LECTURE 01 - Amino Acids

蛋白质化学 - 氨基酸的思维导图，氨基酸，是含有碱性氨基和酸性羧基的有机化合物，化学式是RCH(NH₂)COOH。

Protein Chemistry - Amino Acids

• Property of amino acids:
  • In 20 kinds of common proteinogenic amino acids (all alpha-amino acid, except proline (alpha-imino acid), chiral (alpha-carbon, except glycine);
  • In D/L notation system, L stereoisomers are dominant;
  • Colorless crystalline, high melting point, optical activity (except Gly), some may have two chiral centers, diverse solubility in various solvent.
  • Zwitterions: have isoelectric point(pI), pH>pI, with positive charge and vice versa:
  • pI= 0.5*(pK1+pk2) (average number of two large constant)
• Proteinogenic Amino Acids
  • Aliphatic side chains, nonpolar: Glycine (Gly, G, controversial), Alanine (Ala, A), Valine (Val, V), Leucine (Leu, L), Isoleucine (Ile, I), Methionine (Met, M, thioether), Proline (Pro, P);
  • Aromatic side chains (A280): Phenylalanine (Phe, F), Tyrosine (Tyr, Y, also polar, hydroxyl), Tryptophan (Trp, W);
  • Polar, but uncharged side chains(hydrophilic): Serine (Ser, S, hydroxyl), Threonine (Thr, T, hydroxyl), Cysteine (Cys, C, sulfhydryl, cystine, disulfide bond); Asparagine (Asn, N), Glutamine (Gln, Q) (both with amide groups);
  • Positively charged (basic) side chains at physiological pH (with hydrophilic group): Lysine (Lys, K), Arginine (Arg, R), Histidine (His, H, proton donor/receptor, important in many enzymatic reactions);
  • Negatively charged (acidic) side chains at physiological pH: Aspartic Acid (Aspartate) (Asp, D), Glutamic acid (Glutamate) (Glu, E);
• Uncommon Amino Acids
  • Selenocysteine (Sec, U): SECIS element, opal codon;
  • Pyrrolysine (Pyl, O): amber codon, in methanogenic archaea and bacteria;
  • Ornithine, citrulline: intermediate metabolites in the urea cycle;
  • Transiently modified by phosphorylation, sumoylation, methylation, acetylation, etc.
  • Post-translationally modification: 4-hydroxyproline（important in maintenance of certain structure);
  • D-amino acid also has vital functions in physiological process: neuroplasticity, antibiotic peptide, aging;