BIOMEDICAL IMPORTANCE

• L–α-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
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