Memorize and Draw the 20 Amino Acids
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  • What is the structure of Histidine?

    Answer

    Chemical structure shown in the video (side chain depicted in the displayed structure).

    aminoacid histidine

  • What is the structure of Phenylalanine?

    Answer

    Phenyl group attached to alanine โ€” described in the video as 'Phenyl ON alanine' (structure shown).

    aminoacid phenylalanine nonpolar

  • What is the structure of Tyrosine?

    Answer

    Chemical structure shown in the video (mnemonic shown: 'Tyrosine = Tire').

    aminoacid tyrosine

  • What is the structure of Glycine?

    Answer

    No distinct side chain; only a hydrogen attached to the alpha carbon, so no side chain / R group and the residue is achiral.

    aminoacid glycine special

  • What is the structure of Alanine?

    Answer
    • Methyl side chain (CH3); smallest side chain, aliphatic and hydrophobic
    aminoacid alanine hydrophobic

  • What is the structure of Valine?

    Answer
    • Isopropyl side chain (branched, drawn as an upside-down V); aliphatic and hydrophobic
    aminoacid valine hydrophobic

  • What is the structure of Isoleucine?

    Answer
    • Sec-butyl side chain; constitutional isomer of leucine and contains a second chiral center
    aminoacid isoleucine hydrophobic

  • What is the structure of Methionine?

    Answer
    • Side chain containing a sulfur atom; associated with start codon AUG
    aminoacid methionine sulfur

  • What is the structure of Tryptophan?

    Answer

    Side chain is a bicyclic indole ring (two fused rings); abbreviated Trp, W; described as 'tricky'.

    aminoacid aromatic indole

  • What is the structure of Proline?

    Answer

    Side chain forms a ring that includes the amine nitrogen (cyclic secondary amine); cyclic and hydrophobic; called 'tricky one'.

    aminoacid cyclic hydrophobic

  • What is the structure of Cysteine?

    Answer
    • Side chain: contains a thiol group (-SH).
    cysteine aminoacid thiol

  • What is the structure of Asparagine (Asn, N)?

    Answer
    • Side chain: CH2-CO-NH2 (an amide functional group attached via a methylene).
    asparagine aminoacid amide

  • What is the structure of Glutamine (Gln, Q)?

    Answer
    • Side chain: CH2-CH2-CO-NH2 (a longer amide, two methylenes then an amide).
    glutamine aminoacid amide

  • What is the structure of Arginine (Arg, R)?

    Answer
    • Side chain: contains a guanidinium group (positively charged functional group).
    arginine aminoacid guanidinium

  • What is the structure of histidine's side chain?

    Answer

    An imidazole ring containing a nitrogen that can accept a proton.

    aminoacid histidine sidechain

  • What are the standard abbreviations for histidine?

    Answer
    • His
    • H
    aminoacid histidine abbreviation

  • What is the bonding and lone-pair status of histidine's side-chain nitrogen?

    Answer

    It forms four bonds and has no lone pairs on the side-chain nitrogen.

    histidine bonding structure

  • Which atom in histidine's side chain can accept a proton?

    Answer

    The basic nitrogen in the imidazole ring can accept a proton.

    histidine acidbase sidechain
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Overview

Concise guide to the 20 common amino acids: backbone structure, Rโ€‘group classes (nonpolar, polar uncharged, acidic, basic), abbreviations, mnemonics, and special cases.

Amino acid backbone and general notes

  • General zwitterion form at physiological pH: \(\mathrm{H_3N^+}-\mathrm{CH(R)}-\mathrm{COO^-}\).
  • Each amino acid differs by its side chain (R group). Most side chains begin with \(\mathrm{CH_2}\) or \(\mathrm{CH}\).
  • Chirality: Except glycine, the alpha carbon is chiral; some side chains introduce additional stereocenters (e.g., threonine, isoleucine).

Nonpolar (hydrophobic) amino acids

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  • Glycine (Gly, G): R = H; achiral; smallest; very flexible.
  • Alanine (Ala, A): R = CH3; small, aliphatic, hydrophobic.
  • Valine (Val, V): isopropyl side chain; branched, hydrophobic.
  • Leucine (Leu, L): isobutyl side chain; hydrophobic, common in cores.
  • Isoleucine (Ile, I): secโ€‘butyl side chain; constitutional isomer of leucine; has a second chiral center.
  • Methionine (Met, M): contains a thioether (S); hydrophobic; encoded by start codon AUG.
  • Phenylalanine (Phe, F): benzyl side chain ("phenyl on alanine"); aromatic and hydrophobic.
  • Tryptophan (Trp, W): bicyclic indole ring; large, aromatic, relatively hydrophobic; "tricky".
  • Proline (Pro, P): side chain covalently bonds to backbone N (secondary amine), forms a ring; rigid, introduces kinks.

Polar, uncharged amino acids (mostly hydrophilic)

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  • Serine (Ser, S): CH2OH (alcohol); polar and hydrophilic.
  • Threonine (Thr, T): CH(OH)CH3; alcohol contains a second chiral center; polar.
  • Tyrosine (Tyr, Y): phenolic OH on a benzene ("Tireโ€‘OH"); polar group but bulky aromatic ring makes it borderline hydrophobic.
  • Cysteine (Cys, C): thiol (SH); polar-ish; can form disulfide bonds (\(\mathrm{R-S-S-R}\)) under oxidizing conditions.
  • Asparagine (Asn, N) and Glutamine (Gln, Q): amide side chains (CH2โ€‘CONH2 and CH2โ€‘CH2โ€‘CONH2); polar but not basic.

Acidic and basic amino acids

  • Acid definition: proton donor; Base definition: proton acceptor.
  • Mnemonics: "Acids Give Away" (acidic lose H+) and "Bases Adopt" (bases accept H+).

Acidic (deprotonated at physiological pH): - Aspartate (Asp, D): CH2โ€‘COOโˆ’ (from aspartic acid). - Glutamate (Glu, E): CH2โ€‘CH2โ€‘COOโˆ’ (from glutamic acid); "E" is longer.

Basic (positively charged at physiological pH): - Lysine (Lys, K): long aliphatic chain ending in \(\mathrm{NH_3^+}\); strongly basic. - Arginine (Arg, R): guanidinium group; highly basic and resonanceโ€‘stabilized (described jokingly as "pirate"/anchor). - Histidine (His, H): imidazole ring; side chain nitrogen can accept a proton near physiological pH, making histidine important in enzyme active sites.

Useful tips & special points

  • Many mnemonic clues from the source: "phenyl ON alanine" (Phenylalanine), "Tire" for Tyrosine, "Tricky" for Tryptophan.
  • Proline is sometimes called an "imino acid" because its side chain links back to the backbone N, restricting phi angle.
  • Glycine is the only achiral amino acid and provides flexibility in tight regions of proteins.
  • Cysteine can form disulfide bonds that stabilize tertiary/quaternary structure.
  • Methionine is the encoded start residue (AUG) in eukaryotic and prokaryotic translation.

Quick reference table

Name 3โ€‘letter 1โ€‘letter Side chain / key property
Glycine Gly G H (achiral, flexible)
Alanine Ala A CH3 (small, hydrophobic)
Valine Val V isopropyl (branched)
Leucine Leu L isobutyl (hydrophobic)
Isoleucine Ile I secโ€‘butyl (isomer of Leu)
Methionine Met M thioether (start codon)
Phenylalanine Phe F benzyl (aromatic)
Tryptophan Trp W indole (bicyclic)
Proline Pro P cyclic secondary amine (rigid)
Serine Ser S CH2OH (polar)
Threonine Thr T CH(OH)CH3 (polar, chiral)
Tyrosine Tyr Y phenol (polar + aromatic)
Cysteine Cys C SH (forms disulfides)
Asparagine Asn N CONH2 (amide, polar)
Glutamine Gln Q longer CONH2 (polar)
Aspartate Asp D COOโˆ’ (acidic)
Glutamate Glu E COOโˆ’ (acidic, longer)
Lysine Lys K long chain + NH3+ (basic)
Arginine Arg R guanidinium (very basic)
Histidine His H imidazole (nearโ€‘physiological pKa)

Study/practice suggestions

  • Memorize groups first (nonpolar, polar, acidic, basic) then learn individual side chains and abbreviations.
  • Use the mnemonics provided to recall lists and the short phonetic clues for specific residues.
  • Practice drawing or visualizing the side chains for distinguishing aromatic, sulfurโ€‘containing, and charged residues.