- Amino acid synthesis
"For the non-biological synthesis of amino acids see:
Strecker amino acid synthesis"
Amino acid synthesis is the set of
biochemicalprocesses ( 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
nitrogenin 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 ammoniain 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
transaminationreaction 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 transcarbamylaseand its regulators. Feedback and allosteric mechanisms ensure that all twenty amino acids are maintained in sufficient amounts for protein synthesis and other processes.
Amino acid synthesis
Amino acids are synthesized from
Glutamate, which is formed by aminationof α-ketoglutarate:
Alanineand Aspartateare formed by transaminationof Glutamate.All of the remaining amino acids are then constructed from Glutamateor Aspartate, by transaminationof these two amino acids with one α-keto acid.
Nitrogen Fixation: Microorganisms Use ATP and a Powerful Reductant to Reduce Atmospheric Nitrogen to Ammonia
ATPand 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 glutamineor 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.
Alanineand aspartateare synthesized by the transamination of pyruvateand oxaloacetate, respectively. Glutamine is synthesized from NH4+ and glutamate, and asparagineis synthesized similarly. Prolineand arginineare derived from glutamate. Serine, formed from 3-phosphoglycerate, is the precursor of glycineand cysteine. Tyrosineis 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 coenzymecarries 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. coliis 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 peroxideand organic peroxides by glutathione. Nitric oxide, a short-lived messenger, is formed from arginine. Porphyrinsare 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. Oxidationand side-chain modifications lead to the synthesis of protoporphyrin IX, which acquires an iron atom to form heme. [Biochemistry. Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert. New York: W. H. Freeman and Co. ; c2002]
* [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.TOC&depth=2 NCBI Bookshelf Free Textbook Access]
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