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2.7: Amino Acids

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    3785
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    Amino acids are the building blocks (monomers) of proteins. 20 different amino acids are used to synthesize proteins. The shape and other properties of each protein is dictated by the precise sequence of amino acids in it.

    Each amino acid consists of an alpha carbon atom to which is attached

    • a hydrogen atom
    • an amino group (hence "amino" acid)
    • a carboxyl group (-COOH). This gives up a proton and is thus an acid (hence amino "acid")
    • one of 20 different "R" groups. It is the structure of the R group that determines which of the 20 it is and its special properties. The amino acid shown here is Alanine
    Table 2.7.1: Types of Amino Acids. For each amino acid both three-letter and single letter codes are given
    Alanine Ala A hydrophobic
    Arginine Arg R free amino group makes it basic and hydrophilic
    Asparagine Asn N carbohydrate can be covalently linked ("N-linked) to its -NH
    Aspartic acid Asp D free carboxyl group makes it acidic and hydrophilic
    Cysteine Cys C oxidation of their sulfhydryl (-SH) groups link 2 Cys (S-S)
    Glutamic acid Glu E free carboxyl group makes it acidic and hydrophilic
    Glutamine Gln Q moderately hydrophilic
    Glycine Gly G so small it is amphiphilic (can exist in any surroundings)
    Histidine His H basic and hydrophilic
    Isoleucine Ile I hydrophobic
    Leucine Leu L hydrophobic
    Lysine Lys K strongly basic and hydrophilic
    Methionine Met M hydrophobic
    Phenylalanine Phe F very hydrophobic
    Proline Pro P causes kinks in the chain
    Serine Ser S carbohydrate can be covalently linked ("O-linked") to its -OH
    Threonine Thr T carbohydrate can be covalently linked ("O-linked") to its -OH
    Tryptophan Trp W scarce in most plant proteins
    Tyrosine Tyr Y a phosphate or sulfate group can be covalently attached to its -OH
    Valine Val V hydrophobic

    The Essential Amino Acids

    Humans must include adequate amounts of 9 amino acids in their diet.

    • Histidine
    • Isoleucine
    • Leucine
    • Lysine
    • Methionine (and/or cysteine)
    • Phenylalanine (and/or tyrosine)
    • Threonine
    • Tryptophan
    • Valine

    These "essential" amino acids cannot be synthesized from other precursors. However, cysteine can partially meet the need for methionine (they both contain sulfur), and tyrosine can partially substitute for phenylalanine. Two of the essential amino acids, lysine and tryptophan, are poorly represented in most plant proteins. Thus strict vegetarians should ensure that their diet contains sufficient amounts of these two amino acids. 19 of the 20 amino acids listed above can exist in two forms in three dimensions.

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