Proteins and Amino Acids

Composition of proteins

Polymers of amino acids linked by peptide bonds formed by dehydration

Contains a 3-D Structure of folded polypeptides

Dipeptide vs. Tripeptide

Dipeptide has 2 amino acids, 1 peptide bond

Tripeptide has 3 amino acids, 2 peptide bonds

Types of Protein Structures

1. Primary

2. Secondary

3. Tertiary

4. Quaternary

Amino Acid Structure

• Amine group (NH_x)

• Acid group (COOH)

• Side group (R)

  • Creates unique character (pH, charge, size, shape, etc.)

How many amino acids are there?

20 (9 are essential, 11 nonessential)

Zwitterion

• Molecule containing equal number of + and - charged groups

  • For amino acids, in neutral pH equal number of NH₃⁺ and COO^-

Amino Acid behavior in low pH

Lots of H⁺ atoms, amino group takes it becoming NH₃⁺ (positive molecule)

Amino acid behavior in high pH

Lots of OH^- molecules, COOH donates H becoming COO^- (negative molecule

Isoelectric point

pH at which amino acid is neutral (Zwitterion)

Essential amino acids to know

Lysine, methionine, phenylalanine, tryptophan

Primary protein structure

• COVALENT peptide bond connecting amino acids

  • Pseudo double-bond (strong)

  • Hardest bond to break

Secondary protein structure

• Alpha helices and beta sheets

  • HYDROGEN BONDING between N-H and C=O groups

  • Side chains do not participate in hydrogen bonds

Tertiary Protein Structure

• 3-D folding of alpha and beta sheets with random coils

  • BONDS: ionic, hydrogen, IMFs (hydrophobic interactions)

    • Covalent in the case of disulfide (S-S) bonds in cysteine

Quaternary Protein Structure

Multiple polypeptides coming together to form proteins

Hydrophobic Interactions in Cheese

• Casein in cheese is hydrophobic

  • Need rennet in low pH to cut molecule

  • Culture for bacteria → lactic acid

• Casein + fat + calcium creates curd, cheese

  • Whey leftover contains healthy protein

Ways to denature proteins:

1. Lose secondary, tertiary, or quaternary structures

   1. Not primary due to covalent bonds

2. Treat with urea

3. Change pH to break ionic bonds (3)

4. Heat or freeze to break hydrogen bonds (2 and 3)

5. Change medium to break hydrophobic interactions (3)

6. Mechanical actions

Prion Diseases - Mad cow and CJD

Caused by protein aggregation and conformational changes (wrong folding)

Types of Protein Shapes

1. Globular (sphere)

2. Fibrous (parallel filaments)

Globular Proteins

Spherical proteins that work alone, but can be made into fibrous proteins

Ex. Myoglobin

Fibrous Proteins

Parallel filaments that work together and are very strong, insoluble

Ex. Collagen

Making globular into fibrous proteins:

1. Put in water and base viscous solution

2. Isoelectric precipitation in acid

3. Wash, neutralize, dry

Coagulation

Clots formed by denatured proteins aggregating

All coagulated proteins are denatured, not all denatured proteins aggregate

Functions of Proteins

• Water-binding

• Gelatins

• Emulsification (since proteins can be both hydrophobic/philic)

• Foaming

Protein water-binding

• More water gives more high-quality product

• Depends on isoelectric point/pH

  • Higher/lower pH increases water-binding since proteins aggregate at isoelectric point

Gelatin

• Continuous 3-D network that entraps water

  • interactions between proteins and with water

• Denatured collagen

• Yogurts, custards, hot dogs/sausages, and cheeses

Main protein in eggs

Ovalbumin (denatures first in cooking)

pH of egg whites

The older the eggs, the more basic the egg white is

Importance of egg shell

• CO₂ is acidic and will be lost if shell is cracked, becoming basic

  • Denatures albumin and water moves to yolk

• Mineral oil is sprayed to prevent loss of CO₂

Egg Pasteurization

Heat-treating to kill pathogens like salmonella

Gray ring around yolk

Overheating an egg, causing iron and sulfur to react

Main protein in milk

Lactalbumin (denatures first)

Cooking milk

Using double boiler (water boils first, then milk boils in container by water)

3 categories of protein in meat:

1. Muscle fiber

2. Connective tissue

3. Myoglobin

What makes meat look red?

Myoglobin (protein cells), NOT blood

Cooking meat:

1. Denaturation occurs at 40-65 C

   1. Overcooking toughens meat due to loss of juice/fat

2. Collagen turns into gelatin

3. Meat tenderizes

4. Red → Brown color change

5. Kill microbes

Perfect temperature to cook meat:

• 86 - 93 C for 2-3 hours (simmering)

  • Cooking for too long allows bacteria (thermophiles) to grow

  • Cooking too hot blackens meat

Maillard Reaction

Reaction between glucose/fructose and asparagine (or a carbonyl) at high temperature, producing acrylamide that causes tumors

Why is boiling better than heating food?

It produces less acrylamide, lower risk of tumors

Roles of proteins

1. Growth and maintenance

2. Structure

3. Enzymes

4. Hormones

5. Immune response

6. Fluid Balance

7. Acid-base balance

8. Transport

9. Energy

Proteins and growth/repair

• Growth of bones, muscles, ligaments, etc.

• Replace cells of the intestinal lining every 3 days

Protein and structure

• Collagen gives bones and skin strength

• Keratin gives hair and nails structure

Proteins and enzymes

Act as biological catalysts in the body; ALL are proteins

Hormones

• Mostly proteins that act as messengers to other parts of the body

  • Ex. Insulin, glucagon, adrenaline

  • NOT estrogen, testosterone, etc. (steroid hormones)

Proteins and Immunity

Antibodies are blood proteins that respond to antigens, attacking it and causing release of immune cells and formation of memory to antigen

Proteins and fluid balance

Albumin and globulin attract fluid in capillaries to limit imbalance

Edema

Fluid imbalance due to lack of protein resulting in swelling; often indicates poor diet or malnourishment

Proteins and acid-base balance

Amino acids can pick up H⁺ in acid or lose in base to keep pH at 7.35-7.45

Transport proteins

• Lipoproteins - HDL, LDL, chylomicrons

• Albumin - calcium, zinc, B6

• Transferrin - Iron

• Hemoglobin - oxygen

Sick-cell anemia

RBCs are sickle-shaped and can’t transport enough oxygen because they get stuck in blood vessels; painful and requires transfusions

Proteins as Energy

• Proteins break down when not enough carbs are eaten

  • Into amino acids, deaminated, CHO used to make glucose

• Consuming too much protein = automatically converted into glucose

  • Protein is NOT stored

  4 KCAL/GRAM (lipids 9, alcohols 7, carbs 4)

Protein Digestion in Stomach

• Begins in stomach after chewing stimulates gastrin

  • Secretion of gastrin results in HCl production, reducing pH

• Gastric juice is acidic, antiseptic, and denatures globular proteins

• Pepsin recognizes peptide bonds to be cleaved

Peptidases

Enzymes that hydrolyze proteins by cleaving internal peptide bond or individual amino acids

Endopeptidases

Zymogen that hydrolyzes proteins by cleaving peptide bond

Carboxypeptidase

Exopeptidase enzyme that cleaves amino acid bond at the C terminal

Aminopeptidase

Exopeptidase enzyme that cleaves amino acid at the N terminal

Peptidase Zymogen

Enzyme containing extra amino acid that inactivates it, folds into active enzyme after amino acid is removed

Digestion in the Small Intestine

• Zymogens are released after CCK and secretin is secreted by pancreas

  • Secretin - bicarbonate secretion to neutralize HCl

  • CCK - pancreatic juice with peptidase zymogens

Enzyme Specificity

• Each peptidase is specific to amino acid R groups

  • Ex. Trypsin, chymotrypsin, elastase

Final Protein Digestion

Membrane-bound endo, di, and aminopeptidases result in free amino acids, di and tripeptides

Location of Protein Absorption

Duodenum and jejunum of the small intestine into blood (99% as free amino acids)

How are amino acids, di and tripeptides absorbed and transported?

Absorbed across enterocytes and transported by active, carrier-mediated transporters

Why is it problem consuming too much of one amino acid/supplement?

Some amino acids share the same transporter, inhibiting absorption

What happens when you absorb a WHOLE protein?

Allergic reaction, body recognizes it as an allergen

Protein as Allergens

Immune system attacks body due to presence of protein, death possible

Protein Intolerance

Body doesn’t have enzyme to break down, no risk of death

Examples of allergens

Proteins, gluten, nuts, fish, eggs, etc.

Celiac Disease

• Severe malabsorption of gluten by intestinal mucosa due to deficit of cells

  • Allow entrance of polypeptides

• Immune response with gluten casing local reaction by antibodies

Is protein stored?

No, if the body needs protein it will break down tissue protein

How is protein quality measured?

Amino acid content and digestibility

Protein digestibility

• Animal proteins are absorbed 90-99%

• Plant proteins are absorbed 70-90%

  • Except for soy/legumes - 90%

Complementary Proteins

Proteins eaten together that supply all essential amino acids

Which organ produces nonessential amino acids?

The liver

Limiting amino acids

Essential amino acids less than amount needed to support protein synthesis (Ex. corn missing Tryptophan and Lysine)

High quality protein

• Contains all 9 essential amino acids in correct ratio

  • Animal foods (human is the best but that’s cannibalism :))

  • Plant foods miss 1+ but can combine (complementary)

    • Grains + legumes

    • Nuts/seed + legumes

    • Corn + legumes

Essential Amino Acid Percentage

• Total protein %/food % x 100

• Lowest % amino acid is limiting

  • Chemical score

Types of protein-energy malnutrition (PEM)

• Marasmus

• Kwashiorkor

Marasmus

• Severe deprivation/lack of absorption of protein, other nutrients during infancy

• Weight loss, muscle waste, poor development

• Hair and skin problem

• Still have an appetite

Kwashiorkor

• Inadequate protein intake along with infection

• First child after second child (no breastfeeding - nutrient decrease)

  • Older infants/children

• Edema, fat retention, fatty liver (large stomach)

• Mood problems, loss of appetite, hair and skin problem

Problems with overconsumption of proteins

• Heart disease due to saturated fats

• Cancer

• Osteoporosis due to more calcium excretion

Protein in weight loss

Adequate amount of protein, moderate fat, and enough carbs supports loss

Recommended intake of protein

Assuming healthy, mixed quality 10-35% energy intake

Proteins in Med. Diet

• Emphasis on legumes, fish, eggs, milk/dairy, meat in moderation

  • Legumes with grains give all essential AAs

  • Fish are low in lipids but contain other nutrients

  • Eggs are very important

Med. Diet vs. American

• Olive oil instead of trans fats/seed oils

• Greens vs. potatoes

• Fresh fruits vs canned

• Lightly cooked vs. deep fry/BBQ (more acrylamide)

• Unprocessed vs. processed