Amino acid Study Guide

📖 Core Concepts α‑Amino acid backbone – General formula H₂N‑CHR‑COOH; R = side chain. Chirality – All proteinogenic α‑amino acids (except glycine) are chiral; they exist in the L (S) configuration (cysteine is an exception, R). Side‑chain classification – Polar charged, polar uncharged, hydrophobic non‑polar, and special outliers (glycine, cysteine, proline, selenocysteine, pyrrolysine). Zwitterionic form – At physiological pH, amino acids carry ‑NH₃⁺ and ‑COO⁻ groups, giving a net charge of zero. Isoelectric point (pI) – pH where the net charge is zero; for non‑ionizable side chains: $$\text{pI} = \frac{pKa^{\text{COOH}} + pKa^{\text{NH}3^+}}{2}$$ For ionizable side chains, the side‑chain pKₐ is included in the average. Essential vs. non‑essential – Nine amino acids are essential for humans; cysteine, tyrosine, and arginine are semi‑essential. Peptide bond formation – Condensation of the α‑amino of one residue with the α‑carboxyl of another, releasing H₂O; catalyzed by ribosomes (or by specific enzymes for non‑ribosomal peptides). 📌 Must Remember Twenty‑two protein‑building α‑amino acids (20 encoded directly by the universal genetic code). Charged residues at neutral pH: Asp, Glu (‑), Arg, Lys, His (partial +). Histidine pKₐ ≈ 6.0 → acts as acid or base near physiological pH. pI calculation – use the average of the two relevant pKₐ values; add side‑chain pKₐ for ionizable side chains. Glycine = achiral, most flexible. Cysteine → disulfide (cystine) when oxidized; important for protein stability. Proline = secondary amine (imino acid) → rigid kink in polypeptide chain. Selenocysteine incorporated via SECIS element; pyrrolysine via UAG codon in certain archaea. 🔄 Key Processes Aminoacyl‑tRNA charging – ATP‑dependent aminoacyl‑tRNA synthetase attaches the amino acid to its cognate tRNA. Ribosomal peptide elongation – N‑terminus to C‑terminus; each cycle adds one aminoacyl‑tRNA, forms a peptide bond, and translocates the mRNA. Transamination (catabolism) – Amino group transferred from an amino acid to α‑ketoglutarate → glutamate + α‑keto acid. Urea cycle (nitrogen disposal) – Free NH₃ from deamination → urea for excretion (vertebrates). Post‑translational modifications (PTMs) – Phosphorylation (Ser/Thr/Tyr), methylation (Lys/Arg), acetylation (Lys), disulfide formation (Cys). 🔍 Key Comparisons Aspartate vs. Glutamate – Both negatively charged; Glu has one extra methylene in the side chain. Lysine vs. Arginine – Lys: primary amine (pKₐ ≈ 10.5); Arg: guanidinium group (pKₐ ≈ 12.5, always positively charged). Histidine vs. Lysine – Histidine’s imidazole pKₐ ≈ 6 → partial protonation; Lysine stays fully protonated at physiological pH. Glycine vs. Proline – Glycine: flexible, no side chain; Proline: cyclic, restricts backbone φ angle. Cysteine vs. Methionine – Cys: thiol (can form disulfides, pKₐ ≈ 8.3); Met: thioether (non‑reactive, hydrophobic). ⚠️ Common Misunderstandings “All D‑amino acids are unnatural” – D‑forms exist in bacterial cell walls and some peptides; the L/D label derives from glyceraldehyde reference, not from intrinsic “naturalness.” “pI is always the average of two pKₐ values” – Wrong for ionizable side‑chains; the side‑chain pKₐ must be considered. “Proline is a typical α‑amino acid” – It is an imino acid (secondary amine) and imposes a rigid kink. “All sulfur‑containing amino acids are cysteine” – Methionine also contains sulfur but does not form disulfides. 🧠 Mental Models / Intuition “Water‑fearing vs. water‑loving” – Hydrophobic side chains → buried interior; hydrophilic → exposed surface. Visualize a protein as a “marble cake”: chocolate (hydrophobic) core, vanilla (hydrophilic) frosting. “Charge‑complementarity” – Enzyme active sites often contain opposite charges to the substrate (e.g., DNA‑binding proteins enriched in Lys/Arg). “pI as the ‘balance point’” – Imagine a seesaw: when pH = pI, the positive and negative charges on the molecule are perfectly balanced, so it doesn’t migrate in an electric field. 🚩 Exceptions & Edge Cases Cysteine’s absolute configuration – (R) due to sulfur priority, unlike the (S) configuration of other proteinogenic L‑amino acids. Histidine at physiological pH – Only 10 % protonated; can act as both acid and base in enzyme catalysis. Selenocysteine & Pyrrolysine – Not part of the standard 20; require special recoding mechanisms (SECIS element, UAG reassignment). Proline’s peptide bond geometry – Can adopt a cis peptide bond more frequently than other residues. 📍 When to Use Which Choose a side‑chain for membrane‑spanning helices – Prefer strongly hydrophobic residues (Leu, Ile, Val, Phe, Trp). Design a metal‑binding site – Use Cys (thiolate) or His (imidazole) as ligands; combine with Asp/Glu for bidentate coordination. Select a residue for phosphorylation studies – Use Ser, Thr, or Tyr; Tyr’s phenolic pKₐ ≈ 10 makes it less reactive than Ser/Thr. Predict protein solubility – Enrich surface with polar uncharged (Ser, Thr, Asn, Gln) and charged residues; avoid excessive hydrophobic patches. 👀 Patterns to Recognize “Charge‑pairing pattern” – Salt bridges: Asp/Glu (‑) ↔ Arg/Lys ( + ). “Hydrophobic core pattern” – Repeating leucine/isoleucine/valine motifs in interior helices. “Motif for metal binding” – Cys‑X₂‑Cys or His‑X₃‑His patterns indicate potential Zn²⁺ coordination. “pI clustering” – Acidic proteins (low pI) are rich in Asp/Glu; basic proteins (high pI) contain many Lys/Arg. 🗂️ Exam Traps Mistaking glycine for a chiral amino acid – Glycine has no stereocenter; it is achiral. Using the average of the two backbone pKₐ values for a basic amino acid’s pI – For Lys, Arg, His, the side‑chain pKₐ must be included, shifting pI upward. Assuming all sulfur‑containing residues form disulfides – Only Cys can oxidize to cystine; Met does not. Confusing “essential” with “proteinogenic” – All 20 proteinogenic amino acids are incorporated into proteins, but only nine are essential for humans. Believing that proline is a typical α‑amino acid – Its secondary amine makes it an imino acid; it imposes structural constraints. --- This guide condenses the most exam‑relevant facts from the provided outline. Use it for rapid recall right before the test!