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9.3: Amino Acids

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  • Athletics are very competitive these days at all levels, from school sports to the pros. Everybody is looking for that edge that will make them faster, stronger, and more physically fit. One approach taken by many athletes is the use of amino acid supplements. The theory is that the increase in amino acids in the diet will lead to increased protein for muscles. However, the only real benefit comes to the people who make and sell the pills. Studies have not shown any advantage obtained by the athletes themselves; they are better off just maintaining a healthy diet.

    Amino Acids

    An amino acid is a compound that contains both an amine group \(\left( \ce{-NH_2} \right)\) and a carboxyl group \(\left( \ce{-COOH} \right)\) in the same molecule. While any number of amino acids can be imagined, biochemists generally reserve the term for a group of 20 amino acids which are formed and used by living organisms. The figure below shows the general structure of an amino acid.

    Basic structure of an amino acid
    Figure \(\PageIndex{1}\): An amino acid is an organic molecule that contains an amine group, a carboxyl group, and a side chain \(\left( \ce{R} \right)\), all bonded to a central carbon atom.

    The amino acid and carboxyl group of an amino acid are both covalently bonded to a central carbon atom. That carbon atom is also bonded to a hydrogen atom and an \(\ce{R}\) group. It is this \(\ce{R}\) group which varies from one amino acid to another and is called the amino acid side chain.

    The nature of the side chains accounts for the variability in physical and chemical properties of the different amino acids. Some side chains consist of nonpolar aliphatic or aromatic hydrocarbons. Other side chains are polar, while some are acidic or basic.

    Structure of five biologically relevant amino acids

     

    Figure \(\PageIndex{2}\): Five of the twenty biologically relevant amino acids, each having a distinct side chain \(\left( \ce{R} \right)\). Alanine's side chain is nonpolar, while threonine's is polar. Tryptophan is one of several amino acids whose side chain is aromatic. Aspartic acid has an acidic side chain, while lysine has a basic side chain.

    The table below lists the names of the 20 naturally occurring amino acids along with a three-letter abbreviation which is used to describe sequences of linked amino acids.

    Table \(\PageIndex{1}\): Amino acids and Abbreviations

    Amino Acid Abbreviation Amino Acid Abbreviation
    Alanine Ala Leucine Leu
    Arginine Arg Lysine Lys
    Asparagine Asp Methionine Met
    Aspartic acid Asp Phenylalanine Phe
    Cysteine Cys Proline Pro
    Glutamine Gln Serine Ser
    Glutamic acid Glu Threonine Thr
    Glycine Gly Tryptophan Trp
    Histidine His Tyrosine Tyr
    Isoleucine Ile Valine Val

    Another more recent set of abbreviations employs only one letter. Leucine would be designated by L, serine by S, tyrosine by Y. The advantage of this system comes when listing the amino acid sequence of a protein that may contain over 100 amino acids in its chain.

    Summary

    • An amino acid is a compound that contains both an amine group \(\left( \ce{-NH_2} \right)\) and a carboxyl group \(\left( \ce{-COOH} \right)\) in the same molecule.
    • The nature of an amino acid's side chain accounts for the variability in chemical and physical properties of amino acids.
    • There are 20 amino acids formed and used by living organisms.

    Contributors and Attributions

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

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