The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH2)2CO from ammonia (NH3). Animals that use this cycle, mainly amphibians and mammals, are called ureotelic.
Ornithine is a non-proteinogenic amino acid that plays a role in the urea cycle. Ornithine is abnormally accumulated in the body in ornithine transcarbamylase deficiency. The radical is ornithyl.
Ornithine transcarbamylase (OTC) is an enzyme that catalyzes the reaction between carbamoyl phosphate (CP) and ornithine (Orn) to form citrulline (Cit) and phosphate (Pi). There are two classes of OTC: anabolic and catabolic. This article focuses on anabolic OTC. Anabolic OTC facilitates the sixth step in the biosynthesis of the amino acid arginine in prokaryotes. In contrast, mammalian OTC plays an essential role in the urea cycle, the purpose of which is to capture toxic ammonia and transform it into urea, a less toxic nitrogen source, for excretion.
Carbamoyl phosphate is an anion of biochemical significance. In land-dwelling animals, it is an intermediary metabolite in nitrogen disposal through the urea cycle and the synthesis of pyrimidines. Its enzymatic counterpart, carbamoyl phosphate synthetase I, interacts with a class of molecules called sirtuins, NAD dependent protein deacetylases, and ATP to form carbamoyl phosphate. CP then enters the urea cycle in which it reacts with ornithine to form citrulline.
Aspartate carbamoyltransferase catalyzes the first step in the pyrimidine biosynthetic pathway.
Hyperammonemia is a metabolic disturbance characterised by an excess of ammonia in the blood. It is a dangerous condition that may lead to brain injury and death. It may be primary or secondary.
Pyridoxal phosphate (PLP, pyridoxal 5'-phosphate, P5P), the active form of vitamin B6, is a coenzyme in a variety of enzymatic reactions. The International Union of Biochemistry and Molecular Biology has catalogued more than 140 PLP-dependent activities, corresponding to ~4% of all classified activities. The versatility of PLP arises from its ability to covalently bind the substrate, and then to act as an electrophilic catalyst, thereby stabilizing different types of carbanionic reaction intermediates.
Ornithine transcarbamylase deficiency also known as OTC deficiency is the most common urea cycle disorder in humans. Ornithine transcarbamylase, the defective enzyme in this disorder is the final enzyme in the proximal portion of the urea cycle, responsible for converting carbamoyl phosphate and ornithine into citrulline. OTC deficiency is inherited in an X-linked recessive manner, meaning males are more commonly affected than females.
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.
In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondrial DNA, ribosomes, soluble enzymes, small organic molecules, nucleotide cofactors, and inorganic ions.[1] The enzymes in the matrix facilitate reactions responsible for the production of ATP, such as the citric acid cycle, oxidative phosphorylation, oxidation of pyruvate, and the beta oxidation of fatty acids.
N-Acetylglutamate synthase (NAGS) is an enzyme that catalyses the production of N-acetylglutamate (NAG) from glutamate and acetyl-CoA.
Pyrimidine biosynthesis occurs both in the body and through organic synthesis.
In enzymology, a 3-hydroxymethylcephem carbamoyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a N-acetylornithine carbamoyltransferase (EC 2.1.3.9) is an enzyme that catalyzes the chemical reaction
In enzymology, an oxamate carbamoyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a putrescine carbamoyltransferase is an enzyme that catalyzes the chemical reaction
In molecular biology, the ATCase/OTCase family is a protein family which contains two related enzymes: aspartate carbamoyltransferase EC 2.1.3.2 and ornithine carbamoyltransferase EC 2.1.3.3. It has been shown that these enzymes are evolutionary related. The predicted secondary structure of both enzymes is similar and there are some regions of sequence similarities. One of these regions includes three residues which have been shown, by crystallographic studies to be implicated in binding the phosphoryl group of carbamoyl phosphate and may also play a role in trimerisation of the molecules. The N-terminal domain is the carbamoyl phosphate binding domain. The C-terminal domain is an aspartate/ornithine-binding domain.
Homoarginine is an nonproteinogenic alpha-amino acid. It is structurally equivalent to a one-methylene group-higher homolog of arginine and to the guanidino derivative of lysine. L-Homoarginine is the naturally-occurring enantiomer. Physiologically, homoarginine increases nitric oxide (NO) supply and betters endothelial functions in the body, with a particular correlation and effect towards cardiovascular outcome and mortality. At physiological pH, homoarginine is cationic: the guanidino group is protonated.
N-succinylornithine carbamoyltransferase (EC 2.1.3.11, succinylornithine transcarbamylase, N-succinyl-L-ornithine transcarbamylase, SOTCase) is an enzyme with systematic name carbamoyl phosphate:N2-succinyl-L-ornithine carbamoyltransferase. This enzyme catalyses the following chemical reaction
L-Homocitrulline is an amino acid and a metabolite of ornithine in mammalian metabolism. The amino acid can be detected in larger amounts in the urine of individuals with urea cycle disorders. At present, it is thought that the depletion of the ornithine supply causes the accumulation of carbamyl-phosphate in the urea cycle which may be responsible for the enhanced synthesis of homocitrulline and homoarginine. Both amino acids can be detected in urine. Amino acid analysis allows for the quantitative analysis of these amino acid metabolites in biological fluids such as urine or blood.
