Food Chem Protein Exam

Proteins role in biological systems

• Proteins play a central role in biological systems

• Complex biological macromolecules

• Made up of 20 different amino acids

• Partial double amine bond

• 3D configurations

  4 primary elements
  • C (50-55%)
  • H (6-7%)
  • O (20-23%)
  • N (12-19%
  • May also contain S, P, Fe, Zn, Cu

Proteins are classified based on:

• Source

• Structure

• Composition & Solubility

• Biological Function

Animal vs Plant Proteins

Animal Proteins

• Higher quality proteins with all essential amino acids

• Least variable component – number of muscle fibers fixed at birth
  

Plant Proteins

• Lower-quality proteins with limited essential acids (Met, Lys, Thr and Trp – one or more)
  

• Food Proteins – those that are easily digestible, nontoxic, nutritionally adequate, functionally usable in food products, available in abundance and agriculturally sustainable

What amino acids are in:

• Cereal grains/ millets

• rice and soybeans

• legumes

• nuts

• sunflower seeds

• green leafy vegetables

• Lysine, Threonine

• Methionine

• Methionine, Tryptophan

• Methionine, Lysine, Threonine

• Lysine

• Methionine

Why are milk, eggs and meat a complete protein?

They have all the essential amino acids

Biological Roles and Relationships

• Enzymes

• Storage proteins

  • proteins in grains; casein in milk; ovalbumin in egg white

• Structural proteins

  • collagen, elastin, keratin, proteins in cell membrane and cell wall.

• Contractile proteins

  • Myosin and actin in muscle

• Transport proteins

  • Hemoglobin/oxygen; transferrin/iron; serum albumin/fatty acids

• Defense proteins

  • Antibodies, fibrinogen and thrombin – blood clotting

• regulatory proteins

  • Insulin, growth hormones

  • ATP - regulates chemical reactions

• Toxic proteins

  • Clostridium botulinum toxin, ricin in plants

• Allergens

Proteins are maid of:

• Amino Acids

• 20 for protein construction

• 9 essential (isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine)

• Histidine for children

• PVT TIM HLL

Stereochemistry of Amino Acids

• α-L-amino acids are the most commonly found

• L – “Levo” or Left

• D – “Dextro” or Right

• α- refers to the position of N in relation to the carboxyl group

• All naturally occurring amino acids in proteins are of the L-configuration (except glycine, which doesn’t have a stereocenter)

• D-amino acid L-amino acid

Chiral= four different functional groups that have two mirror forms

what does alpha mean in alpha amino acid

the amine group is attached directly to the carbon and is next to the carboxyl group

What is the orange group

Orange = Non polar/ hydrophobic

• Alanine, Valine, Leucine, Isoleucine,
  Phenylalanine, Tryptophan, Methionine,
  Proline
  • R-Groups contain only C & H
  • Except Methionine with S atom
  • Normally found inside protein structures, away
  from water

  
  

what is the green group?

• Polar / Uncharged

• Glycine, Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine

• Have groups with O and N which can participate in hydrogen bonding - as proton donors or acceptors

what is blue and purple?

Polar / (+) or (-) charged
(+): Lysine, Arginine, Histidine — blue
(-): Aspartic Acid, Glutamic Acid — purple

Groups for each

• Aliphatic hydrocarbons: glycine, alanine, valine, isoleucine, leucine, proline

• R-containing aromatic ring : phenylalanine, tryptophan,
  tyrosine

• R-containing aliphatic –OH group: serine, threonine

• R-containing amide group (-CO-NH2): asparagine, glutamine

• Sulfur containing: cysteine, methionine

• Acid group (-COOH): aspartic acid, glutamic acid

• Basic group (-NH2): arginine, lysine , histidine

Four levels of protein structure

1. Primary

2. .Secondary

3. Tertiary

4. Quaternary
   • Classification of proteins is based on tertiary structure

Primary Structure

Single strands of ordered amino acids bonded by peptide bonds

• Peptide Bonds: α-carboxylic (1st AA) joined to α-amino group (2nd AA)

• “Amine Bond”

• The chain length and sequence act as the code for formation of secondary and tertiary structures

• Also determines protein physiochemical, structural and biological functionality of the protei

Secondary Structure

Strands fold on each other to form:
• α-helices:
• β-sheets:
• Stabilized by hydrogen bonds in the peptide backbone

Tertiary Structure

Helices and sheets further fold on themselves through disulfide
bonding

• Disulfide bridges = covalent bonds between cysteine residues

• Each Cysteine has a –SH (thiol) group

• When two cysteines are close together in a folded protein, they can be oxidized to form a bond

• -S-S- = Disulfide Bridge

Two ways to classify proteins based on tertiary structure

• Fibrous

  • Rod shaped molecules containing twisted linear polypeptide chains

  • Collagens (bone, teeth, skin)

  • Elastins (ligaments)

    • collagen and elastin = texture of meat/ food

  • Keratins (hair, wool, nails)

  • Myosin + Actin (contractile muscles)

  • Fibrin (blood clot)

• Globular

  • Spherical or ellipsoidal shapes

  • Albumins (eggs)

  • Globulins (muscle foods)

Quaternary Structure

• 2+ polypeptide chains united by non-covalent bonds

• 3D arrangement that assembles into ONE functional proteins

• Unraveled = Independent Proteins

  • Myoglobin = stores oxygen

  • Hemoglobin = transports oxygen

Simple Proteins (homoproteins): ONLY amino acid

1. Albumins –
2. Globulins –
3. Glutelin –
4. Prolamines –
5. Scleroproteins

know examples of these

Conjugated Proteins (heteroproteins): protein + prosthetic group

1. Metalloproteins –
2. Glycoproteins/mucoproteins –
3. Lipoproteins –
4. Nucleoproteins –
5. Phosphoproteins –
6. Chromoproteins –

know examples of these

Milk Proteins

Milk contains 30-36 g/L total protein

Major classes:
• Casein
• Whey
• Serum (blood) proteins

Caseins

Represent about 80% of total proteins in milk

Main Types:

• αs1-casein, αs2-casein, β-casein, K-casein, γ-casein

• Each Globular with hydrophobic regions

• High ratios of polar amino acids

• Valine, leucine, isoleucine, phenylalanine, and proline

• Uniquely heat stable – up to 140°C

• Renitt = makes milk coagulate

Casein Micelles

• Micelle = casing + calcium phosphate
  

• Contain majority of casein proteins

  • 95%

  • Plus some calcium

• Size varies 80 – 1,000 nm in diameter

  • Average is 250 nm in bovine milk

• Made up of submiclles (12-15 nm)

  • α- and β-casein concentrated in middle of submicelles

  • K-casein on surface

    • Hydrophilic protruding chain extends from surface

    • ”Hairy Layer”

• Micelles carry calcium

Draw Milk

Whey Proteins

• Hydrophobic, globular proteins

• Well-developed secondary, tertiary and quaternary structures

• Poor heat stability

  • Denature at 75 C

• 2 Primary proteins

  • α-lactalbumin and β-lactoglobulin

• Others include

  • Proteose peptones

    • Derived from hydrolysis of β-casein

    • Considered whey protein because they elute in the whey fraction