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Food Chemistry 1 > P2.1 Reactions + denaturation > Flashcards

P2.1 Reactions + denaturation Flashcards

1
Q

are proteins more sensitive to their environment than lipids and carbs? + explanation

A

yes! can undergo drastic changes in structure (denaturation) under relatively mild conditions –> unfolding of tertiary structure and unraveling of secondary structure

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2
Q

is denaturation good or bad for protein?

A

can be good (ie eggs) and bad (overheat and precipitation) depending on one’s objective

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3
Q

definition of denaturation?

A

change in structure or conformation of a protein that does not involve breaking of peptide bonds (primary structure remains intact)

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4
Q

secondary and tertiary structure involves a balance of a number of ____________ bonds/interactions (4 types?) that hold protein in a space
- most interactions are ______ but their high numbers provide _________
- denaturation results from the __________ of these interactions

A
  • non-covalent
  • hydrophobic interactions, electrostatic interactions, hydrogen bonds, van der waals forces
  • weak, provide stability
  • disruption
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5
Q

sensitivity of proteins to denaturation is a function of many variables: (5)
- all also govern what of protein?

A
  • amino acid composition
  • chain length
  • complexity of its structure
  • prosthetic groups
  • environmental conditions (ie pH)
    all govern functionality of protein
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6
Q

casein has little _________ structure –> consequence? vs egg albumin

A
  • little secondary structure
  • does not denature readily even when boiled, vs globular egg albumin is very sensitive to temperature
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7
Q

when the conformation of a protein is changed, many of its ________ properties are also changed

A

physical

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8
Q

5 common changes observed when denaturation takes place

A

1) unfolding of the tertiary structure/unraveling of the secondary structure –> makes the peptide bonds more accessible to proteolytic enzymes
2) denaturation of protein generally results in a reduction in its solubility (bc hydrophobic core is out) –> can precipitate and form a gel
3) enzymatic activity may be decreased or lost
4) viscosity generally increases, gelation may occur
5) crystallization is no longer possible (no 2°, 3° or 4° structure)

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9
Q

if denaturation is extensive, what happens?
(3)

A
  • globular form can be completely unraveled and its secondary structure badly disrupted
  • many reactive groups like COOH, NH2, SH, etc. are exposed –> capable of reacting with each other, so that polymerization or aggregation takes place
  • some proteins form gels when denatured, while others precipitate out of solution
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10
Q

difference between aggregation and polymerization?

A
  • aggregation: protein coprecipitate with each other –> hydrophobic and sulfite bond
  • polymerization: carboxyl and amino group attach from 2 different proteins –> need high temp and catalyst
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11
Q

most common factor contributing to denaturation? explanation

A

heat!
- increase in kinetic energy of molecules result in disruption of H-bonds, which are relatively weak

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12
Q

other important factors that contribute to denaturation (besides heat and more general) (4 + 1)

A
  • changes in pH
  • changes in ionic strength
  • chemical agents (ethanol, acetone, urea)
  • surface forces (ie. add air to proteins)
  • combinations of above
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13
Q

can the disruption of 1 alpha helix change 3° structure?

A

yes

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14
Q

most proteins tend to be stable over a relatively large/narrow pH range

A

narrow

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15
Q

shift in pH affects ________ ________ of molecule, which affects the __________ bond contributions to the tertiary structure

A
  • overall charge
  • electrostatic bond
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16
Q

proteins with significant quantities of (3 aa) are especially susceptible to changes in pH

A
  • aspartic acid
  • glutamic acid
  • lysine
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17
Q

many soluble proteins _________ when they are subjected to environmental conditions close to their _________ point
- why?

A
  • precipitate
  • isoelectric
  • proteins fall out of solution because they don’t interact with water anymore –> positive and negative charges are balanced
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18
Q

contributors to denaturation (5)

A

1) effect of pH
2) Hydrogen bond breakers
3) Detergents
4) Organic solvents
5) surface forces

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19
Q

compounds that can effectively compete with _______ linkage for H-bonds can readily disrupt both ______ and _______ structure
- ex.: ?

A
  • peptide linkage for H-bond
  • both 2° and 3° structure
  • ex.: urea
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20
Q

urea is commonly used to _______ proteins for analysis by _________, which is based on the fact that migration of proteins in an _______ _______ depends on their ____ and ___ ________ –> if not _________, their shape would be a confounding factor affecting their migration in the _______ ________

A
  • denature
  • by electrophoresis
  • electric field
  • depends on their size and net charge
  • not denatured
  • the electric field
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21
Q

what are good H-bond disruptors? how? mostly used for what?

A

alcohol and acetone
- by partial dehydration of a protein
- however, the degree of structure modification is not as severe as urea
- mostly used to precipitate enzymes from solution for isolation with minimal denaturation

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22
Q

detergents have both (2) moieties
- can also be called a ?
- ex? used in what?

A
  • hydrophobic and hydrophilic moieties
  • a surfactant
  • ex.: sodium dodecyl sulfate (SDS) –> used in electrophoresis
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23
Q

detergents are able to bridge the (2) regions of protein = ________ of the structure of the protein

A
  • hydrophobic and hydrophilic
  • opening
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24
Q

organic solvent can make the environment hydrophobic/hydrophilic –> what does that do to the protein? (2)
- ex of organic solvent?

A
  • hydrophobic
  • can turn the protein inside out
  • can also make enzymes work in totally opposite ways –> make lipase add FFAs to glycerol
  • hexane
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25
Q

many proteins themselves are able to reduce _______ ______ –> results in the formation of a ______

A
  • surface tension
  • foam
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26
Q

in foam, what is trapped within what?

A

air is trapped within a protein/water matrix

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27
Q
  • each air bubble has a thick/thin membrane (interface) that separates which 2 environment?
  • at this interface, what do the proteins do?
  • example
A
  • thin membrane –> separates hydrophilic water and hydrophobic air
  • proteins actively rearrange themselves structurally to reduce their free energy
  • ie: with hydrophobic groups facing air + hydrophilic groups facing water –> can cause denaturation
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28
Q

denaturation can also take place at the ________ of a solution

A

surface

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29
Q

each protein is unique and its behavior depends on (2)

A
  • its purity
  • the presence of other constituents
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30
Q

key point is that proteins are very _______, readily changing their __________ as a function of the __________ conditions

A
  • sensitive
  • conformation
  • environmental
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31
Q

although changes caused by denaturation are complex, they can be relatively ________
describe examples:
1) cooking egg
2) meringues
3) milk + acid
4) heating collagen

A
  • consistent
    1) boiling/frying an egg
    2) meringues - egg white/sugar beaten to produce a denatured foam
    3) milk+ acid produces a curd
    4) heating collagen produces a gel
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32
Q

foods that contain appreciable amounts of protein generally undergo _______ when they are heated under ___-_________ conditions
- this is called the _________ reaction or ?

A
  • browning
  • low-moisture
  • Maillard reaction or non-enzymatic browning
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33
Q

What is the Maillard reaction?

A

reaction of an aldehyde group of a reducing sugar and a free amino group of an amino acid/of a peptide/protein (ie Lysine)

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34
Q

is the Maillard reaction when avocado/bananas turn brown?

A

nope! that’s an enzyme that forms the brown/black

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35
Q

reducing sugar + amino compound (ie lysine) –> (name + structure)

A
  • N glycosylamine
  • reducing sugar –> top has an added NHR + double bonded O becomes OH
36
Q

after initial Maillard reaction (that forms _________), the sugar portion undergoes a complex series of reactions called _________ rearrangement

A
  • glycosylamine
  • Amadori
37
Q

what is the Amadori rearrangement? (2)

A
  • involves loss of water from within sugar portion of the molecule
  • then, sugar portion undergoes scission and breaks up into low-molecular weight compounds (producing flavor/aroma) that readily polymerize into brown pigments termed melanoidins
38
Q

the brown pigments produced from Amadori rearrangement are called?

A

melanoidins

39
Q

in terms of the protein, the net result of the Maillard reaction is that lysine will be ________ –> what does it mean?

A

bound
- amino group is not available because a sugar or a fragment of sugar may be attached

40
Q

Bound lysine causes _______ _________ that limits ________ ________ of the protein

A

steric hindrance
- limits enzymatic digestion

41
Q

what does limiting enzymatic digestion of a protein mean?

A

enzymatic action will stop 1 aa on either side of bound lysine, leaving an indigestible tripeptide, which is poorly absorbed and used by the body
AAAAAAALAAAAAAA
- enzyme can’t cut bond btw AL and LA –> leaving ALA tripeptide

42
Q

why can non-enzymatic browning adversely affect nutritional value of proteins (also called reduction of ?)

A
  • because of loss of lysine –> can’t be digested because stuck between a tripeptide
  • reduction of protein efficiency ratio (PER)
43
Q

what is protein efficiency ratio?

A

PER = weight gained/weight of protein consumed (measured in animal feeding trials)

44
Q

explain how toasted cereals has a decreased nutritional value. solution?

A
  • high heat + lots of sugar + lots of protein –> browning effect –> forms tripeptide that is indigestible = loss of lysine
  • advertisement of eating toasted cereal with milk because milk is a very good source of lysine
45
Q

which amino acids (except lysine) can also undergo reactions with reducing sugars at high temps? (Maillard reaction) (5)

A
  • arginine
  • tryptophan
  • histidine
  • glutamine
  • asparagine
    all have secondary amino group
46
Q

if reducing sugars are mainly absent, what can occur at high temp? by formation of what?

A
  • crosslinking of proteins
  • formation of amide bonds btw COOH and NH2 groups on side chains of aa residues
47
Q

cross-linking reactions –> peptide or amide bond that is formed? can it be hydrolyzed by digestive proteolytic enzymes?

A

amide! because it does not involve the alpha-carbon
- cannot be hydrolyzed because not a peptide bond

48
Q

consequence of crosslinking of proteins?

A

enzymatic action will stop one aa on either side of cross-linked aa, leaving an indigestible hexapeptide

49
Q

preparation of protein concentrates and isolates often calls for treatment of proteins with (2) to modify their properties
- what can be formed?

A
  • alkali
  • in presence of heat
  • lysinoalanine
50
Q

the unnatural aa __________ –> 2 characteristics, having shown to cause _______ failure in rats

A

lysinoalanine
- poorly absorbed + may be toxic
- renal failure

51
Q

is the formation of lysinoalanine to be considered a major consequence?

A

no, unless protein is treated very harshly

52
Q

strongly alkaline environments can lead to____________ of aa –> converting what to what? –> consequence?

A
  • racemization
  • converting L-amino acids to D-L mixtures
  • lose effectivement of aa as building block for protein syntesis
53
Q

2 degradative reactions of proteins?

A
  • proteolysis
  • putrefaction
54
Q

consequence of proteolysis?

A
  • reduction in molecular weight
  • loss in functionality
55
Q

what is proteolysis?

A

attack of proteins by proteolytic enzymes (abundant in living tissue and are secreted by microorganisms)

56
Q

is proteolysis desirable?

A
  • can be: tenderization of meat by action of papain (a proteolytic enzyme)
  • also produces aroma in cheese (from small bitter hydrophobic peptides)
57
Q

putrefaction is associated with ?

A

microbial spoilage of high-protein foods (meat/fish)

58
Q

putrefaction involves what?

A

degradation of free amino acids, produced by proteolytic enzymes secreted by spoilage microorganisms

59
Q

putrefaction: free amino acids are attacked by microbial (2 enzymes)

A
  • deaminases
  • decarbosylases
60
Q

what do deaminases and decarboxylases do?

A
  • deaminase: remove amine groups from free aa
  • decarboxylase: remove carboxyl groups from free aa
61
Q

products produced from deamination and decarboxylation are (3)

A
  • volatile
  • rather repulsive odoriferous compounds
  • with reduced molecular weight
62
Q

decarboxylase and deaminase most active at high or low pH?

A

decarboxylase: most active at low pH
deaminase: most active at high pH

63
Q
  • lysine treated with lysine decarboxylase produces ?
  • ornithine treated with ornithine decarboxylase produces?
  • both enzymes can be inhibited by ?
A
  • lysine –> cadaverine
  • ornithine –> putrescine
  • by DFMO (difluoromethylornithine)
64
Q

what is responsible for high protein systems becoming putrefactive –> becoming spoiled or inedible?

A

microorganisms are responsible because they have a good supply of deaminase and decarboxylase enzymes

65
Q

_____ are especially susceptible of become putrefactive-like compounds because ?

A

fish
- they also undergo enzymatic reactions like conversion of trimethylamine oxide (TMAO = no smell) to trimethylamine (fishy smell) by trimethylamine-N-oxide reductase (when the fish dies)
*trimethylamine smells really bad –> protects you –> don’t eat it!

66
Q

is trimethylamine oxide an aa?

A

no but the end product (trimethylamine) is putrefactive from a sensory standpoint

67
Q

production of trimethylamine correlates very well with (2)

A
  • rise in microbial population
  • overall rise in putrefactive compounds
68
Q

by measuring development of TMA in fish tissue, we have an indication of the _______ of the fish and potentially its ______-_______

A
  • freshness
  • shelf-life
69
Q

most proteins are hygroscopic –> what does it mean?

A

have ability to bind water

70
Q

what capacity of a protein is a very important functional property, especially from a textural standpoint?
2 examples

A

water-binding capacity
- milk proteins absorb moisture + retain it in bread = soft texture
- addition of soy protein to hamburger helps retain moisture, making burger juicier

71
Q

water binding capacity (WBC) is strongly associated with (2)
- may also be associated with presence of a _________ as a prosthetic group

A
  • ionic species present (NH3+ and COO-)
  • hydrogen bonding sites (C=O, N-H) that are readily hydrated
  • polysaccharide
72
Q

water, having a strong _______ ________, is capable of forming a __________ _____ around polar groups

A
  • dipole moment
  • hydration shell
73
Q

what is a moisture sorption isotherm?
- unit?

A
  • illustrates the equilibrium moisture associated with a material as a function of relative humidity at a constant temperature
  • unit: grams of H2O/100g protein
74
Q

what do the 3 regions of the isotherm correspond to? + give characteristics

A

A) bound water: H-bonded to protein or a hydrate of an ionic species –> really an integral part of the molecular structure (monolayer) that in essence cannot be removed
B) water associated with bound water but more mobile –> still somewhat structured bc bound to bound water
C) “free water” –> unstructured, quite mobile and can be removed relatively easily –> region where proteins show their water absorption capacity

75
Q

protein’s ability to bind water is a function of (3)

A
  • its amino acid composition
  • its overall structure (including prothetic groups)
  • its charge
76
Q

processing operations may involve substantial changes in factors that clearly affect protein structure –> 3 factors

A
  • temperature
  • pH
  • ionic strength
77
Q

2 processing operations to remove water

A

freezing and drying

78
Q

freezing has very big/little effect on nutritional quality of proteins –> why?
- however, freezing can cause what?

A
  • little effect bc no heat involved
  • can cause major structural changes in proteins
79
Q

in tissue systems (ie meat and fish), freezing can cause extensive _________ due to changes in (2) caused by (2)

A
  • denaturation
  • changes in ionic strength and pH
  • caused by local concentration effects and the removal of water from microenvironment of protein molecule
80
Q

freezing can cause extensive denaturation –> extent of denaturation is often dependent on what?

A

rate of freezing

81
Q

in case of meat and fish, changes in ionic strength and pH are exemplified by gain/loss of water-binding capacity (manifested by ?) and often result in loss of desirable ________

A
  • loss of WBC
  • manifested by development of freezer drip (crystals btw meat and wrapper)
  • desirable texture
82
Q

deteriorative effects of dehydration processes may be due to (2)

A

denaturation and/or browning

83
Q

what is one of the best methods of drying in terms of maintaining protein functionality, nutritional quality and enzyme activity?
- but ?

A
  • freeze drying
  • but very expensive
84
Q

potential for denaturation and browning during freezing?

A
  • some potential for denaturation
  • little browning due to low temp of sublimation
85
Q

in case of meat, ______ ______ is optimal in terms of retaining _______ _______ _______ of proteins

A
  • freeze drying
  • water binding capacity
86
Q

what type of drying is used extensively for protein solutions?
- examples?
- denaturation?
- browning?

A
  • spray drying
  • milk and egg white
  • some denaturation may take place due to higher temps and to shear at the nozzle
  • some browning for egg white and milk bc of presence of glucose and lactose (Maillard reaction)
87
Q

what is drum drying? causes denaturation and browning?

A
  • fairly harsh treatment with extensive heating
  • extensive denaturation and extensive browning if sugars are present

Food Chemistry 1 (22 decks)

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