Amino Acids: Nitrogen Disposal PDF

Amino acids: Disposal of Nitrogen (page 605 in the book) 1  Learning objectives Learn how the body/the cell deals with amino acids. Learn new terms such as: amino acid pool, transamination and oxidative deamination. Learn about ammonia exchan...

Amino acids: Disposal of Nitrogen (page 605 in the book) 1  Learning objectives Learn how the body/the cell deals with amino acids. Learn new terms such as: amino acid pool, transamination and oxidative deamination. Learn about ammonia exchange process between different tissues. 2 * Introduction: Amino acids are not stored by the body. Therefore, amino acids are obtained from the diet, synthesized de novo, or produced from normal protein degradation. Any amino acids in excess of the biosynthetic needs of the cell are rapidly degraded. 3  Amino acid Pool Free amino acids are present throughout the body and they belong to a single entity, called the amino acid pool. The amino acid pool is supplied by three sources: 1) amino acids provided by the degradation of body proteins, 2) amino acids derived from dietary protein, and 3) synthesis of nonessential amino acids from simple intermediates of metabolism. The amino acid pool is utilized by three routes: 1) synthesis of body protein, 2) amino acids consumed as precursors of essential nitrogen-containing small molecules, and 3) conversion of amino acids to glucose, glycogen, fatty acids or CO2. 4 Protein Turnover 5 Protein Turnover 6  Digestion of Dietary Proteins Amino acid catabolism is part of the larger process of the metabolism of nitrogen-containing molecules. Nitrogen enters the body in a variety of compounds present in food, the most important being amino acids contained in dietary protein. Nitrogen leaves the body as urea, ammonia, and other products derived from amino acid metabolism. 7  Digestion of Dietary Proteins 8  Mechanism of Action for Digestive Proteases → Role of Enteropeptidase ? → Free amino acids are taken into the enterocytes up by a Na+-linked secondary transport system. Di- and tripeptides are taken up by a H+-linked transport system. Amino acids are either metabolized 9 by the liver or released into the general circulation.  Amino acid Catabolism: Removal of Nitrogen A) Transamination: The first step in the catabolism of most amino acids is the transfer of their α-amino group to α-ketoglutarate. The products are an α-ketoacid and glutamate (Slide 11). This transfer of amino groups from one carbon skeleton to another is catalyzed by a family of enzymes called aminotransferases (formerly called transaminases). These enzymes are found in the cytosol and mitochondria of cells throughout the body (Slide 12). 10 * All amino acids, with the exception of Lysine, threonine and proline, participate in transamination at some point in their catabolism. 11 12 * Allaminotransferases require the coenzyme pyridoxal phosphate which is a derivative of vitamin B6. 13 B) Oxidative deamination of amino acids: These reactions occur primarily in the liver and kidney. They provide α-keto acids that can enter the central pathway of energy metabolism, and ammonia, which is a source of nitrogen in urea synthesis. 14 * Glutamate dehydrogenase is unusual in that it can use either NAD+ or NADP+ as a coenzyme. NAD+ is used primarily in oxidative deamination and NADPH is used in reductive amination. * Guanosine triphosphate (GTP) is an allosteric inhibitor of glutamate dehydrogenase, whereas adenosine diphosphate (ADP) is an allosteric activator.15 Amino Acids Catabolism 16 C) Transport of Ammonia to the Liver: 17  Summary PPP is essential for producing NADPH & ribose 5-ph. Proteins are digested mainly in the S.intestine by specialised enzymes. Amino group of free amino acids are removed by transamination and oxidative deamination. To deliver ammonia to the lever for processing; most tissues send glutamine and the muscles send alanine. 18