The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of biochemical reactions to release the energy stored in nutrients through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The chemical energy released is available under the form of ATP. The Krebs cycle is used by organisms that respire (as opposed to organisms that ferment) to generate energy, either by anaerobic respiration or aerobic respiration. In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, that are used in numerous other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest components of metabolism. Even though it is branded as a "cycle", it is not necessary for metabolites to follow only one specific route; at least three alternative segments of the citric acid cycle have been recognized.
Methionine is an essential amino acid in humans.
Lysine is an α-amino acid that is a precursor to many proteins. It contains an α-amino group, an α-carboxylic acid group, and a side chain lysyl, classifying it as a basic, charged, aliphatic amino acid. It is encoded by the codons AAA and AAG. Like almost all other amino acids, the α-carbon is chiral and lysine may refer to either enantiomer or a racemic mixture of both. For the purpose of this article, lysine will refer to the biologically active enantiomer L-lysine, where the α-carbon is in the S configuration.
Threonine is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group, a carboxyl group, and a side chain containing a hydroxyl group, making it a polar, uncharged amino acid. It is essential in humans, meaning the body cannot synthesize it: it must be obtained from the diet. Threonine is synthesized from aspartate in bacteria such as E. coli. It is encoded by all the codons starting AC.
In molecular biology, biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. This process often consists of metabolic pathways. Some of these biosynthetic pathways are located within a single cellular organelle, while others involve enzymes that are located within multiple cellular organelles. Examples of these biosynthetic pathways include the production of lipid membrane components and nucleotides. Biosynthesis is usually synonymous with anabolism.
Homoserine (also called isothreonine) is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH2OH. L-Homoserine is not one of the common amino acids encoded by DNA. It differs from the proteinogenic amino acid serine by insertion of an additional -CH2- unit into the backbone. Homoserine, or its lactone form, is the product of a cyanogen bromide cleavage of a peptide by degradation of methionine.
Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) (EC 1.1.1.49) is a cytosolic enzyme that catalyzes the chemical reaction
Amino acid biosynthesis is the set of biochemical processes by which the amino acids are produced. The substrates for these processes are various compounds in the organism's diet or growth media. Not all organisms are able to synthesize all amino acids. For example, humans can synthesize 11 of the 20 standard amino acids. These 11 are called the non-essential amino acids).
In enzymology, a shikimate dehydrogenase (EC 1.1.1.25) is an enzyme that catalyzes the chemical reaction
In enzymology, a homoserine dehydrogenase (EC 1.1.1.3) is an enzyme that catalyzes the chemical reaction
In enzymology, a glutamate-5-semialdehyde dehydrogenase (EC 1.2.1.41) is an enzyme that catalyzes the chemical reaction
In enzymology, a L-aminoadipate-semialdehyde dehydrogenase (EC 1.2.1.31) is an enzyme that catalyzes the chemical reaction
In enzymology, a N-acetyl-gamma-glutamyl-phosphate reductase (EC 1.2.1.38) is an enzyme that catalyzes the chemical reaction
In enzymology, a 2,4-diaminopentanoate dehydrogenase (EC 1.4.1.12) is an enzyme that catalyzes the chemical reaction
In enzymology, a saccharopine dehydrogenase (NADP+, L-glutamate-forming) (EC 1.5.1.10) is an enzyme that catalyzes the chemical reaction
In enzymology, a saccharopine dehydrogenase (NADP+, L-lysine-forming) (EC 1.5.1.8) is an enzyme that catalyzes the chemical reaction
In enzymology, a diaminopimelate epimerase is an enzyme that catalyzes the chemical reaction
Alpha-aminoadipic semialdehyde synthase is an enzyme encoded by the AASS gene in humans and is involved in their major lysine degradation pathway. It is similar to the separate enzymes coded for by the LYS1 and LYS9 genes in yeast, and related to, although not similar in structure, the bifunctional enzyme found in plants. In humans, mutations in the AASS gene, and the corresponding alpha-aminoadipic semialdehyde synthase enzyme are associated with familial hyperlysinemia. This rare disease is inherited in an autosomal recessive pattern and patients often have no clinical symptoms.
Glutamate-5-semialdehyde is a non-proteinogenic amino acid involved in both the biosynthesis and degradation of proline and arginine, as well as in the biosynthesis of antibiotics, such as carbapenems. It is synthesized by the reduction of glutamyl-5-phosphate by glutamate-5-semialdehyde dehydrogenase.
L-Aspartic-4-semialdehyde is an α-amino acid derivative of aspartate. It is an important intermediate in the aspartate pathway, which is a metabolic pathway present in bacteria and plants. The aspartate pathway leads to the biosynthesis of a variety of amino acids from aspartate, including lysine, methionine, and threonine.
