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Proteins Flashcards

1
Q

When is lipid autoxidation termination favored? What occurs as a result of the reaction?

A

-high levels of radicals present of low oxygen levels
-lipid radical reaction comes to an end, not end of oxidation as a whole
-covalently linked lipids are produced

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2
Q

What three things can initiate lipid autoxidation?

A
  1. transition metals
  2. ionizing radiation (UV light)
  3. heat (reduces energy barrier to break double bond)
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3
Q

Besides autoxidation, name the two other oxidation methods. What happens in them? What are some of their defining features?

A
  1. photosensitizer-induced oxidation = singlet oxygen forms from metal-containing compounds reacting,
    -radical forms at double bond
    -no initiating ROS required
  2. lipoxygenase enzymes = present in many animals/plants
    -contains iron catalytic site to form alkyl radical
    -commonly inactivated by blanching
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4
Q

What five things can inhibit oxidation of lipids?

A
  1. amber bottles = to filter UV light
  2. bleaching = removes photosensitizers
  3. blanching = inactivates oxidizing enzymes
  4. chelators = molecules that bind to metal to make them unable to oxidize lipids
  5. antioxidants = slow rate of oxidation reactions
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5
Q

What are the two antioxidant mechanisms? How do they work?

A
  1. quench singlet oxygen = oxygen returns to triplet/ground state and absorbed energy is dissipated as heat (ex. of quencher = ascorbic acid)
  2. compete for free radicals = antioxidants can neutralize or “quench” free radicals, preventing or slowing down the chain reactions that free radicals can initiate
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6
Q

What kind of compounds are a major class of anti-oxidants? What makes them good anti-oxidants?

A
  1. phenolic compounds
    -form resonance hybrids which stabilizes their radical
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7
Q

What makes an antioxidant good at preventing oxidation?

A

-must easily form radicals, but remain stable

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8
Q

What is a limiting factor in frying? Why are plant lipids not great for frying?

A

-smoke point
-plant lipids oxidize faster

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9
Q

What occurs in a lipid hydrolysis reaction?

A

-can be catalyzed by an acid or base (saponification)
-carbonyl serves as site for nucleophilic attack
-reaction is not stereospecific, all fatty acids are removed

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10
Q

What are the products of a base catalyzed lipid hydrolysis reaction (saponification)?

A

-glycerol and fatty acid soaps

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11
Q

Describe an enzyme-catalyzed lipolysis reactions. What are the products?

A

-lipase enzymes catalyze hydrolysis
-used in lipid digestion
-stereospecific, sn-2 is kept
-produces short-chain fatty acids, monoglycerides, diglycerides, overall amphiphilic molecules

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12
Q

What occurs in an esterification/interesterification lipid reaction? How does it impact the physical properties of lipids?

A

-esterification = condensation reaction (reverse of lipolyis)
-interesterification = fatty acids are removed and randomly reattached to glycerol backbone
-can be carried out by base-catalyzed hydrolysis or lipases as an alternative to partial hydrogenation
-alters crystallization/melting behavior
-creates good shortenings

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13
Q

What occurs in a hydrogenation reaction? What is the product?

A

-a metal catalyst adsorbs diatomic hydrogen and forms hydrogen radicals
-the acyl lipid chain then adsorbs to catalyst by breaking double bond (makes unsaturated fats saturated)
-the hydrogen is added across the double bond
-creates shortenings

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14
Q

What are some of the properties of nonpolar (hydrophobic) uncharged amino acids?

A

-large R-groups = taste bitter
-Tyr and Trp have some minor hydrophilic groups, but they are largely hydrophobic
-smaller R-groups = taste sweet
-Gly and Ala have relatively small interactions with water

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15
Q

What are some of the properties of charged amino acids

A

-bulky but interact well with water
-contain R groups which can be protonated/deprotonated
-acidic = negatively charged, have acidic taste (glutamic and aspartic acid
-basic = positively charged, have bitter taste (arginine, lysine, histidine)

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16
Q

What are some of the properties of polar (hydrophilic) uncharged amino acids?

A

-polar groups are able to form hydrogen bonds
-positive interactions with water
-generally taste sweet
-exception = methionine is hydrophobic and bitter

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17
Q

Describe Aspartame’s structure. What are some of its defining features?

A

-dipeptide of Asp + Phe with methyl groups on carboxylic acid of Phe
-relative sweetness intensity of about 200 (200x sweeter than sucrose)
-used in diet beverages
-hydrolyzes at high temperatures (not good for baking)

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18
Q

Describe some of the features of a peptide bond.

A

-rigid and planar
-limits rotation of bonds about alpha carbon
-almost exclusively in trans-configuration

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19
Q

Protein folding depends on ____________, environmental interactions, and bond formation.

A

-primary structure

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20
Q

Describe the secondary structure of a protein.

A

-alpha helix forms
-intra-coiling hydrogen bonding in backbone
-beta sheets form hydrogen bonds with neighboring portions of strands
-similar R-groups cluster on one side of strands

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21
Q

Describe the tertiary structure of a protein. What three types of proteins fall under this category?

A

-globular = compact, most common food proteins, bury hydrophobic groups in protein core
-fibrous = elongated, mainly hydrophilic, common to have repeating structures linked together
-random coil proteins = less defined structural motifs, often poor water solubility

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22
Q

Describe the quaternary structure of a protein. What are some examples?

A

-can consist of several subunits
-ex. glycinin, hemoglobin, collagen

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23
Q

Describe the general structure of a globular protein? What are some examples?

A

-spherical, hydrophobic core
-highly water soluble
-ex. B-lactoglobulin, alpha-lactalbumin, egg proteins

24
Q

Describe the structure of B-lactoglobulin.

A

-small, B-barrel
-hydrophobic core
-one free cysteine (contributes to gelation)
-can unfold at interface to act as an emulsifier or foam-stabilizer

25
Describe the structure of alpha-lactalbumin.
-small, high helix content -native structure has high heat-stability -binds calcium to deliver it in body
26
What are the two proteins that make up egg white proteins? Describe their structure/features.
-ovalbumin = 2 cysteines available for disulfide bonding, heat denaturation at 70-80 C, forms gel -ovomucin = large, lots of interaction with water (viscous)
27
Describe the structure of egg yolk proteins.
-high content of lipoproteins -rich in phospholipids
28
Why are eggs good for foam formation? What can inhibit this?
-their proteins create disulfide bonds that stabilize air bubbles -the bonds can then rearrange to form more extensive cross-linking -metal ions or acid can inhibit disulfide bond formation (prevents over-whipping)
29
Describe the general structure of a fibrous protein? What are some examples?
-takes on tertiary and quaternary structures -highly hydrophilic -often contains multiple polypeptide chains -ex. collagen, tropomyosin, actin
30
Describe the structure of collagen.
-Gly-X-hPro repeats -triple helix structure -high tensile strength -many structures covalently link to form full collagen molecule
31
Describe the structure of tropomyosin.
-mainly extended helices -hydrophobic seam between helices -elongated fibers
32
What is a main class of unstructured proteins? Describe its structure/properties.
-poorly defined tertiary structure -insoluble, but forms smaller micelles -high protein content -assists in delivery of calcium and phosphate -calcium phosphate stabilizes alpha/beta casein core and k-casein occupies surface
33
What are the structures/properties of B-casein and K-casein?
-B-casein = many prolines, charge, and polar groups prevent globular structure -K-casein = charged, much more hydrophilic, plays important role in micelle stability and coagulation in dairy
34
What are the two water insoluble proteins found in gluten? What are their structures? What is the general structure of gluten?
-gliadin = mainly alpha helices, random coil -glutenin = larger, mainly random coil with few helices provides strength/elasticity -gluten has few charged groups, is rich in glutamine, proline, and has a poorly defined tertiary structure -gluten network forms disulfide bonds in water to create network
35
What effects does protein denaturation have on a protein?
-loss of native functionality -increased entropy -reduced solubility, hydration, and enzyme activity
36
What five features can induce protein denaturation? How do they do this?
1. temperature = proteins unfold and increase in entropy 2. pH = alters surface charge, can cause repulsion 3. salts = binds to charged residues, can cause aggregation 4. interfaces = hydrophobic interfaces (oil, air) can stabilize hydrophobic regions 5. solvents = hydrophobic solvents (ethnol) can reduce hydrophobic interactions
37
Which protein is less soluble and more heat stable? Whey proteins or globular plant proteins?
-globular proteins are less soluble and more heat stable
38
If the pH of a molecule is above the isoelectric point, the NET charge will be _______________.
-positive
39
What does "salting in" mean in the context of denaturing proteins? "Salting out"?
-salting in = moderate salt level can improve solubility of proteins -salting out = too much salt can reduce solubility
40
When are the following proteins soluble? -albumins -globulins -prolamins -glutenin's -fibrous proteins
-albumins = soluble in water at neutral pH -globulins = soluble in dilute salt -prolamins = soluble in alcohol -glutenin's = soluble in a strong acid/base -fibrous proteins = soluble in hot water
41
How are dairy products formed through curdling?
-casein micelle is destabilized -proteins aggregate and trap fat in the protein network -acidification and enzyme modification is used
42
How are whole-muscle proteins impacted by cooking?
-the structural hierarchy of actin/myosin remains intact -water is held between structures by capillary forces -myofilaments can swell, encouraged by salt (myosin helices denature)
43
How are the myofibrillar proteins in processed meat products extracted? How are they impacted by cooking?
-extracted with salt and fine chopping -cooking = proteins denature and crosslink; fats integrate into this gel texture to hold water
44
What is gelatin made from?
-partially hydrolyzed collaged made with strong acidic or basic conditions
45
What does a high bloom strength mean for gelatin?
-higher molecular weight and a stronger gel
46
What two major fractions of globulin proteins are soy proteins high in? What are their structures?
-glycinin = large, 2-3 disulfide bonds -B-conglycinin = no disulfide bonds
47
How is tofu made?
-soybeans are first processed (chopped) and heated -the globular proteins are denatured and enzymes form aggregates -hydrophobic groups are exposed to improve aggregation and gelation -aggregated proteins are coagulated with either salt or acid-induced gels
48
How do salt and acid-induced gels help coagulation in the making of tofu?
1. salt = forms salt bridges, promotes aggregation, but needs divalent cations 2. acid-induced gels = creates fine texture and increases water content (creates silken tofu)
49
What are the three main protein reactions? What do they do?
1. hydrolysis = breakdown of peptide bond between amino acid backbones 2. redox reactions = formation/breaking of disulfide bonds between cysteine residues 3. enzymatic cross-linking = transglutaminase enzyme catalyzes formation of isopeptide bond (between amino acid side chains)
50
What are the consequences of protein hydrolysis?
-loss of native functionality (could impart new properties) -generates flavor compounds -allows for absorption of amino acids in the GI tract
51
What are the consequences of protein redox reactions?
-alters inter- and intra- molecular bonding arrangement -stabilizes (native) tertiary structures -alters the functionality of foods (ex. gluten formation)
52
What are the industrial food applications of enzymatic cross-linking of proteins?
-increased firmness and water holding -restructuring of meats -improved quality
53
What are the three main categories of food enzymes?
1. proteases = break proteins 2. amylases = cleaves polysaccharides 3. lipases = assists in digestion of lipids
54
What are the two types of amylase enzymes? How do they differ in functionality?
1. endoenzymes = cleave within polysaccharide chains into smaller polymers 2. exoenzymes = cleave terminal ends of polysaccharide chains into sugars
55
How do lipase enzymes break down lipids?
-attack glycerol/fatty acid ester bond -enzyme remains in water phase where bile salts can emulsify lipids -hydrophobic region of triglyceride comes to the interface, causes opening of catalysis site for nucleophilic attack

FST 100A (1 decks)

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