Proteins

Question 1

How can you isolate insoluble and soluble dry state proteins?

Answer 1

insoluble: dry or semi-dry separation techniques
soluble: aqueous extraction

Question 2

How can whey proteins and caseins be separated from each other?

Answer 2

enzymatically or by acidification of the milk

Question 3

What are the differences between whey proteins and caseins?

Answer 3

table slide 7

Question 4

What protein in egg yolk has good emulsifying properties? Why?

Answer 4

phosvitin has phosphorylated serine residues, these negatively charged residues confer the protein good emulsifying properties

Question 5

What mechanism makes ovalbumin more heat-stable?

Answer 5

thiol-disulfide exchange

Question 6

How are the different proteins in wheat separated during Osborne fractionation?

Answer 6

scheme slide 9

Question 7

What are 7S and 11S proteins?

Answer 7

vicilins (7S) and legumins (11S), the 2 main proteins in legumins

Question 8

How can you separate the legumin and the vicilin fractions?

Answer 8

By changing the pH or the ionic strength

Question 9

How are SPC and SPI produced from soybeans?

Answer 9

scheme slide 11

Question 10

What are the main classes of protein in potato?

Answer 10

patatin, protease inhibitors

Question 11

What compounds can interact with proteins, potentially affecting their properties?

Answer 11

enzymes + non-proteinaceous compounds

Question 12

What is important about enzymes?

Answer 12

ALL enzymes are proteins

Question 13

Why does repeated exposure to protease inhibitors in plants result in health problems?

Answer 13

protease inhibitors in plants inhibit digestive enzymes in GI tract: proteins cannot be digested and are taken up by the body

Question 14

Give 2 examples on protease inhibitors and their negative impact of the body.

Answer 14

1. lectins: cause problems in gut by interacting with epithelial cells
2. tannins: complexate with proteins and decrease protein digestability

Question 15

Which methods are used to separate proteins based on:
- charge;
- size;
- iso-electric point;
- Mw ?

Answer 15

charge: Ion exchange chromatography
size: SEC
iso-electric point: iso-electric focusing
Mw: SDS-page

Question 16

When do decarboxylation and deamination occur?

Answer 16

during decay and microbial spoilage of protein rich foods

Question 17

What molecules are produced after decarboxylation and deamidation?

Answer 17

volatiles with specific odor

Question 18

What reaction causes pseudo-allergic reaction?

Answer 18

decarboxylation of histidine into histamine

Question 19

When does Strecker degradation occur?

Answer 19

free amino acid + dicarbonyl compounds = aldehyde

Question 20

Where can the amino group involved in Maillard come from?

Answer 20

The amino group can come from the α-amino group of free amino acids, the ε–amino group of lysine or the free amino group of the N-terminal amino acid in a peptide

Question 21

What is formed during Maillard?

Answer 21

• melanoidins
• volatile compounds, - - loss of essential amino acids (lysine, cysteine and methionine), - mutagenic/carcinogenic compounds
• compounds that cause cross-linking of proteins (and consequently cause changes in the functionality of proteins)

Question 22

How can reactions at the functional group of the side chain lead to decreased biological value?

Answer 22

• Degradation of essential amino acids
• Conversion of essential amino acids into derivatives that cannot be metabolised
• Decrease in the digestibility of protein as a result of intra- or interchain cross linking (decreased accessibility for the proteases)

Question 23

What are disulfide bonds?

Answer 23

disulfide bridges within one peptide chain can stabilize the structure of a peptide

Question 24

In what reactions are disulfide bonds involved?

Answer 24

1. Oxidation of sulfhydryl groups or reduction of disulfide bonds
2. Reshuffling of disulfide bonds

Question 25

When does oxidation of methionine occur?

Answer 25

During heating or storage in the presence of peroxides

Question 26

What reaction is responsible for off-flavors in milk when it is exposed to too much light?

Answer 26

oxidation of methionine

Question 27

What have isopeptides been used for?

Answer 27

determine/quantify extent of heat damage, especially LAL

Question 28

What forces stabilize the primary, secondary, tertiary and quaternary structure of proteins?

Answer 28

primary: Electrostatic and hydrophobic forces secondary: hydrogen bonds tertiary + quaternary: Electrostatic and hydrophobic forces + disulfide bonds

Question 29

How are disulfide bonds formed?

Answer 29

Formed when two suflhydryl groups (fromo 2 cysteines) react together

Question 30

How can disulfide bonds be broken?

Answer 30

Nucleophiles, some chemical reactions (β-elimination) and reducing agents (DTT) can break disulfide bonds

Question 31

What is a clathrate structure?

Answer 31

Clathrate structure = water in low energy state around a hydrophobic molecule

Question 32

What are the native, denatured and molten state of a protein?

Answer 32

The native state of proteins refers to the “natural” state of the protein as it is produced by an organism The denatured state of a protein refers to a change in the structure of the protein that can affect its functionality A ”molten globule” state occurs when a protein looses its fixed tertiary structure while still maintaining its secondary structure elements intact.

Question 33

What is measured with intrinsic fluorescence?

Answer 33

Protein unfolding

Question 34

Why are aromatic amino acids used during intrinsic fluorescence?

Answer 34

because their aromatic rings absorb light in the UV range

Question 35

In which form is beta-lactoglobulin present in? What about soy glycinin?

Answer 35

1. beta-lactoglobulin: usually present as a dimer but can also occur as an octamer 2. soy glycinin: usually present as a hexamer (at room temperature, pH 7 and low ionic strength)

Question 36

What factors can alter the structure of proteins?

Answer 36

pH Temperature Ionic strength Denaturement Pressure

Question 37

How does pH affects protein structure?

Answer 37

If a network is formed between oppositely charged groups, they can stabilize the protein structure: this occurs around the iso-electric point Far from the iso-electric point, there will mainly be only + or – charges that will repel each other: the protein will then swell slightly to a more open structure

Question 38

Will the denaturation temperature of proteins be slightly lowered around or far away from the pI?

Answer 38

Far from the iso-electric point, there will mainly be only + or – charges that will repel each other: the protein will then swell slightly to a more open structure Repulsion between parts of the proteins can lead to a lowered denaturation temperature since the protein already is “unfolding”

Question 39

Why are there 2 denaturation temperatures for proteins?

Answer 39

When the value of the enthalpic and entropic contributions are calculated, there are 2 points where lines cross This shows there are 2 denaturation temperatures for proteins: one at lower and one at higher temperatures

Question 40

How do organic solutes weaken the hydrophobic interactions?

Answer 40

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Question 41

How do surfactants contribute to unfolding of protein?

Answer 41

Surfactants molecules have a hydrophilic and hydrophobic part The hydrophobic parts of the surfactant will bind to the hydrophobic parts of the protein The hydrophilic parts of the surfactant will bind to water, thereby “dissolving” the hydrophobic parts

Question 42

When aggregation occurs between proteins, what are the 2 main scenarios?

Answer 42

Either proteins cluster and sediment OR they form gels

Question 43

Which conditions are needed to form transparent gels? and turbid/opaque gels?

Answer 43

transparent gels are formed from linear aggregates (conditions = far from pI, low ionic strength) opposite conditions forms random aggregates and turbid/opaque gels

Question 44

On what do the structure and properties of a gel depend?

Answer 44

The structure and properties of a gel depend on the type of aggregates that form the network and the type of interaction between them Disulfide bridges strengthen aggregation by stabilising the proteins in the aggregate: a free sulfhydryl group attacks a disulfide bridge in another protein molecule

Question 45

What is Circular Dichroism used for?

Answer 45

CD monitors changes in the secondary structure as the protein unfolds

Question 46

Explain electrostatic interactions and the role of pH and salt.

Answer 46

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