Proteins – Human Nutrition Chapter 6

Overview of Chapter 6: Proteins

• Scope: chemical nature of proteins, digestion/absorption, physiological roles, metabolic fate, food sources, health implications, and quantitative requirements.

Protein Basics – Definition & Versatility

• Term “protein” derives from Greek protos = “of prime importance”
• Ubiquity of functions:
• Muscle contraction
• Blood clotting
• Vision
• Catalysis of virtually every biochemical reaction
• Immune defense
• Structural integrity of bone, skin, hair, connective tissue
• Broad functional classes (quick view): structure, movement, transport, immunity, enzymatic catalysis, water- and ion-balance, nutrient carriage, energy backup.

Amino Acids – Building Blocks

• 20 common α-amino acids combine to form virtually all human proteins.
• Shared core: central C atom + H + amino group (NH2)(NH_2) + carboxyl group (COOH)(COOH) + variable side-chain (R-group).
• Side-chain dictates size, shape, polarity, charge ⇒ functional diversity.

Essential vs. Non-essential

• Essential (must come from diet): Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.
• Non-essential (synthesised endogenously): Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Proline, Serine, Tyrosine.
• Conditional essentiality: Arginine, Cysteine, Tyrosine, etc. become essential during growth, illness, or metabolic stress.

Visual examples

• Glycine (simplest R = H) vs. Alanine (R = CH3CH_3) vs. Aspartic acid (acidic side-chain) vs. Phenylalanine (bulky aromatic ring) – illustrates increasing complexity.

Peptide Bond Formation & Protein Architecture

Condensation (Dehydration) Reaction

• Amino acid<em>1+Amino acid</em>2→−H2O\text{Amino acid}<em>1 + \text{Amino acid}</em>2 \xrightarrow{\small -H_2O} dipeptide.
• Extension → tripeptide → oligopeptide (∼2!–!20\sim2!–!20 residues) → polypeptide (>20) → functional protein (typically >50 AAs).

Four Structural Levels
  1. Primary – linear AA sequence (e.g., insulin = 51 AAs, 3 disulphide bridges).
  2. Secondary – local folding via H-bonds → α\alpha-helices & β\beta-pleated sheets (keratin, silk fibroin).
  3. Tertiary – complete 3-D conformation of one chain; stabilised by H-bonds, ionic links, disulphide bridges, hydrophobic clustering, van der Waals (myoglobin).
  4. Quaternary – assembly of ≥2 subunits into a multimeric complex (haemoglobin α<em>2β</em>2\alpha<em>2\beta</em>2; DNA polymerase).
    • Analogy: letters → words → sentences → paragraphs to emphasise hierarchical information encoding.

Digestion & Absorption

Oral Phase

• Mechanical mastication + saliva moisten proteins; no enzymatic proteolysis yet.

Gastric Phase

• HCl denatures tertiary/quaternary structure → exposes peptide bonds.
• Activates pepsinogen → pepsin; pepsin begins endopeptidic cleavage → large → smaller polypeptides + limited free AAs.
• Denaturation ≈ cooking/acid/alkali/enzyme-induced unfolding.

Small-Intestinal Phase

• Pancreatic proteases activated by enteropeptidase:
• Trypsin (from trypsinogen) – cuts Lys/Arg.
• Chymotrypsin – cuts Phe, Tyr, Trp, Met, Asn, His.
• Carboxypeptidases – trim C-terminus.
• Elastase, collagenase – additional cleavage.
• Brush-border peptidases (tripeptidases, dipeptidases, aminopeptidases) → almost complete hydrolysis to single AAs (few oligopeptides may slip through).

Absorptive Mechanisms

• Active transport (ATP-dependent) & Na(^+)/H(^+)-linked symporters bring di-, tri-peptides & AAs into enterocytes.
• Intracellular peptidases finish hydrolysis → free AAs enter portal circulation.

Post-absorptive Handling

• Liver: first pass distribution centre. Uses AAs for:
• Plasma protein synthesis (albumin, clotting factors)
• Energy (via deamination)
• Gluconeogenesis
• Lipogenesis (when dietary surplus).
• No dedicated protein storage – excess cannot be stockpiled as intact protein.

Physiological Roles of Proteins

Structural Material

• Collagen, keratin, elastin: framework of skin, bone, tendon, blood vessels; scar tissue formation; ligament/tendon tensile strength.

Enzymes

• Biological catalysts (e.g., amylase, lipase, protease, DNA polymerase) accelerate reactions by 106!–!1012×10^6!–!10^{12}\times, enabling life-compatible rates.

Hormones & Signalling Peptides

• Protein/peptide hormones: insulin, glucagon, oxytocin, prolactin, growth hormone, ADH, angiotensin, calcitonin, parathyroid hormone, thyroxine-binding proteins; regulate metabolism, fluid–electrolyte, Ca(^ {2+}, lactation, growth.

Transporters

• Hemoglobin (O(_2), lipoproteins (lipids), albumin (hormones, FAs, drugs, osmotic pressure), ferritin (Fe storage), membrane pumps and channels.

Immune Defense

• Antibodies (immunoglobulins), cytokines, complement components.

Movement

• Actin & myosin orchestrate skeletal, cardiac, smooth muscle contraction; tubulin & dynein for intracellular motility.

Fluid & pH Balance

• Plasma proteins maintain oncotic pressure; protein buffers (histidine residues) stabilise pH≈7.35pH \approx 7.35.

Energy Reserve of Last Resort

• 4 kcal/g; utilised during fasting/starvation when carbohydrate & lipid stores depleted.

Protein Metabolism

Protein Turnover & Amino-acid Pool

• Continuous synthesis⇌degradation ((~250\ g/day) internal flux).
• Free intracellular + extracellular AA pool ≈ AA “bank” for rapid cellular access.

Nitrogen Balance Concept

• Equation: N balance=N<em>in−N</em>out\text{N balance} = N<em>{in} - N</em>{out}
• Balance (0): healthy adults.
• Positive: growth, pregnancy, tissue repair, anabolic training.
• Negative: starvation, trauma, burns, infection, inadequate protein/energy intake, hyperthyroidism, cortisol excess.

Fates of Amino Acids
  1. Synthesis of body proteins & non-protein nitrogen compounds (neurotransmitters, melanin from tyrosine, creatine, purines/pyrimidines).
  2. Gluconeogenesis & ATP production when CHO scarce.
  3. Conversion to triacylglycerol for storage when energy/protein intake > demand.
  4. Transamination → non-essential AA synthesis (requires vitamin B(_6 cofactor).
Deamination & Urea Cycle

• Deamination ⇒ ammonia + keto-acid.
• Keto-acids feed into TCA cycle, ketone synthesis, cholesterol, or FA synthesis.
• Ammonia toxic ⇒ hepatocytes convert to urea via ornithine cycle; urea excreted renally.
• High-protein diet raises urea production (≤250 g protein/day manageable) → increased water requirement to avert dehydration.
• Pathology:
• Liver failure → hyperammonaemia.
• Kidney failure → azotaemia (high blood urea).

Dietary Protein – Needs & Food Sources

Who Needs More?

• Rapid growth (infancy, childhood, adolescence).
• Pregnancy & lactation.
• Recovery from illness/injury.
• Athletes (muscle repair & hypertrophy).

Animal Sources

• Seafood – lean, rich in omega-3 (e.g., salmon).
• White-meat poultry (remove skin to cut \uparrow saturated fat).
• Eggs – economical complete protein (AHA: 1 egg/day permissible for healthy adults).
• Milk, yoghurt, cheese – protein + calcium + vit D (choose low-fat/skim).
• Lean beef – comparable sat. fat to skinless chicken, offers haem iron, Zn, B(_{12}.

Plant Sources

• Legumes (beans, lentils, soy) – protein + fibre; 0.5 cup beans ≈ 1 oz meat; soy 50 g/day ↓ LDL (~3\%).
• Nuts & seeds.
• Whole grains.
• Benefit: no cholesterol, lower saturated fat, added minerals/fibre.

Health Effects of Protein Intake

Deficiency & Protein-Energy Malnutrition (PEM)

• Symptoms: hunger, muscle wasting, oedema, immune suppression, poor wound healing, brittle hair/nails, fatty liver, fractures risk.
• Kwashiorkor – severe protein deficit with moderate energy intake → oedema, enlarged liver, distended abdomen.
• Marasmus – severe energy & protein deficiency → severe wasting “skin & bone”.

Chronic Disease Relationships

• Cardiovascular: plant & fish proteins protective; excess red/processed meat ↑ CHD risk.
• Cancer: high red/processed meat ↔ colorectal & others; plant proteins (legumes, soy) may lower cancer risk; poultry/fish neutral or protective.
• Osteoporosis: adequate protein works synergistically with Ca2+Ca^{2+} & vitamin D + weight-bearing exercise to preserve BMD, especially in elderly women/anorexia.
• Weight control: high-protein meals ↑ satiety, preserve lean mass during hypocaloric dieting, modestly boost thermogenesis; excessive calories from protein still → adiposity.

Recommended Intakes & Assessment

Dietary Reference (Adults ≥19 y)

• RDA: 0.8 g×body weight (kg)0.8\,\text{g} \times \text{body weight (kg)}.
• Example: 70 kg adult → 0.8×70=56 g/day0.8 \times 70 = 56\,\text{g/day}.

Special Populations

• Pregnancy/Lactation: 1.1!–!1.3 g/kg1.1!–!1.3\,\text{g/kg}.
• Children/Teens: higher, age-graded.
• Older adults (≥65 y): 1.0!–!1.2 g/kg1.0!–!1.2\,\text{g/kg} to counter sarcopenia.

Protein Balance Graph

• Positive ↔ growth, pregnancy, recovery, anabolic hormone milieu (insulin, GH, testosterone).
• Negative ↔ fasting, catabolic disease states, essential AA deficiency, prolonged bed rest, burns, infection, cortisol/thyroid excess.

Vegetarian Patterns

• Motivations: ethics, health, religion.
• General observation: vegetarians tend to be leaner with lower chronic-disease prevalence.
• Types:
• Vegan (no animal products)
• Lacto- (adds dairy)
• Ovo- (adds eggs)
• Lacto-ovo.
• Pescatarian (adds fish)
• Flexitarian (mostly plant-based, occasional meat)
• Raw-food & fruitarian subsets.
• Vegetarian Food Pyramid: emphasises legumes/soy, whole grains, vegetables, fruits, nuts/seeds; optional eggs/dairy; minimal sweets; mandatory B(_{12} supplementation if strict vegan.

Quick-Reference Equations & Numbers

• RDA equation: \text{Protein}{g/day} = 0.8\,\frac{g}{kg} \times \text{BW}{kg} (modify per life stage).
• Nitrogen balance: N{bal} = N{in} - N_{out};eachgofdietaryprotein≈; each g of dietary protein ≈0.16\,g nitrogen.
• Urea production upper physiological capacity ≈ 250\,\text{g protein/day}.
• Energy yield: 1\,\text{g protein} = 4\,\text{kcal}.

Integration & Key Takeaways

• Proteins are indispensable macromolecules with multi-level structural complexity translating into vast functional repertoire.
• Adequate digestive enzymology ensures liberation of amino acids, subsequently channelled into synthesis, energy, or excretory pathways.
• Nitrogen equilibrium reflects dynamic turnover; imbalances signal anabolic or catabolic states.
• Balanced intake emphasising variety – lean animal & diverse plant proteins – supports growth, maintenance, performance and mitigates chronic disease risk.
• Special needs arise during growth, gestation, ageing, illness, and specific lifestyle choices (e.g., vegetarianism) necessitating tailored planning to secure all essential amino acids and vitamin B(_{12}.
• Excessive or insufficient protein both bear health consequences; moderation aligned with RDA and individual context remains cornerstone.