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The urea cycle converts highly toxic ammonia to urea for excretion. [1] This cycle was the first metabolic cycle to be discovered by Hans Krebs and Kurt Henseleit in 1932, [2] [3] [4] five years before the discovery of the TCA cycle. The urea cycle was described in more detail later on by Ratner and Cohen.
The structure of the molecule of urea is O=C(−NH 2) 2.The urea molecule is planar when in a solid crystal because of sp 2 hybridization of the N orbitals. [8] [9] It is non-planar with C 2 symmetry when in the gas phase [10] or in aqueous solution, [9] with C–N–H and H–N–H bond angles that are intermediate between the trigonal planar angle of 120° and the tetrahedral angle of 109.5°.
In the brain, the NAD+/NADH ratio in brain mitochondria encourages oxidative deamination (i.e. glutamate to α-ketoglutarate and ammonia). [1] In bacteria, the ammonia is assimilated to amino acids via glutamate and aminotransferases. [2] In plants, the enzyme can work in either direction depending on environment and stress.
The Khan Academy website also hosts content from educational YouTube channels and organizations such as Crash Course and the Museum of Modern Art. [30] It also provides online courses for preparing for standardized tests, including the SAT , AP Chemistry , Praxis Core and MCAT [ 31 ] and released LSAT preparation lessons in 2018. [ 32 ]
Argininosuccinate synthase or synthetase (ASS; EC 6.3.4.5) is an enzyme that catalyzes the synthesis of argininosuccinate from citrulline and aspartate. In humans, argininosuccinate synthase is encoded by the ASS gene located on chromosome 9. ASS is responsible for the third step of the urea cycle and one of the reactions of the citrulline-NO ...
Argininemia is an autosomal recessive urea cycle disorder where a deficiency of the enzyme arginase causes a buildup of arginine and ammonia in the blood. Ammonia, which is formed when proteins are broken down in the body, is toxic if levels become too high; the nervous system is especially sensitive to the effects of excess ammonia. [2] [6]
He found that citrulline acted as a catalyst in the metabolic reactions of urea from ammonia and carbon dioxide. He and Henseleit published their discovery in 1932. Thus the urea cycle (or "ornithine cycle") was established, and it was the first metabolic cycle to be discovered. [20] [31]
[5] [6] [7] It has the formula H 2 NC(O)NH(CH 2) 3 CH(NH 2)CO 2 H. It is a key intermediate in the urea cycle, the pathway by which mammals excrete ammonia by converting it into urea. Citrulline is also produced as a byproduct of the enzymatic production of nitric oxide from the amino acid arginine, catalyzed by nitric oxide synthase. [8]