Amino acids have important roles in living rebecca/153A/W11/Lectures/153A_W11_Lec04¢ ...

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Transcript of Amino acids have important roles in living rebecca/153A/W11/Lectures/153A_W11_Lec04¢ ...

  • Amino acids have important roles in living

    organisms

    Subunits (building blocks) of peptides and proteins

    Neurotransmitters Metabolic intermediates

    C

    NH3 +

    O

    O -

    glutamate γ-aminobutyric acid

    (GABA)

  • Proteins are synthesized from 20 ‘standard’ α-amino

    acids. Their names have 3- and 1-letter abbreviations.

    Alanine Ala A Methionine Met M

    Cysteine Cys C Asparagine Asn N

    Aspartate Asp D Proline Pro P

    Glutamate Glu E Glutamine Gln Q

    Phenylalanine Phe F Arginine Arg R

    Glycine Gly G Serine Ser S

    Histidine His H Threonine Thr T

    Isoleucine Ile I Valine Val V

    Lysine Lys K Tryptophan Trp W

    Leucine Leu L Tyrosine Tyr Y

  • Amino acid pKa’s vary because of attached functional

    groups (microenvironment influences acidity)

  • Most amino acids are chiral, and are designated

    ‘D’ or ‘L’ based on Emil Fischer’s nomenclature

  • The ‘Fischer projection’ is a simplified way to

    depict stereochemistry

    Vertical bonds point

    away from viewer

    (dashed wedges)

    Horizontal bonds

    point toward viewer

    (filled wedges)

    Chiral carbons may

    be left out entirely

    Fisher projection

  • How to identify the stereochemistry of an amino

    acid (D or L?)

    The CORN rule

    Fisher projection

    1. Orient the structure to look

    down the C α -H bond

    2. Follow the other C α

    substituents to spell CORN

    (carboxylate, R-group,

    amino): counterclockwise is L,

    clockwise is D

    L-amino acid: D-amino acid:

    L-amino acid: D-amino acid: COO

    -

    R

    H + H3N

    COO -

    R

    NH3 +

    H

    If projection is aligned with:

    • carbon chain vertical

    • carboxylate on top

    amino group on left = L

    amino group on right = D

    C

    NH3 +

    R COO -

    H C

    NH3 +

    - OOC R

    H

  • Nonstandard amino acids are found in modified

    proteins and as free metabolites