Chapter 6 – Proteins & Amino Acids

Basic Protein Structure & Function

• Composition
  • Made of carbon, hydrogen, oxygen, and nitrogen (distinguishes proteins from CHO & lipids)
  • Organized as strands of amino acids (AAs)
• Universal backbone (Fig. 6-1)
  • Central (α) carbon + attached amine group (\text{\,-NH_2}) + acid (carboxyl) group (\text{\,-COOH})
  • Distinctive side-chain (R-group) confers identity, size, shape, & electrical charge
  • Charge categories: positive, negative, neutral → influences solubility & interactions
• Inventory of AAs
  • 20 total; 8 indispensable/essential for adults (histidine may be conditionally essential, esp. for infants/illness) ❗ (Table 6-1)
  • Conditional essentiality: non-essential AA becomes essential when endogenous synthesis cannot meet need (e.g., premature infants, trauma)
• Peptide bonds link AAs (Fig. 6-2, 6-7)
  • 2\,\text{AA}=\text{di-peptide}
  • 3\,\text{AA}=\text{tri-peptide}
  • >3\,\text{AA}=\text{poly-peptide}

Building Protein: Shape & Attraction

• Side-chain chemistry → intra-chain attractions/repulsions
  • Hydrophilic vs hydrophobic, charged vs uncharged segments
• Primary coil (α-helix) emerges as chain folds (Fig. 6-3)
• Further folding →
  • Globular proteins (e.g., enzymes, hemoglobin)
  • Fibrous proteins (e.g., collagen in connective tissue)

Sequence, Shape & Function Relationship

• DNA dictates AA order (gene → mRNA → ribosome) (Fig. 6-6)
• Specific 3-D conformation enables task-specific activity
  • Working proteins: enzymes, antibodies, hormones, oxygen carriers, transporters
  • Structural proteins: tendons, ligaments, muscle fibers, bone matrix, teeth, hair, nails

Protein Synthesis Errors

• Hereditary template = “standard sequence” for each protein
• Single AA substitution can impair function → disease
  • Classic example: sickle-cell anemia (valine replaces glutamic acid in β-chain of hemoglobin) (Fig. 6-5)

Protein Digestion & Absorption

• Stomach (Fig. 6-8)
  • Gastric HCl denatures tertiary structure → uncoils protein
  • Pepsin cleaves specific peptide bonds → shorter polypeptides
• Small intestine lumen
  • Pancreatic proteases further hydrolyze → di- & tri-peptides
• Brush-border membrane
  • Peptidases split di/tri-peptides → single AAs (plus a few small peptides)
• Absorption
  • Enterocytes actively transport AAs & some di/tripeptides
  • Larger peptides occasionally absorbed → may provoke immune response → basis of some food allergies
• Post-absorption routing
  • Portal bloodstream → liver
  • Liver options: use for its own protein, synthesize plasma proteins, convert to other compounds, or release into systemic circulation

Functions of Proteins

• Growth & maintenance
  • Provide building blocks for new tissue; constant turnover (e.g., RBC lifespan 90!–!120\,\text{days})
• Hormones & enzymes (Fig. 6-9, 6-10)
  • Peptide hormones: insulin, growth factors, glucagon etc.
  • Enzymes catalyze \ge 1000s reactions
• Immune protection
  • Antibodies (immunoglobulins) recognize antigens; AA deficiency → impaired immunity
• Fluid & electrolyte balance (Fig. 6-11)
  • Plasma proteins attract water; protein deficiency → edema
• Acid–base balance
  • Protein side chains accept/release \text{H}^+ → act as buffers
• Blood clotting
  • Fibrin (protein) forms clot matrix
• Energy & glucose source (Fig. 6-12)
  • Secondary/last-resort role
  • No dedicated AA storage → body sacrifices functional protein when dietary energy or CHO is insufficient

Protein Utilization & AA Fate

• Priority hierarchy
  1. Build body proteins
  2. Synthesize other N-containing compounds (e.g., tryptophan → niacin, serotonin)
  3. Transamination → make non-essential AAs
• Deamination (remove amine group)
  • Carbon skeletons →
  • Gluconeogenesis: \text{C skeleton} \rightarrow glucose \rightarrow blood
  • Lipogenesis: \text{C skeleton} \rightarrow fatty acids/triglyceride (storage or fuel)
  • Amine → ammonia → liver converts to urea → kidneys excrete
• Wasting scenarios (Protein Utilization 2)
  • Energy deficit
  • Excess total protein
  • Oversupply of single AA (supplements)
  • Limiting essential AA (low protein quality)
• Prevention of waste
  • Adequate quantity of protein and full complement of essential AAs
  • Sufficient dietary energy (CHO + fat) to spare protein

Protein in Food & Intake Recommendations

• Present in all food groups; density & digestibility vary
• Dietary Reference Intake (DRI)
  • RDA: 0.8\,\text{g}/(\text{kg body wt}) per day
  • Average female: 46\,\text{g·d}^{-1}
  • Average male: 56\,\text{g·d}^{-1}
  • Acceptable Macronutrient Distribution Range (AMDR): 10\% \le \text{Energy}_{protein} \le 35\%

Protein Quality & Quantity

• Needs rise with
  • Malnutrition (atrophied gut → ↓ enzyme secretion & absorption)
  • Infection/trauma (immune protein synthesis)
  • Deficiency of supportive micronutrients (vitamins & minerals required for efficient use)
• Determinants of quality
  1. Digestibility (↑ with moist heat)
  • Animal ≈ >90\%, Plant ≈ 70!–!90\%
  1. AA composition
  • High-quality = all essential AAs in adequate amounts
• Complementary proteins (Fig. 6-13, 6-14)
  • Two+ sources whose AA patterns complete each other (e.g., legumes + grains)
  • Must be eaten within same day for max effect
• Measuring: PDCAAS
  • Combines digestibility + AA profile; scale 0!–!100
  • Egg white, beef, chicken, fat-free milk, tuna = 100 (reference)
  • Soy = 94; Wheat = 25
  • Complementation boosts composite score

Nitrogen Balance (Fig. 6-15)

• \text{N Balance}=\text{N}{intake}-\text{N}{excretion}
  • Equilibrium: healthy adult (intake ≈ excretion)
  • Positive balance (retaining N): growth, pregnancy, strength training, recovery
  • Negative balance (losing N): injury, illness, starvation, astronauts (microgravity-induced muscle loss)

Protein-Energy Malnutrition (PEM)

• Marasmus
  • Age: <2 y; total diet deficiency (kcal & protein)
  • Slow onset; severe wasting of muscle & fat; <60 % weight-for-age
  • No edema, no fatty liver; anxious, apathetic; variable appetite; hair/skin problems
• Kwashiorkor
  • Age: 1–3 y; protein deficiency with adequate kcal (often after weaning)
  • Rapid onset; some weight/muscle loss (60-80 % weight-for-age)
  • Edema, fatty liver; apathy/irritability; loss of appetite; skin/hair lesions
• Global burden: ≈33 000 child deaths/day → public-health, ethical, economic implications
• In North America: seen with chronic disease, poverty, eating disorders

Protein Excess & High-Protein Diets

• Definition: >35\% of total calories from protein
• Observed issues (esp. high animal protein)
  • Higher saturated fat → obesity, CVD risk
  • Possible renal & hepatic stress in animals; mixed evidence in humans
  • Increased urinary calcium → bone mineral loss (especially purified protein)
• Kidney considerations
  • Extra urea elimination ↑ kidney workload; can exacerbate existing kidney disease → therapeutic strategy: restrict protein
• Weight-loss (very-low-CHO) diets
  • CHO <130\,\text{g·d}^{-1} ⇒ depleted glycogen, ketosis
  • AA catabolism produces glucose but costs loss of muscle protein (post-absorptive phase)
  • Initial fast weight drop = water loss; long-term: ↓ muscle + fat
  • Diet often high in sat. fat & cholesterol, low in Ca, fibre, some micronutrients → long-term health concerns

Vegetarian Diets (Controversy 6)

• Motivations: health, cultural/religious beliefs, environmental & ethical (animal welfare, sustainability), personal economics
• Terminology (Table C6-1)
  • Vegetarian: plant-based; excludes some/all animal products
  • Vegan: excludes all animal-derived foods; Raw vegan: ≥75 % uncooked
  • Lacto-ovo-vegetarian: includes milk & eggs, excludes flesh/seafood
  • Lacto-vegetarian: dairy yes; eggs & flesh no
  • Ovo-vegetarian: eggs yes; dairy & flesh no
  • Partial vegetarian: limits red meat; Pesco-vegetarian: includes fish, excludes other meat
  • Fruitarian; Macrobiotic (extreme restriction → risk of malnutrition/death)
• Positive health aspects
  • Lower risk of obesity, heart disease, hypertension, type-2 diabetes, certain cancers
  • High intakes of fruit/veg, fibre, phytochemicals, vitamins A,C,\text{folate}; lower sat. fat; generally healthier lifestyle (↑ physical activity, ↓ smoking/alcohol)
  • Leaner body composition; satiety from bulky, fibre-rich foods may ease calorie control
• Potential pitfalls
  • Increased requirements during pregnancy, lactation, growth, illness require careful planning
  • Possible shortfalls: \text{B}_{12}, D, Ca, Fe, Zn, \omega!-
    3 (EPA/DHA) especially for strict vegans, infants, children, seniors
  • Need for complementary proteins & fortified foods/ supplements
• Bottom line
  • Any dietary pattern (omnivore or vegetarian) must be adequate, balanced, calorie-controlled, varied
  • Moderation with foods high in saturated fat, sugar, salt is still essential