3. Proteins

Question 1

What is the structure of an amino acid?

Answer 1

Question 2

Why are proteins complex compared to polysaccharides?

Answer 2

• Side groups on amino acids vary from one amino acid to the next
• A protein is made up of about 20 different amino acids, each with a different side group
• Polysaccharide is usually made up of the same repeating monosaccharide units or combo of a few different units

Question 3

Which amino acids have carbon rich side chains making them HYDROPHOBIC?

Answer 3

Question 4

Which amino acids are hydrophilic or polar and therefore interact well with water?

Answer 4

Question 5

What pH’s do the charged amino acids interact at?

Answer 5

Question 6

What is the primary structure of proteins?

Answer 6

The pimary structure of a protein is the linear sequence of amino acids as encoded by DNA

Question 7

What are the two types of secondary structure that proteins have?

Answer 7

Alpha helices or beta sheets

Question 8

How is the alpha helix stabilised?

Answer 8

Question 9

How are beta sheets stabilised?

Answer 9

Beta sheets are formed when hydrogen bonds stabilize two or more adjacent strands

Question 10

What is the tertiary structure of the protein?

Answer 10

Question 11

Why are the charged amino acids important for structure in proteins such as ubiquitin?

Answer 11

Question 12

How do membrane bound proteins make use of charged amino acids to inform their shape?

Answer 12

Question 13

What are three factors the native structure of a protein depends on?

Answer 13

1. Repulsive and attractive forces
2. Interactions among the amino acid side groups of proteins
3. The environment the protein is in.

Question 14

Which levels of protein structure are disrupted when the protein is denatured?

Answer 14

• Protein denaturation refers to changes in the secondary and tertiary structure of the protein, while the primary structure remains unchanged.

Question 15

What are some of the forces that stabilise protein structure?

Answer 15

• Hydrophobic interactions
• Electrostatic interactions
  
  • Van der Waals interactions
  • Hydrogen bonds
  • Ionic interactions
• COvalent bonds or chemical cross links
  
  • Disulfide bonds

Question 16

What happens to a protein when it is denatured?

Answer 16

• Undergo physical and chemical changes
• Lose its biological activity
• Lose solubility
• Increase tendency to aggregate

Question 17

What happens to an enzyme where it is denatured?

Answer 17

• Loss of enzyme activity
• Changes in reaction rate
• Changes in affinity for its substrate
• Changes to its optimum pH or temperature to funciton

Question 18

In what ways is protein denaturation beneficial for food processing?

Answer 18

Question 19

What are denaturants?

Answer 19

Factors that contribute to the changes in the environment surrounding the protein which result in disruption of interactions that stabilise the secondary and tertiary structure.

Question 20

What are three physical denaturants?

Answer 20

• Temperature induced
• Sheer induced
• High pressure induced

Question 21

What are the three chemical denaturants?

Answer 21

• pH induced
• Organic solvent induced
• Salts and other additives

Question 22

How does temperature act as a denaturant?

Answer 22

• Both high and low temp can cause proteins to denature, and the denaturation temperature of a protein is unique to the protein
• Thermal stability of proteins is determined by the amino acid composition
• Hydrophobic amino acid residues (Val, Ile, Leu, Phe) provide for higher stability than proteins with more hydrophilic amino acids
• Some proteins also denature at cold temperatures, usually reverisble

Question 23

How can mechanical sheer acts as a denaturant?

Answer 23

Denaturant of food proteins can results from exposure to mechanical shear forces generate by processes such as shaking, whipping, mixing, sonicaiton, vortexing, flow through conduits, centrifugation, and texturisation.

Question 24

What are the sources of food proteins?

Answer 24

Question 25

What proteins are sourced from plants?

Answer 25

Gliadin, horedein, karifin, secalin, and zein are PROLINE and GLUTAMINE rich storage proteins found in wheat, barley, sorghum, rye and corn respectively

Question 26

How can plant proteins contribute to allergies?

Answer 26

* These proteins are known to contribute to intolerance and allergenicity in certain individuals * the choice of the processing of food can amplify the allergenicity of some of these proteins * Eg. although dry roasting can impart flavour to peanuts, it can augument immunoreactivity and amplify the formation or unveiling of new allergenic epitopes in peanut proteins

Question 27

What broad categories of factors are there that influence funcitonal properties of proteins?

Answer 27

* Intrinsic properties of protein * Extrinsic factors coming from the food system

Question 28

What are the intrinsic properties of food proteins that affect funcitonal properties?

Answer 28

Question 29

What are the external properties that influence the functional properties of proteins?

Answer 29

Question 30

Answer 30

Question 31

Answer 31

Question 32

Answer 32

Question 33

Answer 33

Question 34

How does the characteristic of soy protein influence its use in bread?

Answer 34

Soybean proteins have neither gliadin nor glutenin, the unique proteins of wheat gluten. As a result, soy flour cannot be incorporated into bread without the use of special additives that improve loaf volume.

Question 35

How do we get from soybeans to soy products?

Answer 35

Question 36

What four groups of proteins does soybean contain?

Answer 36

* Storage proteins * Enzymes and enzyme inhibitors ( \>1% of the total protein) involved in metabolism * Structural proteins including both ribosomal and chromosomal * Membrane proteins

Question 37

What is soy protein isolate?

Answer 37

* When you acidify defatted soybeal meal to pH 4.5-4.8 you prepare SPI and whey * SPI includes the mahor soybean isolation proteins * Classified into four categories 2S, 7S, 11S and 15S which sediment at different gravitational forced when solution is subjected to a centrifugation.

Question 38

What is the basic 7S globin structure from soybean?

Answer 38

Question 39

What is the basic 11S globulin structure from soybean?

Answer 39

Question 40

How does a protein gel form?

Answer 40

* When partially unfolded proteins develop uncolied polypeptide segments that interact at specific points to form a 3D cross linked network * Partial unfolding with slight changes in secondary structure is required for gelation * heating, treatment with acids, alkali can contribute to partial unfolding of the native structure

Question 41

What interactions result in the formation of a protein gel?

Answer 41

* 3D protein network forms because of protein-protein and protein-solvent (water interactions) * These interaction and gelation are accelerated at high protein concentrations because of more intense intermolecular contacts * Higher amound of cross linking provides fluidity, elasticity and flow behaviour of gels

Question 42

Answer 42

Question 43

How is tofu formed?

Answer 43

* Tofu is a bean curd made by coagulating soymilk * Washed soybeans are ground with water. Then soymilk is obtained by filtration before or after heating * Ca induced and heat induced gelling mechanisms are used to form tofu. Salt coagulants are added while heating the soymilk to achieve this

Question 44

What is happening in this picture?

Answer 44

* 7S and 11S proteins exist in the solution, and soy milk is made up of proteins and oil globule in the aqueous solution * Then salts are added, may decrease pH * The oil globules interact with the proteins and pH change partially denatures the protein exposing hydrophobic groups that interact with oil globules * Coagulation forms the network

Question 45

How can a good emulsion differ?

Answer 45

Question 46

What is an emulsion made up of?

Answer 46

Question 47

How do proteins prevent coalescence of foams and emulsions?

Answer 47

* They have an amphipillic nature so they place themselves at the air-water and oil-water interface. * partial unfolding of the proteins enable it to interact with both oil and water

Question 48

How does the composition of soy protein affect its emulsification properties?

Answer 48

* more 7S has better emulsifying properties than fraction with 11S * Better emulsifying properties of 7S globulins give higher rate of diffusion to the interface * Disulfide bonding in the 11S globulin may inhibit unfolding and decrease interactions at the oil/water interface

Question 49

how does the stability of emulsions prepared with mature or immature beans compare?

Answer 49

* The stability of emulsions prepared with soy isolate from mature beans was higher than those from immature ones. This was due to the increased 7S fraction in the mature soybeans. * Preparing blends of soy proteins and caseins or whey protein diminished the undesirable off-flavor of soy protein.