Biochemistry

that the ornithine plays the main role in it. A prerequisite for creating a theory of urea synthesis was the discovery in the liver of ornithine amino acid and arginase enzyme that breaks down arginine into ornithine and urea. According to Krebs theory, urea synthesis begins by the interaction of ornithine with ammonia and carbon monoxide. In this case, citrulline is formed, which interacts with another ammonia molecule with the release of arginine. The arginine is hydrolyzed by arginase to ornithine and urea. Ornithine in these reactions acts as a catalyst. X. A. Krebs was awarded the Nobel Prize for opening of this cycle. The Krebs urea formation cycle is shown in Figure 8.1. A modem view of urea synthesis is shown in Figure 8.2. The closed cycle of urea synthesis can be divided into three stages. At the first and second stages, two ammonia molecules are bonded into a compound harmless to the body, and at the third stage, urea is formed. First stage. Due to the energy of ATP, carbomoylphosphate is synthesized from an ammonia molecule and carbon monoxide. Then there occurs a condensation of carbomoylphosphate with ornithine, as a result of which citrulline is synthesized. Inorganic phosphate is released. The citrulline synthesis is provided by the energy accumulated in the carbomoylphosphate bond. % /"*■ c CH. /NH. + glutamate + 2ADP + Fn CH + CCh + 2 АТФ carbamoylphosphatc- CH NH. synlhc,asc 1 <C=0 X O-PO,H. carbamoyl- COOH phosphate Glutamine Thus, one molecule of ammonia is fixed in citrulline. NH, NH, NH, 1 c=o 1 CO 1 CH, 1 NH 1 1 0 + 1 1 (CH,), 1 -* (CH,), + 1 0- 1 1 HO-P=O 1 H-C-NH, 1 1 H-C-NH, 1 1 HO-P=O 1 1 OH Carbomoylphosphate 1 COOH Ornithine 1 COOH Citrulline 1 OH Phosphate 174