Overview
This lecture covers the role of buffers in biological systems, especially blood, and introduces the structure, properties, and ionization of amino acids as building blocks of proteins.
Biological Buffers in Blood
- Blood pH is maintained near 7.4 by phosphate (pKa ≈ 6.9) and bicarbonate buffer systems.
- Phosphate buffers effectively between pH 5.9 and 7.9.
- Bicarbonate buffer relies on CO₂ equilibrium; removing CO₂ by breathing helps keep blood pH at 7.4.
- Hyperventilation causes respiratory alkalosis, raising blood pH above 7.4.
Amino Acids: Structure and Stereochemistry
- Proteins are polymers of 20 alpha amino acids defined by the genetic code.
- Each amino acid has a central (alpha) carbon, a carboxylate (COO⁻), an amino group (NH₃⁺), a hydrogen, and an R group.
- Most amino acids are chiral (L-stereoisomers); glycine is achiral.
- Only L-amino acids are incorporated into proteins.
Amino Acid Categories & Properties
- Nonpolar aliphatic: hydrophobic, contain hydrocarbon chains (e.g., glycine, alanine, proline, valine, leucine, isoleucine, methionine).
- Aromatic: contain rings; phenylalanine, tyrosine (can ionize at high pH), tryptophan (largest, non-ionizing).
- Polar uncharged: contain groups like -OH or -SH (e.g., serine, threonine, cysteine (pKa ≈ 8, partially ionized at biological pH), asparagine, glutamine).
- Positively charged (basic): lysine (pKa ≈ 10), arginine (pKa ≈ 12.5), histidine (pKa ≈ 6, partially protonated at pH 7.4).
- Negatively charged (acidic): aspartate and glutamate (pKa ≈ 2, deprotonated at pH 7).
Ionization & Isoelectric Point
- Amino acids have at least two ionizable groups: carboxylate (pKa ≈ 2) and amino (pKa ≈ 9).
- At pH values between pKa₁ and pKa₂, amino acids exist as zwitterions (net charge zero).
- The isoelectric point (pI) is the pH where net charge is zero; for simple amino acids, pI = average of pKa values.
- Amino acids with ionizable side chains require pI calculation between side-chain pKa values flanking the zwitterionic form.
Peptide Bond Formation and Ionizable Groups in Peptides
- Peptide bonds form via condensation (loss of H₂O), joining amino acids into chains.
- In peptides, only the terminal amino and carboxyl groups and any ionizable side chains contribute to overall charge.
- Peptide residue loses its original carboxylate or amino group upon bond formation.
- Number and type of ionizable groups affect a peptide’s isoelectric point.
Calculating Isoelectric Points
- Sum the charges of all ionizable groups at selected pH values to find where net charge is zero.
- The isoelectric point is the average of the pKa values on either side of net-zero charge.
Key Terms & Definitions
- Buffer — substance that minimizes pH changes by absorbing H⁺ or OH⁻.
- pKa — the pH at which half of a group is ionized.
- Zwitterion — molecule with both positive and negative charges, overall neutral.
- Isoelectric point (pI) — the pH where net charge of a molecule is zero.
- Residue — the part of an amino acid remaining after forming a peptide bond.
- Chirality — property of a molecule having a non-superimposable mirror image.
- L-stereoisomer — left-handed configuration of amino acids found in proteins.
Action Items / Next Steps
- Memorize and recognize the 20 amino acids and their properties.
- Practice determining ionizable groups and calculating isoelectric points for amino acids and peptides.
- Complete the quiz due Wednesday before the next lecture.