Overview
This lecture covers amino acids and proteins, including their structures, properties, classifications, functions, and related disorders. Key concepts focus on amino acid structure, types of proteins, peptide bonds, protein folding, denaturation, and consequences of protein deficiency.
Structure and Properties of Amino Acids
- Amino acids are the building blocks of proteins and consist of a central (chiral) carbon, amino group, carboxyl group, hydrogen, and an R group (side chain).
- The amino group provides basic properties; the carboxyl group provides acidity.
- The R group determines the identity, size, polarity, and pH behavior of each amino acid.
- Most amino acids are optically active (can rotate polarized light), except glycine.
- Amino acids exist as left-handed (L/levorotatory) or right-handed (D/dextrorotatory) forms.
- Amino acids are amphoteric, acting as both acids and bases.
- Zwitterion: an amino acid with both a positive (amino) and negative (carboxyl) charge, making it electrically neutral overall.
- Isoelectric point (pI): the pH at which an amino acid has no net electrical charge and is least soluble in water.
Classification of Amino Acids
- By R group: non-polar (e.g., glycine, alanine, valine, leucine, isoleucine), acidic (glutamic & aspartic acid), basic (lysine, arginine, histidine), aromatic (phenylalanine, tyrosine, tryptophan), sulfur-containing (methionine, cysteine), hydroxyl-containing (serine, threonine), cyclic (proline), amide-containing (asparagine, glutamine).
- By nutritional importance: essential (must be obtained from diet — "private TIM HALL": phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, arginine, leucine, lysine) and non-essential (body can synthesize).
- Conditionally essential: become essential during physiological stress.
- By metabolism: ketogenic (leucine, lysine), ketogenic and glucogenic (isoleucine, phenylalanine, threonine, tyrosine, tryptophan), glucogenic (remaining 14).
Protein Structure and Classification
- Proteins are formed by amino acids joining via peptide bonds (condensation reaction).
- Classified by hydrolysis product (simple, conjugated, derived), function (transport, storage, contractile, structural, protection, catalytic), and shape (fibrous, globular, mixed).
- Peptides are short chains of amino acids; longer chains (>100) are polypeptides/proteins.
- Peptide bond is strong and exhibits geometric isomerism (cis/trans).
Protein Structure Levels
- Primary: sequence of amino acids (e.g., insulin).
- Secondary: local folding into alpha-helices or beta-pleated sheets (e.g., keratin).
- Tertiary: 3D folding pattern due to side chain interactions (e.g., myoglobin).
- Quaternary: multiple polypeptide chains form a functional protein (e.g., hemoglobin).
Protein Denaturation and Deficiency Disorders
- Denaturation: unfolding of proteins due to heat, chemicals, or pressure; leads to loss of function.
- Physical agents: heat, UV, high pressure; chemical agents: acids, bases, solvents.
- Deficiency disorders: Kwashiorkor (protein malnutrition, edema, liver enlargement), marasmus (severe protein and nutrient lack, muscle wasting), atrophy (muscle wasting), sickle cell anemia (mutation replaces glutamic acid with valine in hemoglobin).
Common Natural Peptides
- Glutathione: antioxidant in cells.
- TRH: regulates thyroid hormone release.
- Angiotensin: increases blood pressure.
- Bradykinin: vasodilator.
- Oxytocin: induces uterine contractions.
- Somatostatin: inhibits growth hormone.
- Endothelin: vasoconstrictor.
- Glucagon: raises blood sugar.
- Insulin: lowers blood sugar.
- Melittin: anti-inflammatory, found in bee venom.
Key Terms & Definitions
- Amino Acid — Building block of proteins with a central carbon, amino, carboxyl, hydrogen, and R group.
- Optical Activity — Ability to rotate polarized light; left (L) or right (D) forms.
- Amphoteric — Can act as both acid and base.
- Zwitterion — Molecule with both positive and negative charges.
- Isoelectric Point (pI) — pH where an amino acid has no net charge.
- Peptide Bond — Covalent bond between amino acids.
- Primary/Secondary/Tertiary/Quaternary Structures — Levels of protein organization from sequence to fully folded complexes.
- Denaturation — Structural loss and functional inactivation of proteins.
- Essential Amino Acids — Cannot be synthesized by the body, must be obtained from diet.
Action Items / Next Steps
- Complete activities and check-for-understanding (CFU) assignments.
- Transfer answers to the provided Google Form in Google Classroom.
- Review and memorize essential amino acids (private TIM HALL).
- Read module details as instructed for further clarification.