Editing Metabolism (section)

===Proteins===
{{further|Protein biosynthesis|Amino acid synthesis}}

Organisms vary in their ability to synthesize the 20 common amino acids. Most bacteria and plants can synthesize all twenty, but mammals can only synthesize eleven nonessential amino acids, so nine [[essential amino acid]]s must be obtained from food.<ref name="Nelson-2005"/> Some simple [[parasite]]s, such as the bacteria ''[[Mycoplasma pneumoniae]]'', lack all amino acid synthesis and take their amino acids directly from their hosts.<ref>{{cite journal | vauthors = Himmelreich R, Hilbert H, Plagens H, Pirkl E, Li BC, Herrmann R | title = Complete sequence analysis of the genome of the bacterium Mycoplasma pneumoniae | journal = Nucleic Acids Research | volume = 24 | issue = 22 | pages = 4420–49 | date = November 1996 | pmid = 8948633 | pmc = 146264 | doi = 10.1093/nar/24.22.4420 }}</ref> All amino acids are synthesized from intermediates in glycolysis, the citric acid cycle, or the pentose phosphate pathway. Nitrogen is provided by [[glutamate]] and [[glutamine]]. Nonessensial amino acid synthesis depends on the formation of the appropriate alpha-keto acid, which is then [[Transaminase|transaminated]] to form an amino acid.<ref>{{cite book | vauthors = Guyton AC, Hall JE |title=Textbook of Medical Physiology |url=https://archive.org/details/textbookmedicalp00acgu |url-access=limited |publisher=Elsevier |year=2006 |location=Philadelphia |pages=[https://archive.org/details/textbookmedicalp00acgu/page/n889 855]–6 |isbn=978-0-7216-0240-0}}</ref>

Amino acids are made into proteins by being joined in a chain of [[peptide bond]]s. Each different protein has a unique sequence of amino acid residues: this is its [[primary structure]]. Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked in varying sequences to form a huge variety of proteins. Proteins are made from amino acids that have been activated by attachment to a [[transfer RNA]] molecule through an [[ester]] bond. This [[aminoacyl-tRNA]] precursor is produced in an [[Adenosine triphosphate|ATP]]-dependent reaction carried out by an [[aminoacyl tRNA synthetase]].<ref>{{cite journal | vauthors = Ibba M, Söll D | title = The renaissance of aminoacyl-tRNA synthesis | journal = EMBO Reports | volume = 2 | issue = 5 | pages = 382–7 | date = May 2001 | pmid = 11375928 | pmc = 1083889 | doi = 10.1093/embo-reports/kve095 | url = http://www.molcells.org/home/journal/include/downloadPdf.asp?articleuid={A158E3B4-2423-4806-9A30-4B93CDA76DA0} | url-status = dead | archive-url = https://web.archive.org/web/20110501181419/http://www.molcells.org/home/journal/include/downloadPdf.asp?articleuid=%7BA158E3B4-2423-4806-9A30-4B93CDA76DA0%7D | archive-date = 1 May 2011 }}</ref> This aminoacyl-tRNA is then a substrate for the [[ribosome]], which joins the amino acid onto the elongating protein chain, using the sequence information in a [[messenger RNA]].<ref>{{cite journal | vauthors = Lengyel P, Söll D | title = Mechanism of protein biosynthesis | journal = Bacteriological Reviews | volume = 33 | issue = 2 | pages = 264–301 | date = June 1969 | pmid = 4896351 | pmc = 378322 | doi = 10.1128/MMBR.33.2.264-301.1969 }}</ref>