Amino Acids (1.LF UK, NT)

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Peptide bond

Amino acids are the basic building blocks of proteins. Chemically, they are organic compounds connected to each other by a peptide bond. At least one primary amino group –NH 2 and at the same time at least one carboxyl group –COOH must be present in the amino acid. Chemically, they are substituted derivatives of carboxylic acids.

• 2–100 amino acids (monomers – peptides
• 100 or more amino acids – proteins

More than 700 different AMKs have been demonstrated in nature. That is why we also divide AMK according to their occurrence:

• amino acids found in all living organisms
  • bound in proteins (21 proteinogenic AMK), peptides or as free AMK
• amino acids found only in some organisms
  • bound in peptides or as free AMK
  • they are not components of proteins

Proteinogenic amino acids, or coded ones, occur in proteins as L-alpha-amino acids (the exception is glycine). This is due to the chemical arrangement that is necessary for biogenic function. Specific types of amino acids, their sequence and spatial structure then give proteins their biological properties.

Structure

• amino group (-NH₂, free, substituted)
• carboxyl group (-COOH)
• other functional groups
  • hydroxyl -OH
  • sulfhydryl (mercapto group) -SH
  • sulphide -S-R
  • guanidyl
    

    guanidyl
  • phenyl etc.
    

    phenyl

Classification

• according to the structure of the side chain and functional groups
• according to side chain polarity
  • polar
  • non - polar
• according to importance in human nutrition
  • essential = the human organism is unable to create them endogenously
    • valin, leucine, isoleucine, phenylalanine, lysine, methionine, tryptophan, threonine
  • conditionally essential = essential in the absence of precursors or immaturity of enzymatic systems
    • arginine, histidine
  • completely non-essential
    • 

      glycine
      glycine, alanine, serine, cysteine, aspartic acid a asparagine, glutamic acid and glutamine, selenocysteine, tysorine, proline
      

      valin
      

      alanine
      

      leucine
      

      isoleucine

cysteine

threonine

serine

methionine

aspartic acid

Extended classification

primary amino group 19 α-amino acid with a primary amino group (-NH2) 1 α-amino acid with a secondary amino group (-NH-) n=0, pyrrolidin secondary amino group 18 amino acids = chiral compounds of the L series trivial names, systematic names, symbols (three-letter, one-letter) Classification of essential amino acids By side chain structure and functional groups aliphatic with an unsubstituted chain glycin , alanin , valin , leucin , isoleucin aliphatic hydroxyamino acids serin , threonin aliphatic sulfur cystein , methionin with a carboxyl group in the side chain (monoaminodicarboxylic, acidic) kyselina asparagová , kyselina glutamová their monoamides (with a carboxamide group in the side chain) asparagin , glutamin with basic groups in the side chain amino group guanidyl group imidazoyl cycle lysin , arginin , histidin with an aromatic (heterocyclic) side chain fenylalanin , tyrosin , tryptofan , prolin According to the polarity of the side chain and its ionic form (in a neutral environment) non-polar, hydrophobic Val, Leu, Ile, Phe, Tyr, Met, Pro; sometimes Gly, Ala, Trp (amphiphilic) glutamic acid polar, hydrophilic Ser, Thr, Cys, Asp, Glu, Asn, Gln, Lys, Arg, His Hydrophilic (according to the ionic form of the side chain in a neutral environment) neutral (has no electrical charge): most acidic (negative charge): Asp, Glu basic (positive charge): Lys, Arg, His

phenylalanine

arginine

lysine

glutamine

asparagine

histidine

tyrosine

tryptophan

proline

L-cystine

4-hydroxy-L-proline

5-hydroxy-L-lysine

3-methyl-L-histidine

O-phospho-L-serine

sarkosine

N,N,N-trimethylglycine

L-carnitine

Representatives

Derivatives of basic proteinogenic amino acids the emergence of specific modifications L-cystin (CySSCy) 4-hydroxy-L-prolin (Hyp) 5-hydroxy-L-lysin (Hyl) 3-methyl-L-histidin O-phospho-L-serine Other non-protein amino acids N-substituted α-amino acids N-methylglycin (sarkosin) , N,N-dimethylglycin , N,N,N-trimethylglycin L-carnitine (3-hydroxy-4-trimethylaminobutyrate, vitamin Bt) ß-alanine (3-aminopropionová kyselina) , γ-aminobutyric (4-aminobutyric) acid (GABA) Sulfur amino acids S-alk(en)yl-L-cysteiny , S-alk(en)yl-L-cysteinsulfoxidy N,N-dimethylglycine Basic amino acids and related compounds L-ornithin (n = 2) L-citrulline (n = 2, karbamoylderivát ornithinu) creatine-phosphate Aromatic amino acids tetrajodthyronin (thyroxin), R = R1 = R2 = R3 = I 3,4-dihydroxy-L-fenylalanin (DOPA) Essential Amino Acids foods deficient in certain amino acids Lys − cereals (plant proteins in general) Met − milk, meat Thr − wheat, rye Trp − casein, corn, rice

S-alk(en)yl-L-cysteinesulfoxid

S-alk(en)yl-L-cysteine

ß-alanine

L-ornithine

L-citrulline

creatine phosphate

thyroxine

DOPA

Physico-chemical properties

Physico-chemical properties acid-base (pK and pI) optical sensory Acid-base properties (Gly) ion I1 (kation) ion I2 (amfion) ion I3 (anion) free charge +1 free charge 0 free charge -1 pH < 2 pH ≈ 6 pH > 10 Dependence of the ionic forms of Gly on pH cation (I1) → amphion (I2) → anion (I3) Optical properties Gly = exception majority = chiral atom Cα... 2 optical isomers (enantiomers) some 2 chiral centers... Ile, Thr, Hyp, CySSCy L- and D-amino acids, L-amino acids = (S)-stereoisomers,výjimka: L-cysteine ​​= (R)-stereoisomer D-amino acids = (R)-stereoisomers Content L-amino acid ie.: (S)-amino acid D-amino acid ie.: (R)-amino acid Diastereoisomers of amino acids L-isoleucine (2S, 3S)-isoleucine D-isoleucine (2R, 3R)-isoleucine L-allo-isoleucine (2S, 3R)-isoleucine D-allo-isoleucine (2R, 3S)-isoleucine Organoleptic properties sweet - Gly, Ala, Thr, Pro acidic - Asp, Glu bitter - Leu, Ile, Phe, Tyr, Trp indifferent - others Unique properties = umami taste natrium-hydrogen-glutamát

Graph gly-ph

L-amino acid

D-amino acid

L-isoleucine

D-isoleucine

L-allo-isoleucine

D-allo-isoleucine

sodium hydrogen glutamate

Gly on pH

Links

Related articles

• Peptidy (1. LF UK, NT)
• Bílkoviny (1. LF UK, NT)
• Aminokyseliny, peptidy, bílkoviny (1. LF UK, NT)

Sources

• LF1. 2. AMINOKYSELINY, PEPTIDY, BÍLKOVINY [online]. [cit. online]. <https://el.lf1.cuni.cz/p51525121/>.

Použitá literatura

• VELÍŠEK, Jan – HAJŠLOVÁ, Jana. Chemie potravin 2. 3. edition. 2009. ISBN 978-80-86659-17-6.

• SVAČINA, Štěpán – ET AL.,. Klinická dietologie. 1. edition. 2008. ISBN 978-80-247-2256-6.