Amino Acid
<p >An alpha-amino acid has the generic <a href="http://en.wikipedia.org/wiki/Chemical_formula" title="Chemical formula" >formula</a> H<sub >2</sub>NCHRCOOH, where R is an organic <a href="http://en.wikipedia.org/wiki/Substituent" title="Substituent" >substituent</a>;<sup id="cite_ref-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Amino_acid#cite_note-0" >[1]</a></sup> the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (the <a href="http://en.wikipedia.org/wiki/Alpha_carbon" title="Alpha carbon" class="mw-redirect" >α–carbon</a>). Other types of amino acid exist when the amino group is attached to a different carbon atom; for example, in gamma-amino acids (such as<a href="http://en.wikipedia.org/wiki/Gamma-amino-butyric_acid" title="Gamma-amino-butyric acid" class="mw-redirect" >gamma-amino-butyric acid</a>) the carbon atom to which the amino group attaches is separated from the carboxylate group by two other carbon atoms. The various alpha-amino acids differ in which <a href="http://en.wikipedia.org/wiki/Side-chain" title="Side-chain" class="mw-redirect" >side-chain</a> (R-group) is attached to their alpha carbon, and can vary in size from just one<a href="http://en.wikipedia.org/wiki/Hydrogen" title="Hydrogen" >hydrogen</a> atom in <a href="http://en.wikipedia.org/wiki/Glycine" title="Glycine" >glycine</a> to a large <a href="http://en.wikipedia.org/wiki/Heterocycle" title="Heterocycle" class="mw-redirect" >heterocyclic group</a> in <a href="http://en.wikipedia.org/wiki/Tryptophan" title="Tryptophan" >tryptophan</a>.</p>
<p>
Amino acids serve as the building blocks of <a href="http://en.wikipedia.org/wiki/Protein" title="Protein" >proteins</a>, which are linear chains of amino acids. Amino acids can be linked together in varying sequences to form a vast variety of proteins.<sup id="cite_ref-NIGMS_1-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Amino_acid#cite_note-NIGMS-1" >[2]</a></sup>
Twenty amino acids are naturally incorporated into polypeptides and are called proteinogenic or standard amino acids. These 20 are encoded by the universal <a href="http://en.wikipedia.org/wiki/Genetic_code" title="Genetic code" >genetic code</a>. </p>
<p>
Nine standard amino acids are called "<a href="http://en.wikipedia.org/wiki/Amino_acid#In_human_nutrition" title="Amino acid" >essential</a>" for humans because they cannot be created from other <a href="http://en.wikipedia.org/wiki/Chemical_compound" title="Chemical compound" >compounds</a> by the human body, and so must be taken in as food.</p>
<p>
Amino acids are important in <a href="http://en.wikipedia.org/wiki/Nutrition" title="Nutrition" >nutrition</a> and are commonly used in nutrition supplements, fertilizers, <a href="http://en.wikipedia.org/wiki/Food_technology" title="Food technology" >food technology</a> and <a href="http://en.wikipedia.org/wiki/Industry" title="Industry" >industry</a>. In industry, applications include the production of <a href="http://en.wikipedia.org/wiki/Biodegradable_plastic" title="Biodegradable plastic" >biodegradable plastics</a>, <a href="http://en.wikipedia.org/wiki/Drug" title="Drug" >drugs</a>, and<a href="http://en.wikipedia.org/wiki/Asymmetric_catalysis" title="Asymmetric catalysis" class="mw-redirect" >chiral catalysts</a>.</p>
<p>
When taken up into the human body from the diet, the 22 standard amino acids either are used to synthesize proteins and other biomolecules or are oxidized to <a href="http://en.wikipedia.org/wiki/Urea" title="Urea" >urea</a> and <a href="http://en.wikipedia.org/wiki/Carbon_dioxide" title="Carbon dioxide" >carbon dioxide</a> as a source of energy.<sup id="cite_ref-30" class="reference" ><a href="http://en.wikipedia.org/wiki/Amino_acid#cite_note-30" >[31]</a></sup> The oxidation pathway starts with the removal of the amino group by a <a href="http://en.wikipedia.org/wiki/Transaminase" title="Transaminase" >transaminase</a>, the amino group is then fed into the <a href="http://en.wikipedia.org/wiki/Urea_cycle" title="Urea cycle" >urea cycle</a>. The other product of transamidation is a <a href="http://en.wikipedia.org/wiki/Keto_acid" title="Keto acid" >keto acid</a> that enters the <a href="http://en.wikipedia.org/wiki/Citric_acid_cycle" title="Citric acid cycle" >citric acid cycle</a>.<sup id="cite_ref-31" class="reference" ><a href="http://en.wikipedia.org/wiki/Amino_acid#cite_note-31" >[32]</a></sup><a href="http://en.wikipedia.org/wiki/Glucogenic_amino_acid" title="Glucogenic amino acid" >Glucogenic amino acids</a> can also be converted into glucose, through <a href="http://en.wikipedia.org/wiki/Gluconeogenesis" title="Gluconeogenesis" >gluconeogenesis</a>.<sup id="cite_ref-32" class="reference" ><a href="http://en.wikipedia.org/wiki/Amino_acid#cite_note-32" >[33]</a></sup></p>
<p>
Pyrrolysine trait is restricted to several microbes, and only one organism has both Pyl and Sec. </p>
<p>
Of the 22 standard amino acids, 9 are called <a href="http://en.wikipedia.org/wiki/Essential_amino_acid" title="Essential amino acid" >essential amino acids</a> because the <a href="http://en.wikipedia.org/wiki/Human_body" title="Human body" >human body</a> cannot <a href="http://en.wikipedia.org/wiki/Biosynthesis" title="Biosynthesis" >synthesize</a> them from other <a href="http://en.wikipedia.org/wiki/Chemical_compound" title="Chemical compound" >compounds</a> at the level needed for normal growth, so they must be obtained from food.<sup id="cite_ref-33" class="reference" ><a href="http://en.wikipedia.org/wiki/Amino_acid#cite_note-33" >[34]</a></sup> </p>
<p>
In addition, <a href="http://en.wikipedia.org/wiki/Cysteine" title="Cysteine" >cysteine</a>, <a href="http://en.wikipedia.org/wiki/Taurine" title="Taurine" >taurine</a>, <a href="http://en.wikipedia.org/wiki/Tyrosine" title="Tyrosine" >tyrosine</a>, and <a href="http://en.wikipedia.org/wiki/Arginine" title="Arginine" >arginine</a> are semiessential amino-acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed.<sup id="cite_ref-34" class="reference" ><a href="http://en.wikipedia.org/wiki/Amino_acid#cite_note-34" >[35]</a></sup><sup id="cite_ref-35" class="reference" ><a href="http://en.wikipedia.org/wiki/Amino_acid#cite_note-35" >[36]</a></sup> The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids.
</p>
<table class="wikitable" ><tbody>
<tr>
<th >Essential</th>
<th >Nonessential</th>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Histidine" title="Histidine" >Histidine</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Alanine" title="Alanine" >Alanine</a></td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Isoleucine" title="Isoleucine" >Isoleucine</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Arginine" title="Arginine" >Arginine</a>*</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Leucine" title="Leucine" >Leucine</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Asparagine" title="Asparagine" >Asparagine</a></td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Lysine" title="Lysine" >Lysine</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Aspartic_acid" title="Aspartic acid" >Aspartic acid</a></td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Methionine" title="Methionine" >Methionine</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Cysteine" title="Cysteine" >Cysteine</a>*</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Phenylalanine" title="Phenylalanine" >Phenylalanine</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Glutamic_acid" title="Glutamic acid" >Glutamic acid</a></td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Threonine" title="Threonine" >Threonine</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Glutamine" title="Glutamine" >Glutamine</a>*</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Tryptophan" title="Tryptophan" >Tryptophan</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Glycine" title="Glycine" >Glycine</a></td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Valine" title="Valine" >Valine</a></td>
<td ><a href="http://en.wikipedia.org/wiki/Ornithine" title="Ornithine" >Ornithine</a>*</td>
</tr>
<tr>
<td > </td>
<td ><a href="http://en.wikipedia.org/wiki/Proline" title="Proline" >Proline</a>*</td>
</tr>
<tr>
<td > </td>
<td ><a href="http://en.wikipedia.org/wiki/Selenocysteine" title="Selenocysteine" >Selenocysteine</a>*</td>
</tr>
<tr>
<td > </td>
<td ><a href="http://en.wikipedia.org/wiki/Serine" title="Serine" >Serine</a>*</td>
</tr>
<tr>
<td > </td>
<td ><a href="http://en.wikipedia.org/wiki/Taurine" title="Taurine" >Taurine</a>*</td>
</tr>
<tr>
<td > </td>
<td ><a href="http://en.wikipedia.org/wiki/Tyrosine" title="Tyrosine" >Tyrosine</a>*</td>
</tr>
</tbody></table>
<p><br class="Apple-interchange-newline" />
</p>