Proteins & Amino Acids
Proteins are large biomolecules made of amino acid chains linked by peptide bonds. They constitute about 50% of cellular dry weight and perform the widest variety of functions of any biomolecule.
Amino Acids — Structure
All 20 standard amino acids are L-α-amino acids (except Glycine, which has no chiral center). Each has an α-carbon bearing: amino group (-NH₂), carboxyl group (-COOH), hydrogen, and a variable R-group (side chain).
Classification of Amino Acids
- Non-polar/Hydrophobic: Ala, Val, Leu, Ile, Pro, Met, Phe, Trp (tend to be buried in protein interior)
- Polar/Uncharged: Gly, Ser, Thr, Cys, Tyr, Asn, Gln
- Positively charged: Lys, Arg, His (basic)
- Negatively charged: Asp, Glu (acidic)
- Essential (must come from diet): PVT TIM HALL — Phe, Val, Thr, Trp, Ile, Met, His, Arg, Leu, Lys
Levels of Protein Structure
- Primary: Sequence of amino acids joined by covalent peptide bonds. Determines all higher order structure.
- Secondary: Local folding stabilized by hydrogen bonds between backbone C=O and N-H. Patterns: α-helix (right-handed coil) and β-pleated sheet (parallel or anti-parallel).
- Tertiary: Overall 3D shape of a single polypeptide. Stabilized by: H-bonds, disulfide bridges (-S-S-), hydrophobic interactions, ionic bonds.
- Quaternary: Association of multiple polypeptide subunits (e.g., Hemoglobin: 2α + 2β chains).
Denaturation
Disruption of secondary/tertiary/quaternary structure (NOT primary). Caused by heat, pH extremes, organic solvents, heavy metals. Often irreversible. Loses biological function.
Special Proteins
- Collagen: Most abundant protein in body; triple helix; rich in Gly-Pro-Hyp; Vitamin C required for hydroxylation (scurvy = deficiency)
- Hemoglobin & Myoglobin: Oxygen-binding heme proteins; Hb shows cooperativity (sigmoidal O₂ curve); Mb is monomeric (hyperbolic curve)
- Immunoglobulins: Antibodies with specific antigen-binding sites
Clinical
- Sickle cell anemia: Glu→Val substitution at position 6 of β-globin chain (point mutation)
- Kwashiorkor: Protein deficiency with edema (low oncotic pressure)
- Marasmus: Protein + calorie deficiency