BIOMEDICAL IMPORTANCE

  • L𝛼-Amino acids
  • 腎臟每天超過濾出50克的游離胺基酸,但大部份都回收

PROPERTIES OF AMINO ACIDS

The Genetic Code Specifies 20 L-𝛼-Amino Acids

  • 大自然有超過300種胺基酸,絕大多數蛋白質由20種胺基酸組成

Selenocysteine, the 21st Protein L-𝛼-Amino Acid

  • 人類有兩打的selenoproteins,包含iodothyronine deiodinases(把T4轉成T3)
  • UGA為stop codon,但若伴隨著selenocysteine insertion element (stem-loop structure),則此UAG會被tRNASec辨視,進而轉譯selenocysteine上去
  • Serine的氧變硫就是cysteine、硫變硒就是selenocysteine

Stereochemistry of the Protein Amino Acids

  • 除了glycine外,胺基酸的𝛼-carbon為chiral;雖然胺基酸有左旋及右旋,但全部都有L-glyceraldehyde的結構,因此才會稱為L-𝛼-amino acids
  • R/S來自右手命名法則
  • D/L來自L-glyceraldehyde
  • d/l or (+)/(-)來自光學性質
  • 大部份的胺基酸是(R),但L-cysteine是(S)

Posttranslational Modifications Confer Additional Properties

  • 某些prokaryotes會使用pyrrolysine
  • 植物可使用azetidine-2-carboxylic acid,此為proline analogue
  • 在collagen,proline會變成4-hydroxyproline,lysine會變成5-hydroxylysine
  • 凝血因子的glutamyl residues經carboxylation變𝛾-carboxyglutamyl residues
  • Histones的lysine可經acetylation及methylation;arginine可經deamination

Extraterrestrial Amino Acids Have Been Detected in Meteorites

  • 𝛼-amino acids: Ala, Asp, Glu, Gly, Ile, Leu, Phe, Ser, Thr, Tyr, Val, N-methylglycine (sarcosine), 𝛽-alanine
  • D-及L-isomers混合之3-5碳胺基酸

L-𝛼-Amino Acids Serve Additional Metabolic Roles

  • Tyrosine為甲狀腺素之原料
  • Tyrosine及phenylalanine會被代謝成epinephrine, norepinephrine及dihydroxyphenylalanine (DOPA)
  • Glutamine為neurotransmitter,也是𝛾-aminobutyric acid (GABA)之原料

Certain Plant ʟ-𝛼-Amino Acids Can Adversely Impact Human Health

  • Lathyrus(山黧豆)造成neurolathyrism,來自L-homoarginine及𝛽-N-oxalyl-L-𝛼,𝛽-diaminopropionic acid (𝛽-ODPA)
  • 𝛾-glutamyl-𝛽-aminopropionitrile (BAPN)為osteolathyrogen,來自sweet pea種子
  • 𝛼,𝛾-diaminobutyric acid為ornithine相似物,會抑制hepatic urea cycle的ornithine transcarbamylase,造成ammonia中毒
  • Cycad(蘇鐵)種子含L-𝛽-methylaminoalanine,會讓amyotrophic lateral sclerosis-Parkinson demantia complex的風險上升。

D-Amino Acids

  • 人腦組織天然存在D-serine及D-aspartate
  • 革蘭氏陽性菌的細胞壁有D-amino acids
  • Bacillus subtilis會分泌D-methionine, D-tyrosine, D-leucine, D-tryptophan以引發生物膜的分解
  • Vibrio cholera的peptidoglycin含D-leucine及D-methionine

PROPERTIES OF THE FUNCTIONAL GROUPS OF AMINO ACIDS

Amino Acids May Have Positive, Negative, or Zero Net Charge

  • 在生理環境,carboxyl group幾乎以R-COO⁻之型式存在,amino group幾乎以R-NH₃⁺之型式存在