reactions
2. Amino group reactions
3. Thiol group reactions
4. Other reactions between amino acids (e.g. discolouration in low acid, high protein products and oxidation during freezing)

Question 22

How can carboxyl group reactions be used in food science?

Answer 22

To decarboxylate histadine into histamine.

(Histadine -> food poisoning, allergic reactions)

Question 23

How can amino group reactions be used in food science?

Answer 23

Acetylation increases the solubulity of proteins -> emulsions

Formation of nitrosamines (nitrates added in meat curing - flavour, colour, protection from bacteria)

Maillard browining - reaction with sugars giving desirable colour and flavour of bread, meats and coffee beans when toasted/cooked/roasted.

Question 24

What are the cons of carboxyl and amino group reactions?

Answer 24

AAs are modified, becoming unavailable.

Production of toxic/carcinogenic compounds.

Question 25

How are Thiol group reactions useful in food science?

Answer 25

Formation of disulphide bonds - increases elasticity of dough and improves texture.