= Amino acid
{wiki}
An <amino acid> combines <carboxylic acid> and <amine> functionality in the same <molecule>; the name alone does not specify their separation. An <alpha-amino acid> has its amino <nitrogen> attached to the <carbon> immediately adjacent to the carboxyl <carbon>, the <amino acid alpha carbon>. For example, $\mathrm{H_2N{-}CH_2{-}COOH}$ is <glycine>, whereas $\mathrm{H_2N{-}CH_2{-}CH_2{-}COOH}$ has an extra intervening <carbon> and is not an <alpha-amino acid>. These <chemical formulas> describe connectivity, not the predominant <protonation> state in <water>.
Most common <proteinogenic amino acids> share the skeleton $\mathrm{H_2N{-}CH}(R)\mathrm{{-}COOH}$, with $R$ the <amino acid side chain>. In <glycine>, $R$ is <hydrogen>; in <alanine>, $R=\mathrm{CH_3}$. <Proline> is a useful exception to the displayed primary-<amine> skeleton: its amino <nitrogen> is part of a ring and has a second <carbon> substituent. The shared backbone permits assembly into <polypeptides>, while different <amino acid side chains> give the resulting <proteins> diverse chemistry. <Nomenclature and Symbolism for Amino Acids and Peptides (Recommendations 1983) by IUPAC-IUB Joint Commission on Biochemical Nomenclature>[3AA-1, Table 1: glycine, alanine and proline structures; 3AA-2.2.5, backbone and side chain]{ref}
In a <chemical solution>, <protonation> changes the <electric charges> without changing this backbone. Free <glycine> near neutral <pH> is predominantly $\mathrm{H_3N^+{-}CH_2{-}COO^-}$, a <zwitterion>. Its positive and negative groups are joined within one <molecule>; zero net <electric charge> does not mean absence of local <electric charges>, nor does it mean the uncharged drawing $\mathrm{H_2N{-}CH_2{-}COOH}$ predominates. <Glycine protonation> explains how lowering or raising <pH> changes the balance. <Organic Chemistry by John McMurry>[§26.1, opening zwitterion and acid/base interconversion discussion]{ref}
Building a <protein> joins the carboxyl <carbon> of one <amino acid> to the amino <nitrogen> of another through a <peptide bond>, an <amide> linkage. The formal net condensation loses the elements of <water>; cellular <translation> uses activated substrates at a <ribosome>, rather than simply mixing free <amino acids> in <water>. Incorporated units are <amino acid residues>. The interior <amide> groups no longer have the same acid/base behavior as the two free terminal groups of <glycine>, so its free-<molecule> <glycine titration> cannot be copied independently at every chain position. <Organic Chemistry by John McMurry>[§26.4, opening peptide formation and amide resonance discussion]{ref}
<Chirality (chemistry)> distinguishes most common <proteinogenic amino acids> from <glycine>. The other nineteen members of the conventional twenty have a <stereocenter> at their <amino acid alpha carbon>; ordinary <translation> incorporates their L forms. <Glycine>'s two identical <hydrogen> substituents make it achiral. Thus being an <amino acid>, or even an <alpha-amino acid>, does not by itself imply <chirality (chemistry)>. <Nomenclature and Symbolism for Amino Acids and Peptides (Recommendations 1983) by IUPAC-IUB Joint Commission on Biochemical Nomenclature>[3AA-3.2, note: glycine itself is achiral; 3AA-3.3 and 3AA-3.5, protein-source L configuration]{ref}
Back to article page
