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22-AUGUST-2008 06:13:22 - synthesis June 2008 For the non-biological synthesis of amino acids see: Strecker amino acid synthesis Amino acid synthesis is the set of biochemical processes metabolic pathways by which the various amino acids are produced from other compounds. The substrates for these processes are various compounds in the organism's diet or growth media. Not all organisms are able to synthesise all amino acids, for example humans are only able to synthesise 12 of the 20 standard amino acids. A fundamental problem for biological systems is to obtain nitrogen in an easily usable form. This problem is solved by certain microorganisms capable of reducing the inert N≡N molecule nitrogen gas to two molecules of ammonia in one of the most remarkable reactions in biochemistry. Nitrogen in the form of ammonia is the source of nitrogen for all the amino acids. The carbon backbones come from the glycolytic pathway, the pentose phosphate pathway, or the citric acid cycle. In amino acid production, one encounters an important problem in biosynthesis - namely, stereochemical control. Because all amino acids except glycine are chiral, biosynthetic pathways must generate the correct isomer with high fidelity. In each of the 19 pathways for the generation of chiral amino acids, the stereochemistry at the α-carbon atom is established by a transamination reaction that involves pyridoxal phosphate. Almost all the transaminases that catalyze these reactions descend from a common ancestor, illustrating once again that effective solutions to biochemical problems are retained throughout evolution. Biosynthetic pathways are often highly regulated such that building blocks are synthesized only when supplies are low. Very often, a high concentration of the final product of a pathway inhibits the activity of enzymes that function early in the pathway. Often present are allosteric enzymes capable of sensing and responding to concentrations of regulatory species. These enzymes are similar in functional properties to aspartate transcarbamylase and its regulators. Feedback and allosteric mechanisms ensure that all twenty amino acids are maintained in sufficient amounts for protein synthesis and other processes. Contents 1 Amino acid synthesis 2 Nitrogen Fixation: Microorganisms Use ATP and a Powerful Reductant to Reduce Atmospheric Nitrogen to Ammonia 3 Amino Acids Are Made from Intermediates of the Citric Acid Cycle and Other Major Pathways 4 Amino Acid Biosynthesis Is Regulated by Feedback Inhibition 5 Amino Acids Are Precursors of Many Biomolecules 6 References 7 External links Amino acid synthesis Amino acids are synthesized from Glutamate, which is formed by amination of α-ketoglutarate: \alpha -ketoglutarate + NH_4^+ \rightleftarrows Glutamate Afterwards, Alanine and Aspartate are formed by transamination of Glutamate. All of the remaining amino acids are then constructed from Glutamate or Aspartate, by transamination of these two amino acids with one α-keto acid. Nitrogen Fixation: Microorganisms Use ATP and a Powerful Reductant to Reduce Atmospheric Nitrogen to Ammonia Microorganisms use ATP and reduced ferredoxin, a powerful reductant, to reduce N2 to NH3. An iron-molybdenum cluster in nitrogenase deftly catalyzes the fixation of N2, a very inert molecule. Higher organisms consume the fixed nitrogen to synthesize amino acids, nucleotides, and other nitrogen-containing biomolecules. The major points of entry of NH4+ into metabolism are glutamine or glutamate. Amino Acids Are Made from Intermediates of the Citric Acid Cycle and Other Major Pathways Human beings can synthesize 11 of the basic set of 20 amino acids. These amino acids are called nonessential, in contrast with the essential amino acids, which must be supplied in the diet. The pathways for the synthesis of nonessential amino acids are quite simple. Glutamate dehydrogenase catalyzes the reductive amination of α-ketoglutarate to glutamate. A transamination reaction takes place in the synthesis of most amino acids. At this step, the chirality of the amino acid is established. Alanine and aspartate are synthesized by the transamination of pyruvate and oxaloacetate, respectively. Glutamine is synthesized from NH4+ and glutamate, and asparagine is synthesized similarly. Proline and arginine are derived from glutamate. Serine, formed from 3-phosphoglycerate, is the precursor of glycine and cysteine. Tyrosine is synthesized by the hydroxylation of phenylalanine, an essential amino acid. The pathways for the biosynthesis of essential amino acids are much more complex than those for the nonessential ones. Tetrahydrofolate, a carrier of activated one-carbon units, plays an important role in the metabolism of amino acids and nucleotides. This coenzyme carries one-carbon units at three oxidation states, which are interconvertible: most reduced-methyl; intermediate-methylene; and most oxidized-formyl, formimino, and methenyl. The major donor of activated methyl groups is S-adenosylmethionine, which is synthesized by the transfer of an adenosyl group from ATP to the sulfur atom of methionine. S-Adenosylhomocysteine is formed when the activated methyl group is transferred to an acceptor. It is hydrolyzed to adenosine and homocysteine, the latter of which is then methylated to methionine to complete the activated methyl cycle. Amino Acid Biosynthesis Is Regulated by Feedback Inhibition Most of the pathways of amino acid biosynthesis are regulated by feedback inhibition, in which the committed step is allosterically inhibited by the final product. Branched pathways require extensive interaction among the branches that includes both negative and positive regulation. The regulation of glutamine synthetase from E. coli is a striking demonstration of cumulative feedback inhibition and of control by a cascade of reversible covalent modifications. Amino Acids Are Precursors of Many Biomolecules Amino acids are precursors of a variety of biomolecules. Glutathione γ-Glu-Cys-Gly serves as a sulfhydryl buffer and detoxifying agent. Glutathione peroxidase, a selenoenzyme, catalyzes the reduction of hydrogen peroxide and organic peroxides by glutathione. Nitric oxide, a short-lived messenger, is formed from arginine. Porphyrins are synthesized from glycine and succinyl CoA, which condense to give δ-aminolevulinate. Two molecules of this intermediate become linked to form porphobilinogen. Four molecules of porphobilinogen combine to form a linear tetrapyrrole, which cyclizes to uroporphyrinogen III. Oxidation and side-chain modifications lead to the synthesis of protoporphyrin IX, which acquires an iron atom to form heme. 1 References ^ Biochemistry. Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert. New York: W. H. Freeman and Co. ; c2002 External links NCBI Bookshelf Free Textbook Access v d e Protein biosynthesis Biochemical Processes Amino acid synthesis - tRNA synthesis Molecular Biology Processes Transcription - Post-transcriptional modification - Translation - Regulation of gene expression v d e Protein metabolism: protein metabolism Protein synthesis - Amino acid synthesis - Catabolism Nucleotide metabolism: Purine metabolism - Nucleotide salvage - Pyrimidine metabolism v d e Metabolism: amino acid metabolism - synthesis and catabolism enzymes essential in CAPS K→acetyl-CoA LYSINE→ Saccharopine dehydrogenase - Glutaryl-CoA dehydrogenase LEUCINE→ Branched chain aminotransferase - Branched-chain alpha-keto acid dehydrogenase complex - Isovaleryl coenzyme A dehydrogenase - Methylcrotonyl-CoA carboxylase - Methylglutaconyl-CoA hydratase - 3-hydroxy-3-methylglutaryl-CoA lyase TRYPTOPHAN→ Indoleamine 2,3-dioxygenase/Tryptophan 2,3-dioxygenase - Arylformamidase - Kynureninase - 3-hydroxyanthranilate oxidase - Aminocarboxymuconate-semialdehyde decarboxylase - Aminomuconate-semialdehyde dehydrogenase PHENYLALANINE→tyrosine→ see below G→pyruvate glycine→serine→ Serine hydroxymethyltransferase - Serine dehydratase alanine→ Alanine transaminase cysteine→ D-cysteine desulfhydrase threonine→ L-threonine dehydrogenase G→glutamate HISTIDINE→ Histidine ammonia-lyase - Urocanate hydratase - Formiminotransferase cyclodeaminase proline→ Proline oxidase - Pyrroline-5-carboxylate reductase - 1-Pyrroline-5-carboxylate dehydrogenase/ALDH4A1 - PYCR1 arginine→ Ornithine aminotransferase - Ornithine decarboxylase - Agmatinase →alpha-ketoglutarate→TCA Glutamate dehydrogenase G→propionyl-CoA VALINE→ Branched chain aminotransferase - Branched-chain alpha-keto acid dehydrogenase complex - Enoyl-CoA hydratase - 3-hydroxyisobutyryl-CoA hydrolase - 3-hydroxyisobutyrate dehydrogenase - Methylmalonate semialdehyde dehydrogenase ISOLEUCINE→ Branched chain aminotransferase - Branched-chain alpha-keto acid dehydrogenase complex - 3-hydroxy-2-methylbutyryl-CoA dehydrogenase METHIONINE→ Methionine adenosyltransferase - Adenosylhomocysteinase - Cystathionine beta synthase - Cystathionine gamma-lyase methionine synthesis: MTR/Homocysteine methyltransferase THREONINE→ Threonine aldolase →succinyl-CoA→TCA Propionyl-CoA carboxylase - Methylmalonyl CoA epimerase - Methylmalonyl-CoA mutase G→fumarate PHENYLALANINE→tyrosine→ Phenylalanine hydroxylase - Tyrosine aminotransferase - 4-Hydroxyphenylpyruvate dioxygenase - Homogentisate 1,2-dioxygenase - Fumarylacetoacetate hydrolase G→oxaloacetate asparagine→aspartate→ Asparaginase/Asparagine synthetase - Aspartate transaminase Other tyrosine→melanin Tyrosinase cysteine+glutamate→glutathione Gamma-glutamylcysteine synthetase - Glutathione synthetase - Gamma-glutamyl transpeptidase glycine→creatine Guanidinoacetate N-methyltransferase - Creatine kinase glutamate→glutamine Glutamine synthetase - Glutaminase proline+lysine→collagen Prolyl hydroxylase - Lysyl hydroxylase see also disorders, intermediates This article about metabolism is a stub. Retrieved from http://en..org/wiki/Amino_acid_synthesis Categories: Biochemistry stubs | MetabolismHidden category: Articles needing additional references from June 2008 Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages 日本語 This page was last modified on 23 July 2008, at 01:03
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