2.2 Proteins: Structure, Function, and Metabolism

Amino acids are the building blocks of proteins, forming complex structures that determine their function. From essential amino acids we must consume to non-essential ones our bodies produce, these molecules play crucial roles in maintaining our health and supporting vital processes.

Proteins serve diverse functions in our bodies, from providing structure to catalyzing reactions and transporting molecules. Understanding how we digest and metabolize proteins is key to grasping their importance in nutrition and overall health.

Amino Acids and Protein Structure

Structure and role of amino acids

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• Amino acid structure forms building blocks of proteins
  • Central carbon atom (alpha carbon) bonds to four groups
  • Amino group (-NH2) provides basic properties
  • Carboxyl group (-COOH) contributes acidic properties
  • Side chain (R group) determines unique amino acid properties (hydrophobic, polar, charged)
• Peptide bond formation links amino acids
  • Condensation reaction between amino and carboxyl groups releases water molecule
  • Creates chain of amino acids forming polypeptides
• Protein structure levels determine function
  • Primary: sequence of amino acids defines protein identity
  • Secondary: alpha helices and beta sheets stabilize structure through hydrogen bonds
  • Tertiary: three-dimensional shape forms through various interactions (disulfide bonds, hydrophobic interactions)
  • Quaternary: multiple polypeptide chains associate to form functional proteins (hemoglobin)

Essential vs non-essential amino acids

• Essential amino acids cannot be synthesized by human body
  • Must be obtained through diet to maintain health
  • Nine essential amino acids in humans include leucine, isoleucine, valine
• Non-essential amino acids synthesized by body
  • Not required in diet but crucial for protein synthesis
  • Examples include alanine, aspartic acid, glutamic acid
• Conditionally essential amino acids become essential under specific circumstances
  • Needed in higher amounts during illness, stress, or growth
  • Examples include arginine (important for wound healing), cysteine (limited synthesis in premature infants)

Protein Functions and Metabolism

Functions of proteins in body

• Structural components provide support and protection
  • Collagen strengthens connective tissue in skin, bones, tendons
  • Keratin forms protective layers in hair, nails, skin
• Enzymes catalyze biochemical reactions
  • Accelerate chemical processes in cells (amylase, lipase)
  • Regulate metabolism and cellular functions
• Transport proteins move molecules throughout body
  • Hemoglobin carries oxygen in red blood cells
  • Albumin transports fatty acids and maintains blood osmotic pressure
• Hormones regulate physiological processes
  • Insulin controls blood glucose levels
  • Growth hormone stimulates cell growth and reproduction
• Immune system components defend against pathogens
  • Antibodies recognize and neutralize foreign substances
  • Complement proteins enhance immune response
• Energy source utilized when carbohydrates and fats depleted
  • Provides 4 kcal/g during prolonged fasting or intense exercise

Protein digestion and metabolism

• Protein digestion breaks down proteins into absorbable units
  1. Begins in stomach: pepsin cleaves peptide bonds
  2. Continues in small intestine: pancreatic enzymes (trypsin, chymotrypsin) further break down peptides
  3. Brush border enzymes complete digestion into individual amino acids
• Absorption transports amino acids into bloodstream
  • Occurs in small intestine through active transport mechanisms
  • Amino acids enter intestinal cells and move to portal vein
  • Distributed to tissues for protein synthesis or energy production
• Protein metabolism maintains balance between synthesis and breakdown
  • Protein turnover: continuous process of synthesis and degradation
  • Transamination transfers amino groups between amino acids
  • Deamination removes amino groups for energy production or glucose synthesis
  • Urea cycle converts excess nitrogen to urea for excretion
• Protein synthesis builds new proteins
  1. Transcription: DNA information copied to mRNA
  2. Translation: mRNA sequence decoded to amino acid sequence
  3. Post-translational modifications: protein folding, additional processing
• Nitrogen balance reflects overall protein status
  • Positive balance: protein synthesis exceeds breakdown (growth, pregnancy)
  • Negative balance: protein breakdown exceeds synthesis (malnutrition, illness)
  • Equilibrium: synthesis equals breakdown (healthy adults)