Chapter 11: Proteins: Amino Acids and Peptides Flashcards
Describe PKU.
Individuals with PKU must not consume phenylalanine (since it is not synthesized). Tyrosine is a byproduct of phenylalanine. When no Phe is available, additional Tyr must be consumed to meet the body’s needs (conditionally essential AA).
Which plant source is known for its high quality? How is it treated? Which amino acid is limiting?
- Soybeans
- Must be heat processed for several hours to destroy toxic compounds that prevent the complete digestion of proteins
- Low in methionine
Which amino acid is often limiting in cereal grain? Which others are limiting in plant-based products?
Cereal: Lysine
Others: Tryptophan, Threonine
Name some combinations that provide adequate amounts of AA.
- Whole-wheat bread + peanut butter
- Rice and red beans
- Refried beans and corn tortilla
- Hummus (chickpeas + sesame seeds)
Which amino acids are used in lipoprotein clusters?
AA with nonpolar side chains, positioned toward the outsides of the molecules, allowing them to attract cholesterol molecules
Which AA have a hydroxyl group?
Serine, threonine, tyrosine (causes a polar nature)
What are proteins composed of?
Carbon, Hydrogen, Oxygen, Nitrogen, usually Sulfur
What else can be contained in protein composition?
Iron, Copper, Phosphorus, Zinc
What is the basic subunit of proteins?
Amino acids
Name the 3 parts of the amino acid.
- Side chain of C and H
- Carboxyl group (COOH)
- Amine group (NH2)
Which part of the amino acid acts as the acid? As the base?
Acid: carboxyl group
Base: amine group
How do amino acids combine?
carbonyl carbon and amide nitrogen combine and form a peptide bond through condensation (released water)
Define polypeptide.
Chain of amino acids bound together by peptide bonds.
How many AA are in most proteins?
100 to 150 AA
How are AAs classified?
1) By their nutritional use
2) By the chemical nature of their side chain
Compare dispensable and indispensable AA.
Dispensable/nonessential: made by the body (11)
Indispensable/essential: not made by the body and must be supplied by the diet (9)
Name the 9 essential amino acids.
Histidine, Isoleucine, Leucine, Lysine, Valine, Methionine, Phenylalanine, Threonine, Tryptophan
What causes conditionally indispensable AA?
Certain conditions prevent the body from producing enough dispensable AA and they have to be obtained from the diet
(not in healthy individuals)
Compare complete and incomplete protein sources.
Complete: contain all the indispensable AA
Incomplete: grains and vegetables are short one or more of the essential AA
How do vegetarians get complete proteins?
Combining sources (ex: beans + rice)
Name the 4 types of amino acid side chains.
- Nonpolar
- Uncharged polar
- Positively charged
- Negatively charged
Which amino acid side chains are less soluble in water?
Nonpolar sidechains
Which amino acid side chains will form hydrogen bonds and are attracted to polar molecules?
AA with neutral sidechains
Which amino acid side chains can function as buffers?
Positively and negatively charged
Name 3 reasons why protein structure is complex.
1) Number of amino acids
2) Order in which they combine
3) Interaction of the sidechain
Define primary structure.
The order the amino acids occur in the sidechain; results from chain of peptide bonds.
Define secondary structure.
Refers to the shape of sections of the protein
Define tertiary structure.
Refers to the 3D structure of an entire AA chain
Name the 3 types of secondary structure
1) Helix: repeating coil
2) Random coil: tangled and twisted
3) Pleated sheet: like a paperfan
Differentiate the 2 types of tertiary structure.
1) Globular proteins do not form gel networks (do not hold water)
2) Fibrous proteins form gel networks, long elastic, usually made from helix-shaped strands
How do H-bonds form in proteins?
Form between the H-atom of one chain and the hydroxyl group of another
Name the 2 functions of H-bonds in proteins.
- Basic to the stability of 2nd and 3rd structures
- Make some proteins water soluble
Define disulfide cross-links
Covalent bonds between 2 protein molecules at side chains with sulfur
What is the function of disulfide cross-links?
The more stable disulfide bonds, the more stable the molecule
How can we break disulfide bonds?
Sodium hydroxide (Lye)
How do hydrophobic interactions form in proteins?
Form between sidechains that are nonpolar
Name 2 examples of hydrophobic interactions in food.
- Caseins in milk (cheese curds)
- Whey, by-product of cheese production
Define myoglobin.
The iron binding protein pigment in muscle that provides colour
When is myoglobin bright red, purplish and brown?
Bright Red: when O molecule is attached
Purplish: when O molecule is not attached
Brown: after prolonged exposure to O
Define oxidation and reduction reactions.
The reversible process of adding and removing oxygen.
Oxidation adds oxygen, Reduction removes it
When are nitrites used?
added during the curing process to preserve meats
Define cured meats
ham and bacon, very stable, pink-red colour
Define denaturation
Any change in the shape of a protein molecule without breaking the peptide bonds
Name the 3 characteristics of denaturation.
- Loosening or unfolding of the tertiary and sometimes secondary structure
- Sometimes reversible
- Involves only H-bonds
The breaking of which reaction is irreversible?
Breaking disulfide cross-links
Define coagulation. What does it result from?
Permanent denaturation, which results when liquid or semiliquid proteins form solid or semisoft clots
Name 3 effects of coagulation a protein.
- Changes its physical characteristics
- Alters the ability to bind with water
- Interferes with biological interactions of enzymes
Name a denatured protein that can return to its original state?
Beaten egg whites
What kind of protein holds their new shape?
Coagulated proteins, ex: cooked eggs
Name the 3 methods to physically denature proteins
1) Temperature changes
2) Mechanical actions
3) Sound waves and irradiation
How do heat and cold affect proteins?
Heat: speeds up denaturation
Cold: may cause curdling
What mechanical actions disrupt protein structures? What strengthens gluten?
Disrupt: beating, rolling, kneading
Strengthens gluten: kneading
How do sound waves/irradiation disrupt protein structures?
Prolonged exposure at high levels is needed
Name the 2 methods to chemically denature proteins.
1) Change in pH
2) Exposure to mineral salts or metals
Name examples of products in which acids have denatured milk proteins.
Sour cream, buttermilk, yogurt
What kind of salts can denature proteins?
Sodium and potassium salts
What are the 4 factors food scientists examine to determine how effective proteins work in a product?
- Water absorption
- Solubility
- Viscosity
- Stability in acids and alkalis
Name the 5 functions of protein in food.
1) Proteins form gels
2) Proteins change texture
3) Proteins emulsify
4) Proteins form foams
5) Proteins develop gluten
Define a protein gel
Mixture of fluids locked in a tangled mesh of denatured and coagulated protein
What are the 2 parts of protein gels?
1) 3D molecular structure
2) Liquid attracted to the protein
What do acids do to gelatin? What can develp?
Acids soften gelatin, which may develop syneresis if stored or cooked for too long
What increases gel stability?
Mineral salts, hard water, more gelatin, slow coolin rates
What decreases gel stability?
Acids, sugars, food pieces, rapid cooling
How is protein gel formed from muscle tissue?
When salt is added to destabilize proteins
How can globular proteins change texture?
Spun into fibers under the right conditions
Name 2 examples of texturizing.
- Soy protein to create meat substitutes by denaturation or heat-coagulation under pressure
- To produce processed cheeses
What kind of protein can act as an emulsifier?
Denatured protein
How do proteins act as emulsifiers?
Polar side chain attracts water-based liquid and nonpolar side attracts oil
How can casein act as an emulsifier?
Pressure used in homogenization of milk ENABLES casein to act as an emulsifier
What makes extraction of oils from seeds more difficult?
Protein’s ability to form an emulsion
Define a foam.
Gas suspended in liquid or semi-solid
How are foams formed?
By bubbling gas through a mixture, beating or whipping, and depressurization
Give examples of foams.
Meringue, foam cakes, marshmallows, soufflés, bread
Give examples of good foaming agents.
Albumin in egg whites and milk
What is gluten?
Strongly cohesive and elastic protein
How is gluten formed?
When a high protein flour is combined with moisture and stirred or kneaded
What does gluten’s strength result from?
Disulfide cross-links formed during kneading
What happens to gluten when baked?
Gluten coagulates when baked
What damages high protein foods?
High temperature and prolonged cooking
What happens when high protein foods are damaged?
Protein molecules shrink and squeeze out H2O, resulting in a dry, rubbery, tough product
Name 6 functions of protein as a nutrient.
1) Support growth and repair
2) Fight disease
3) Maintain fluid and mineral balance
4) Maintain pH balance
5) Control bodily functions
6) Provide energy
How do proteins function as buffers?
Pick up and release acids and bases (side chain)
How do proteins control bodily functions?
Proteins are apart of hormones that maintain body functions and enzymes (chem reactions)
How do proteins provide energy? What is a byproduct? How does it affect health?
AA -> Energy (produces ketones and ammonia - strains kidneys)
Name 2 ways food scientists are developing affordable proteins.
1) Developed grains (triticale)
2) Using biotechnology
What is a health concern of protein?
Allergies
